y e n o m d n a
Guide Book 4 REFRIGERATION
y g r e n e e v a s o t w o H
S T R A T E G Y
ENERGY EFFICIENCY EARNINGS
3E
STRATEGY
M
I
N
Netherlandss Ministery of EconomicAffairs Netherland EUROPEAN COMMISSION
E R A
G R
Y
TSI
E
L S
A N N D
EN
Technical Services International
HOW TO SAVE ENERGY AND MONEY IN REFRIGERA REFRIGERATION TION
This booklet is part of the 3E strategy series. It provides advice on practical ways of improving energy efficiency in industrial refrigeration applications. Prepared for the European Commission DGXVII by: The Energy Research Institute Department of Mechanical Engineering Engineering University of Cape Town Private Bag Rondebosch 7701 Cape Town South Africa www.eri.uct.ac.za This project is funded by the European Commission and co-funded by the Dutch Ministry of Economics, the South African Department of Minerals and Energy and Technical Services International (ESKOM), with the Chief contractor being ETSU. Neither the European Commission, nor any person acting on behalf of the commission, nor NOVEM, ETSU, ERI, nor any of the information sources is responsible for the use of the information contained in this publication. The views and judgements given in this publication do not necessarily represent the views of the European Commission. Commission.
HOW TO SAVE ENERGY AND MONEY IN REFRIGERATION
3E
STRATEGY
HOW TO SAVE ENERGY AND MONEY IN REFRIGERATION
Other titles in the 3E strategy series:
HOWTO HOWTO SAVE SAVE ENERGY ENE RGY AND MONEY MON EY::THE 3E STRATEGY HOWTO HOWTO SAVE SAVE ENERGY AND MONEY MONE Y IN ELECTRICITY ELECTRI CITY USE HOWTO HOWTO SAVE SAVE ENERGY AND MONEY MONE Y IN BOILERSAND BOILERSAND FURNACES HOWTO HOWTO SAVE SAVE ENERGY AND MONEY MONE Y IN COMPRESSED COMPRESS EDAIR AIR SYSTEMS HOWTO HOWTO SAVE SAVE ENERGY AND MONEY IN STEAM SYSTEMS HOWTO HOWTO SAVE SAVE ENERGY ENE RGY AND MONEY MONE Y INSULATION INSULATION SYSTEMS SYS TEMS Copies of these guides may be obtained from: The Energy Research Institute Department of Mechanical Engineering Engineering University of Cape Town Private Bag Rondebosch 7701 Cape Town South Africa Tel No: +27 (0) 21 650 3892 Fax No: +27 (0) 21 686 4838 E-mail:
[email protected] Website: http://www.3e.uct.ac.za http://www.3e.uct.ac.za
ACKNOWLEDGEMENTS The Energy Research Institute would like to acknowledge the following for their contribution in the production of the guide:
Energy Technology Support Unite (ETSU), UK, for permission to use information from the ‘’Energy Efficiency Best Parctice’’ series of handbooks.
Energy Conservation Branch, Department of Energy, Mines and Resources, Canada, for permission to use information from the ‘’Energy Management’’ series of manuals.
TLV Co, Ltd, for permission to use figures from their set of handbooks on steam. Wilma Walden for graphic design work (
[email protected]). Doug Geddes of South African Breweries Breweries for the cover colour photography. photography.
Guide Book Essentials QUICK 'CHECK-LIST' FOR SAVING ENERG E NERGY Y AND MONEY IN REFRIGERA REFRIGERATION TION SYSTEMS This list is a selected summary of energy and cost savings opportunities outline in the text. Many more are detailed in the body of the booklet. These are intended to be a quick 'checklist'. EQUIPMENT MAINTENANCE (Chapter 3):
Ensure that there is good and regular maintenance of all equipment.
Avoid blockage of air flow through and around heat exchanges (e.g. evaporators and condensers).
Make sure that fouling of primary and secondary refrigeration circuits is kept to a minimum.
Maintain isolation standards where appropriate.
EFFICIENT USE OF THE REFRIGERATION SYSTEM (Chapter 5):
Keep operating hours to a minimum.
Ensure that the cooling load is kept to a minimum.
Avoid operating refrigeration plant under part-load conditions.
Investigate the possibility of improving control functions.
Reschedule production cycles to reduce peak electrical demand.
ALTERATIONS ALTERATIONS TOTHE EXISTING PLANT (Chapters 3 and 5):
Utilise waste heat where possible. Where appropriate, retrofit plant with more energy efficient components.
Increase evaporator temperature to increase system COP.
Reduce condensing temperature to increase system COP
Upgrade automatic controls in refrigeration plants to provide accurate and flexible operation.
Replace high-maintenance, centrifugal compressors with compressors selected for high efficiency when operating at part load conditions.
Upgrade insulation on primary and secondary refrigerant piping circuits.
REFRIGERANTS (Chapter 4):
Review energy efficiency when replacing CFC with ozone benign refrigerants. (This might not have an energy saving effect).
AUDITING (Chapter 5) Refrigeration efficiency is usually expressed as the coefficient of performance (COP), defined as: COP
=
Cooling effect (kW) Power input to compressor (kW)
Once the system performance has been established it is useful to identify the contribution of each plant component to the total system power input. Suitable electricity submeters can be installed for this purpose. The main contributors are normally:
compressors (typically 65%);
condenser pumps (typically 5%);
condenser fans (typically 10%);
evaporator pumps (typically 15%);
lights (typically 5%).
The next stage is to divide the total cooling load amongst the various process requirements. This should allow the loads that significantly affect costs to be highlighted.
3E
STRATEGY
Table of contents ........................................... ............................................. ............................................. ............................................ ............................................. ............................................. ..............................1 ........1 1. INTRODU INTRODUCTION CTION..................... 1.1 Purpose.......... Purpose................................. ............................................. ............................................. ............................................. ............................................. ............................................. ............................................. ....................................1 .............1 2.THE REFRIGER R EFRIGERA ATION PROCESS PROCESS..................... ........................................... ............................................. ............................................. ............................................ ...........................................2 .....................2 2.1 The vapour compressio compression n cycle ..................... ........................................... ............................................ ............................................. ............................................. ...........................................2 .....................2 2.2. Reverse Carnot Cycle............. Cycle.................................... ............................................. ............................................. ............................................. ............................................ ............................................. ........................4 .4 2.2.1 Coefficient of Performance....... Performance............................. ............................................. ............................................. ............................................. ............................................. ..............................4 ........4 2.3 Theoretical Vapour Compressio Compression n Cycle Cycle...................... ............................................ ............................................. ............................................. ............................................5 ......................5 2.3.1 Model Coefficient of Performance....... Performance............................. ............................................. ............................................. ............................................. ......................................6 ...............6 2.3.2 Practical Considerations...................................... Considerations............................................................ ............................................. ............................................. ............................................. ............................7 .....7 2.4 Absorption Cycle............. Cycle.................................... ............................................. ............................................. ............................................. ............................................. ............................................. ..............................11 ........11 2.5 Special Refrigeration Systems ...................... ............................................ ............................................. ............................................. ............................................. ..........................................13 ...................13 2.6 Variations on the simple Carnot circuit................ circuit....................................... ............................................. ............................................. ............................................. ............................13 ......13 2.6.1 Suction/liquid heat exchanger........... exchanger................................. ............................................. ............................................. ............................................. .........................................13 ..................13 2.7 Multiple evaporator circuits circuits...................... ............................................. ............................................. ............................................. ............................................. ............................................. .......................14 14 2.7.1 Multiple compressor Systems Systems.................... ........................................... ............................................. ............................................. ............................................. ...............................15 .........15 2.7.2 Cascade Systems ...................... ............................................ ............................................. ............................................. ............................................ ............................................. ..................................17 ...........17 2.7.3 Heat Pump Systems ....................... ............................................. ............................................. ............................................. ............................................. ............................................. ..........................18 ....18 .......................................... ............................................. ............................................. ............................................. ............................................. ............................................. ........................................20 .................20 3. EQUIPMEN EQUIPMENT T .................... 3.1 Compressors Compressors...................... ............................................ ............................................. ............................................. ............................................. ............................................. ............................................. ...............................20 ........20 3.1.1 Types of compressor housing .................... ........................................... ............................................. ............................................. ............................................. ...............................20 .........20 3.1.2 Hermetic and semi-hermetic compressors ..................... ........................................... ............................................. ............................................. .........................20 ...20 3.1.3 Open compressors .................... .......................................... ............................................. ............................................. ............................................. ............................................. ..............................20 ........20 3.1.4 Reciprocating compressor compressors..................... s........................................... ............................................. ............................................. ............................................. ....................................21 .............21 3.1.5 Screw compressors. compressors........................ ............................................. ............................................. ............................................. ............................................. ............................................. ..........................21 ....21 3.1.6 Scroll compressors ..................... ............................................ ............................................. ............................................ ............................................. ............................................. ..............................22 ........22 3.1.7 Compressor performance data .................... .......................................... ............................................. ............................................. ............................................. ............................22 .....22 3.1.8 Capacity control........... control.................................. ............................................. ............................................. ............................................. ............................................. ............................................. .......................22 .22 3.2 Evaporators.... Evaporators.......................... ............................................. ............................................. ............................................. ............................................. ............................................. ............................................. .............................23 .......23 3.2.1 Direct expansion ..................... ............................................ ............................................. ............................................. ............................................. ............................................. ..................................23 ...........23 3.2.2 Flooded....... Flooded.............................. ............................................. ............................................. ............................................. ............................................. ............................................. ............................................. .......................24 24 3.2.3 Oil control in evaporators..... evaporators........................... ............................................. ............................................. ............................................ ............................................. .................................25 ..........25 3.2.4 Energy efficient operation of evaporators .................... ........................................... ............................................. ............................................. .............................27 ......27 3.2.5 Defrosting.......... Defrosting................................. ............................................. ............................................. ............................................. ............................................. ............................................. .....................................27 ...............27
3.3 Expansion devices.............. devices.................................... ............................................. ............................................. ............................................. ............................................. ............................................ .............................28 .......28 3.3.1 Thermostatic expansion valves valves.................... ........................................... ............................................. ............................................. ............................................. ............................28 ......28 3.3.2 Float valve systems.......... systems................................. ............................................. ............................................. ............................................. ............................................. ..........................................30 ...................30 3.4 Condensers...... Condensers............................ ............................................. ............................................. ............................................. ............................................. ............................................. ............................................. ...........................32 .....32 3.4.1 Air-cooled condensers...... condensers............................. ............................................. ............................................. ............................................. ............................................. .....................................32 ..............32 3.4.2 Water-cooled condensers......................................... condensers............................................................... ............................................. ............................................. .......................................32 .................32 3.4.3 Evaporative condensers...... condensers............................. ............................................. ............................................ ............................................. ............................................. ...................................33 .............33 3.4.4 Loss of condenser efficiency due to air in system .................... .......................................... ............................................. ......................................38 ...............38 ............................................. ............................................. ............................................. ............................................. ............................................. ............................................. ..............................35 .......35 4. REFRIG REFRIGERANTS ERANTS ....................... 4.1 Desirable Characteristics Characteristics....................... ............................................. ............................................. ............................................. ............................................. ............................................. ...........................35 .....35 4.2 Common Refrigerants - Vapour Compression Cycles....................................................... Cycles.............................................................................. .........................38 ..38 4.3 Common Refrigerants - Absorption Cycle....................................................... Cycle............................................................................. ............................................. ...........................38 ....38 4.4 Brines and Secondary Coolants....... Coolants.............................. ............................................. ............................................. ............................................. ............................................. ..............................38 .......38 ............................................ ............................................. ............................................. .................................39 ...........39 5. ENERGY MANAGEMENT MAN AGEMENT OPPOR OPPO RTUNITI TUNITIES ES ...................... 5.1 Housekeeping Opportunities.... Opportunities.......................... ............................................. ............................................. ............................................. ............................................. .....................................39 ...............39 5.1.1 General maintenance......................................... maintenance................................................................ ............................................. ............................................ ............................................. ............................39 .....39 5.1.2 Plant operation ..................... ........................................... ............................................. ............................................. ............................................. ............................................. ......................................40 ................40 5.1.3 Instrumentation.... Instrumentation.......................... ............................................. ............................................. ............................................. ............................................. ............................................. ................................40 .........40 5.1.4 Trouble shooting ..................... ........................................... ............................................ ............................................. ............................................. ............................................. ...................................42 ............42 5.1.5 Housekeeping Worked Examples......... Examples................................ ............................................. ............................................. ............................................. .................................42 ...........42 5.2 Low Cost Opportunities..... Opportunities........................... ............................................. ............................................. ............................................. ............................................. ............................................. ........................45 .45 5.2.1 Low Cost Worked Examples.......... Examples................................ ............................................. ............................................. ............................................. ...........................................46 ....................46 5.3 Retrofit Opportunities...... Opportunities............................ ............................................. ............................................. ............................................. ............................................. ............................................. ............................47 .....47 APPENDIX 1: GLOSSARY GLOSSARY OF TERMS..................... TERMS ........................................... ............................................. ............................................. ............................................. ....................................49 .............49 APPENDIX 2: ENERGY, ENERGY,VOLUME AND MASS MAS S CONVERS C ONVERSION ION FACT FACTORS ORS ............................................57 APPENDIX 3: EXAMPLE OF MEASURING MEASUR ING COP DIRECTL DIREC TLY Y....................... ............................................. ............................................. .............................58 ......58
3E
STRATEGY
1. INTRODUCTION
Throughout history, humans have used various
1.1
PURPOSE
forms of refrigeration. Simple cooling arrangements, such as those provided by iceboxes
The following summarizes the purpose of this
and root cellars, allowed long term storage of
guide.
perishable foods. These, and other simple techniques, though largely supplanted by
mechanical refrigeration equipment, are still used
Introduce the subject of Refrigeration and Heat Pumps as used in the Industrial,
by campers, cottagers and people in remote or less
Commercial and Institutional Sectors.
developed areas.
Make building owners and operators aware of the potential energy and cost
Mechanical refrigeration systems were first built in
savings available through the implemen-
the late nineteenth century, but did not become
tation of Energy Management Oppor-
commonplace until the 1940s. Although
tunities.
mechanical refrigeration provides benefits such as refrigerated storage independent of season or
·
climate, and better living and working
Prov Provid ide e meth method odss of calc calcul ulat atin ingg the the pote potent ntia iall energy and cost savings, using simple worked
environments, the energy costs related to
examples.
operation of these systems are significant. This guide examines refrigeration and heat pump systems and identifies where energy consumption and costs may be reduced.
1
2. THE REFRIGERATION PROCESS
The majority of refrigeration systems are driven by
The temperature at which refrigerant boils
a machine, machi ne, which compresses compr esses and pumps
varies with its pressure; the higher the
refrigerant vapour around a sealed circuit. Heat is
pressure, the higher the boiling point;
absorbed and rejected through heat exchangers.
When refrigerant liquid boils, changing its
These systems work on what is called a vapour
state to a gas, it absorbs heat from its
compression cycle.
surroundings;
Refrigerant can be changed back from a gas
There are other types of plant which can be used to
to a liquid by cooling it, usually by using air
obtain a cooling effect, such as absorption cycle
or water.
systems, but these are not in common use and are only economica economically lly viable viable where where there are large
Note: Note:
In the refrigera refrigeration tion industry industry the term
supplies of waste heat.
evaporation is used instead of boiling. Also, if a gas is heated above its boiling point it is said to be superheated and if liquid is cooled below its
2.1
THE VAPOUR
condensing temperature it is sub-cooled.
COMPRESSION
To enable the refrigerant to be condensed it has to
CYCLE
be compressed to a higher pressure, and it is at this point that energy has to be used to drive the machine that performs this task. The machine is
Heat can only flow naturally from a hot to a colder
called a compressor and it is usually driven by an
body. In refrigeration system the opposite must
electric motor.
occur. This is achieved by using a substance called a refrigerant, which absorbs heat and hence boils or
The operation of a simple refrigeration system is
evaporates at a low pressure to form a gas. This gas
shown in Figure 1. The diagram shows the
is then compressed to a higher pressure, such that it
refrigerant pressure (bars) and its heat content
transfers the heat it has gained to ambient air or
(kJ/kg).
water and turns back into a liquid (condenses). In this way heat is absorbed, or removed, from a low
The refrigeration cycle can be broken down into
temperature source and transferred to one at a
the following stages:
higher temperature.
1-2
There are a number of factors, which make the
Low Low pres pressu sure re liqu liquid id refr refrig iger eran antt in the the
operation of the vapour compression cycle
evaporator absorbs heat from its
possible:
surroundings, usually air, water or some
2
other process liquid. During this process it
cooling for this process is usually achieved
changes its state from a liquid to a gas, and
by using air or water. A further reduction in
at the
temperatu temperature re happens happens in the pipe work and
evap orat or exit
is slig htly ht ly
superheated.
liquid receiver (3b - 4), so that the refrigerant liquid is sub-cooled as it enters
2-3
The The super superhea heated ted vapo vapour ur enters enters the
the expans expansio ion n devi device. ce.
compressor where its pressure is raised. There There will also also be a big increase increase in
4-1
temperature, because a proportion of the
through the expansion device, which both
energy put into the compression process is
reduces its pressure and controls the flow
transferred to the refrigerant. 3-4
The high high pressu pressure re sub-co sub-coole oled d liquid liquid passe passess
into the evaporator.
The high pressure pressure superheat superheated ed gas passes passes
It can be seen that the condenser condenser has to be capable capable
from the compressor into the condenser.
of rejecting the combined heat inputs of the
The initial part of the cooling process (3 -
evaporator and the compressor; i.e. (1 - 2) + (2 - 3)
3a) desuperheats desuperheats the gas before before it is then
has to be the same as (3 - 4). There is no heat loss or
turned back into liquid (3a - 3b). The
gain through the expansion device.
Figure 1: Single stage vapour compression circuit and pressure enthalpy diagram (source: ETSU)
3
2.2. REVERSE CARNOT
3 to 4 is constant entropy (ideal) expansion from a higher to a lower
CYCLE
pressure through the throttling device.
The Carnot Cycle is a theoretical model
From the diagram, the concept of Coefficient of
representing the basic processes of a heat engine. A
Performance (COP) is derived. The COP is the
heat engine is a devide which produces work from
ratio of the cooling or Refrigeration Effect (RE), to
heat. The Reverse Carnot cycle produces a transfer
the work required to produce the effect.
of heat from work. From the model, the maximum theoretical performance can be calculated, establishing criteria to which real refrigeration
2.2.1 COEFFICIENT OF
cycles can be compared.
PERFORMANCE
The following processes occur in the Reverse The refrigeration effect is represented as the area
Carnot Cycle (Figure 2).
under the process line 4 - 1.
4 to 1 is the absorption of heat at the RE = TL × (s1 - s4)
evaporator, a constant temperature boiling boiling process process at TL.
Where, RE = Refrigeration effect (kJ)
1 to 2 is constant entropy (ideal)
TL = Temperatu Temperature re (K)
compression. Work input is required and
s1, s4 = Entrop Entropyy [kJ/kg [kJ/kg·K ·K)J )J
the temperature of the refrigerant increases.
2 to 3 is heat rejection at the condenser, a
The theoretical work input (W (WS) (i.e. energy
constant constant temperat temperature ure process process at TH .
requirement) for the cycle is represented by the
Figure 2: Reverse Carnot Cycle (source: CEMET)
4
area "within" the cycle line 1-2-3-4-1.
Example: two refrigeration machines of similar capacity are compared. One has a COP of 4.0 while
W S= (T H- T L) × (s 4 s 1) kJ/kg
the second a COP of 3.0 at the same operating conditions. The first machine with the higher COP
The equation for coefficient of performance (COP)
is the most efficient, producing 1.33 times the
is obtained by dividing the refrigeration effect (RE)
refrigeration effect for the same work input of the
by the theoretical theoretical work work input (WS ).
second machine. The figures above show the effect
COP =
of evaporator and condenser temperatures on the
TL x (s1 - s4 ) RE = W S (TH - TL ) x (s1 - s4 )
COP for various types of chillers.
The coefficient of performance for this theoretical
The theoretical COP can also be expressed in
system is temperature dependent and can be
terms of enthalpy, where the difference in energy
reduced to:
content of the refrigerant at various points of the
T L COP (Ideal) = (TH - T L )
cycle define the cooling effect and the work input.
Actual systems are not as efficient as the ideal or
COP =
theoretical model (i.e. lower COP), but the
(h1 - h4 ) (h2 - h1 )
following general conclusion applies: The smaller the temperature difference between the heat sink
2.3
and the the heat heat source, source, (T (TH - TL ) the greater greater the
THEORETICAL THEORETI CAL VAPOUR COMPRESSION CYCLE
efficiency of the refrigeration (or heat pump) system. The COP, a measure of the energy required to produce a given refrigeration effect, is
The Carnot cycle, although a useful model to assist
an excellent means of comparing the efficiencies of
in the understanding of the refrigeration process,
similar equipment.
has certain limitations. One limitation is the lack of
Figure 3: Effects of evaporator evapo rator and an d condensing conden sing temperatur temp erature e on chiller COP CO P. (source: (sour ce: CEMET)
5
accounting for changes of state. The figure below
condenser. Step 2 2' is the initial de-superheating
shows a vapour compression cycle approximating
of the hot gas at the condenser or intermediate
the effect of the cycle on the refrigerant, assuming
equipment, and 2' - 3 is the condensation process.
ideal equipment, where:
1 - 2 Compression.
2 - 2' Desuperheating.
2' - 3 Constant Temperature
2.3.1 MODEL COEFFICIENT OF PERFORMANCE
Condensation.
As in the Reverse Carnot cycle, the coefficient or
2 - 4' Throttling.
4' - 1 Constant Temperature
performance is:
Evaporation.
COP(refrig) = refrigeration effect Work input
Assuming that the compression process starts at COP(refrig) =
point 1 as a saturated vapour, energy added in the form of shaft work will raise the temperature and pressure. Ideally, this is a constant entropy process
T L h -h = 1 4 (TH - T L ) h2 - h1
Where h 4 = h3 '
represented by a vertical line on the T-s diagram. The net result is superheating of the vapour to
Departures from the ideal Carnot cycle are
point 2. Process 2 2' 3 is heat rejection at the
apparent.
Figure 4: Basic Refrigeration Cycle. (source: CEMET)
6
[h2 - h1](theoreti ](theoretical) cal) is larger larger than than [h2 -
limitatio limitations ns such as equipment equipment size, system system pressure, pressure,
h1](Carn ](Carnot) ot)..
and design temperatures at the evaporator and
[h1 - h4](theo ](theoret retica ical) l) is smal smaller ler than than [h [h1 -
condenser, reduce the effectiveness of actual
h4](Carn ](Carnot) ot)..
systems. Actual COPs are 20 to 30 per cent of the
1
theoretical COP based on the Carnot cycle operating at the same conditions. Individual
The net effect is a COP reduction.
components, such as the compressor, may have an effectiveness of 40 to 60 per cent of the theoretical
The throttling process reduces the refrigerant
COP (Figure below). These limitations, and
pressure from the condensing (high) pressure side
techniques used to reduce their input on cycle
to the evaporator (low) pressure side. By definition,
efficiency, are now discussed.
throttling is a constant enthalpy process. The enthalpy at point 3 is equal to that at point 4', thus h3 = h4'. Energy is degraded degraded in this process, therefore therefore
2.3.2.1 Heat Transfer
the entropy must increase from point 3' to 4.
Operating temperatures in actual cycles are established to suit the temperatures required at the
2.3.2 PRACTICAL
cold medium and the temperature acceptable for
CONSIDERATIONS
the heat sink. The practical temperature gradient required to transfer heat from one fluid to another
Refrigeration and heat pump cycles are more
through a heat exchanger is in the range of 5 to 8ºC.
complex than the theoretical vapour compression
This means that the refrigerant entering the
cycle discussed in the previous sector. Practical
evaporator should be 5 to 8ºC colder than the
Figure 5: Effectiveness of Reciprocating compressors. (source: CEMET) 1
An example of measuring COP directly directly is given in Appendix 3
7
Figure: 6: Heat exchanger limitations and the effects of superheating. (source: CEMET)
desired medium temperature. The saturation
When the superheating occurs at the evaporator,
0
temperature at the condenser should should be 5 to 8 C
the enthalpy of the refrigerant is raised, extracting
above the temperature of the heat rejection
additional heat and increasing the refrigeration
medium (Figure below).
effect of the evaporator. When superheating occurs in the compressor suction piping, no useful cooling occurs.
The area enclosed by line l - 2 - 3 - 4' - l, which describes the cycle, has increased because of the temperature difference required to drive the
The increase in refrigeration effect, caused by
transfer process. There has been an increase in the
superheating in the evaporator, is usually offset by a
work required to produce the refrigeration effect
decrease in refrigeration effect at the compressor.
because the temperature difference has increased,
Because the volumetric flow rate of a compressor is
(TH - TL).
constant, the mass flow rate and refrigerating effect are reduced by decreases in refrigerant density caused by the superheating. The relative effects of increases in enthalpy and decreases in density must
2.3.2.2 Superheat
be calculated in detail. A study of the system design In the refrigerant cycle, refrigerant gas becomes
may be practical only for systems over 500 kW in
superheated at the evaporator and at the
capacity. There is a loss in refrigerating capacity of
compressor (Figure 6). During the evaporation
about one per cent for every 2.5ºC of superheat in
process the refrigerant is completely vaporized
the suction line of a reciprocating compressor.
part-way through the evaporator. As the cool
Insulation on suction lines will minimize the
refrigerant vapour continues through the
undesirable heat gain.
evaporator, additional heat is absorbed which superheats the vapour. Pressure losses, caused by
Refrigerant superheating also occurs at the
friction, further increase the amount of superheat.
compressor. The refrigerant enters the compressor
8
as a saturated vapour. Increasing the pressure will
gas) leaving the compressor will reduce the
increase the temperature and cause superheat.
required condenser capacity, and provide a high-
Friction, system inefficiency and the work added,
grade heat source for other process use. A typical
raise the entropy and superheat above that
application would be the preheating of boiler make-
occurring in the theoretical cycle. Superheat, Superheat,
up or process water. The total amount of heat
caused by the compression process, process, does not
available as superheat can be difficult to predict, as
improve cycle efficiency, but results in larger
the superheat fluctuates with changes in load
condensing equipment and large compressor
conditions. If a use can be found for low-grade heat,
discharge piping.
the total condensing load can be reclaimed. This can result in substantial energy savings.
Desuperheating is the process of removing excess
heat from superheated refrigerant vapour, and when accomplished by means external to the cycle,
2.3.2.3 FLASH GAS AND
can be beneficial to system performance.
SUBCOOLING
Desuperheating the suction gas is often impractical because of the low temperatures (less than 10 ºC) and the small amount of available energy. Some
Liquid subcooling occurs when a liquid refrigerant is
superheat is required to prevent slugs of liquid
cooled at constant pressure to below the
refrigerant from reaching the compressor and
condensation temperature (Figure 7). When
causing serious damage. At design conditions,
subcooling occurs by a heat transfer method
superheat can account for 20 per cent of the heat
external to the refrigeration cycle, the refrigerating
rejected at the condensers, and often raises
effect of the system is increased because the
condensing temperatures above 45ºC.
enthalpy of of th the su subcooled liliquid is is le less th than th the enthalpy of the saturated liquid. Subcooling of the
Desuperheating the high-pressure refrigerant (hot
liquid upstream of the throttling device also reduces
Figure 7: Effect of Subcooling (source: CEMET)
9
flashing in the liquid piping. The work input is
cent for an 8 cylinder unit. For centrifugal centrifugal
reduced, and the refrigeration effect is increased
equipment, the bypass varies with the load and
because (h1 h4) is less than (h1 h4') ' ).
impeller characteristics.
Subcooling refrigerant R-22 by 13ºC increases the refrigeration effect by about 11 per cent. If
2.3.2.5 EVAPORATOR FROSTING
subcooling is obtained from outside the cycle, each degree increase in subcooling will improve system
When a refrigeration system operates with the
capacity by approximately one per cent. Subcooling
evaporator temperature close to 0ºC, or less,
from within the cycle may not be as effective
frosting of the evaporator coil is inevitable.
because of offsetting effects in other parts of the
Examples of this would be the frosting of heat
cycle.
pump evaporator coils during winter operation, or freezer evaporators. Ice buildup on the coils lowers
Subcooling capacity can be increased by providing
the heat transfer rate, effectively reducing the
additional cooling circuits in the condenser or by
refrigeration effect. The suction temperature will
immersing the liquid receiver in a cooling tower
fall as the heat transfer rate falls, further increasing
sump. Most systems provide 5 to 7ºC subcooling at
the rate of ice buildup. For systems operating under
the condenser to improve system efficiency.
these conditions defrosting accessories are available from the equipment manufacturer.
2.3.2.4 HOT GAS BYPASS
Defrost is performed by reversing the refrigerant flow, so that the system operates in an air-
Hot gas bypass is a method of placing an artificial
conditioning mode, using the evaporator as the
heat load on the refrigeration system to produce
condenser to reject heat through the t he frosted coils.
stable suction pressures and temperatures, when
In a heat pump system used for heating, a back-up
the refrigeration load is very low. The heat load is
heating system is required to prevent chilling the
produced by bypassing hot gas from the
space during the defrost mode. Defrosting is a
compressor discharge to the evaporator inlet or
major consumer of energy. It is important that the
the compressor suction. While permitting stable
controls optimise the defrost cycle to avoid
compressor operation at low load, hot gas bypass
unnecessary defrosting while preventing unwanted
wastes energy. Bypass is required to maintain
ice build-up.
evaporator temperature above freezing, and prevent frosting of the coil, freezing of the chilled water, and compressor cycling.
2.3.2.6 HEAT PUMP CYCLE
The total refrigeration load on a compressor with hot gas bypass will be equal to the actual (low) load
The heat pump is a separate class of compression
plus the amount of hot gas bypass. Typically, the hot
refrigeration equipment whose main purpose is to
gas bypass on a reciprocating machine is 25 per cent
transfer heat from a low temperature heat source
of the nominal refrigeration capacity for a 4 cylinder
to a higher temperature heat sink for heating, rather
unit, 33 per cent for a 6 cylinder unit and 37.5 per
than for cooling. The coefficient of performance in
10
the heating configuration is: Refrigeration effect plus work input COP(Heat Pump) = Net work input
The steps in an absorption refrigeration cycle are: 2
1.
Liquid refrigerant is vaporized in the evaporator absorbing heat from the
=
TH
medium to be cooled
( T H - TL)
2.
The su suction ef effect ne necessary to to draw the vapour through the system is ac-
In a heat pump system where both heating and
complished by bringing the refrigerant into
cooling are required, a special four-way valve is
contact with a solvent. The solvent's affinity
used to reverse the functions of the evaporator and
for the refrigerant causes the refrigerant to
condenser. In this manner, the coil or exchanger is
be absorbed by the solution, reducing the
used to supply heating or cooling as required.
pressure of the refrigerant vapour. The
Alternatively, the piping or ductwork system
absorption process releases heat which
external to the heat pump can be provided with
must be removed from this portion of the
valves or dampers to reverse the primary air or fluid
cycle. The solution of refrigerant and
flows, without the reversing valve. The heat pump
solvent solvent (weak (weak liquor) liquor) is
cycle is identical to a standard refrigeration cycle on
pu mp ed fr om
the absorber at low pressure, to the
a T-s diagram (Figure 2).
generator at a high pressure. 3.
Heat is added to the weak liquor to drive the refrigerant out of solution. A heat
2.4
exchanger is located between the
ABSORPTION CYCLE
absorber and generator. Heat is removed from the strong liquor (solution with high solvent
The absorption refrigeration cycle is similar to the
and low refrigerant concentrations) leaving
vapour compression cycle, however instead of
the generator, and is added to the weak
using a compressor, high pressures are obtained by
liquor entering the generator, reducing the cycle
applying heat to a refrigerant solution.
heat input.
The system operates on the principle that variations
4.
Further heat ad added to the weak liquor in
in refrigerant solubility can be obtained by changing
the generator drives the refrigerant out of
solution temperatures and pressures. Absorption
solution providing a high pressure
systems in industry often use ammonia as the
refrigerant vapour. The hot solvent, still
refrigerant in a water solvent, whereas in
containing some refrigerant (strong liquor),
commercial and institutional applications water is
returns to the absorber through the heat
used as the refrigerant in a lithium bromide solvent.
exchanger where the solvent cycle repeats.
The basic components of an absorption system are
5.
Vapour at at hi high-pressure an and te temperature
the vapour absorber, solution transfer pumps, and a
flows to the condenser where heat is
vapour regenerator (solvent concentrator) in
rejected through a coil or heat exchanger
addition to the evaporator and condenser.
during the condensation process.
2
i.e. 'Heat 'pumped' to the hot surface.
11
Figure 8: Absorption Refrigeration Cycle. (source: CEMET)
6.
The pr pressure of of th the liliquid re refrigerant is is reduced by passing through a throttling device before returning to the evaporator section. The complete cycle is shown in Figure 8.
The generator may be equipped with a rectifier for selective distillation of refrigerant from the solution. This feature is common in large ammonia systems. Performance of an absorption chiller is measured by the COP, the ratio of actual cooling or heating effect, to the energy used to obtain that effect. The best ratios are less than one for cooling and 1.2 t o 1.4 for a heat pump application. Compared to compression cycles this is low, but if high temperature waste heat can be utilized to regenerate the refrigerant, refrigeration can be
Figure 9: Diagram of a Two-Shell Lithium Bromide Cycle Water Chiller.
obtained at reasonable costs.
(source: CEMET)
System performance is affected by: The flow diagram of a two-shell lithium bromide
Heat source temperature.
chiller is shown in Figure 9. Figure 10 shows an
Temperature of medium being cooled.
alternative configuration of an absorption machine
Temperature of the heat sink.
using only a single shell. Actual installations vary
12
considerably in layout, number of components and
Well water , or any other clean water below l5ºC,
accessories, ap applicatio tion an and re refrigerant ty type.
can be be us used fo for co cooling or or pr precooling ve ventilation ai air, or a process.
2.5
SPECIAL 2.6
REFRIGERATION
VARI ARIA ATIO TIONS NS ON THE SIMPLE CARNOT
SYSTEMS
CIRCUIT Steam jet refrigeration systems use steam ejectors to
reduce the pressure in a tank containing the return
2.6.1 SUCTION/LIQUID HEA HEAT T
water from a chilled water system. Flashing a
EXCHANGER
portion of the water in the tank reduces the liquid temperature. The chilled water is then used directly or passed through an exchanger to cool another
The cooling effect of an evaporator is proportional
heat transfer fluid.
to the length of the line between points 1 and 2 in
Figure 10: Single shell configuration. (source: CEMET)
13
Figure Figure 1. Additiona Additionall cooling cooling can be obtained obtained by
temper atures . In genera l, the evapor ating
increasing the amount of subcooling at the inlet to
temperature below which a suction/liquid suction/liquid heat
the expansion device.
exchanger no longer becomes viable is about 15º C. Care must also be taken when using these heat
The temperature of the refrigerant leaving the
exchangers on systems with R22 and R717
evaporator will be lower than that of the liquid
(ammonia) refrigerants, where the increased
entering the expansion expansion device. Therefore, it is is
suction temperature at the compressor could result
possible to reduce the liquid temperature by using a
in an excessive discharge temperature.
heat exchanger between these two pipes. A schematic layout showing how a suction/liquid heat
2.7
exchanger can be incorporated into a refrigeration circuit is given in Figure 2.
MULTIPLE EVAPORATOR CIRCUITS
It is often desirable to operate more than one evaporator on the same system. This is not a problem if all if the evaporators are working it the same temperature, as they can simply be connected in parallel. If, however, one evaporator is required to work it a lower temperature than the others, it will be necessary to operate the compressor(s) at the pressure required by the lower temperature. The other evaporators will then have to be controlled at a higher pressure by installing evaporator pressure regulators between the exit of the evaporator and the suction into the compressor(s). The disadvantage of this is that operating the system at Figure 11: The suction line heat exchanger.
the lower suction pressure will reduce the
(source: ETSU)
compressor's efficiency and capacity. If the main load has the lower temperature, then the cost of
It must be remembered that there will be a
installing an additional system for the small higher
corresponding increase in the suction gas
temperature load would probably not be
temperature entering the compressor, which will
economic, despite the increase in efficiency which
reduce its capacity as the gas will be less dense and,
would result. If the opposite case exists, it will
therefore, a lower mass of refrigerant will be
almost certainly be better to put the small low
pumped by the compressor. Experience has shown
temperature load on its own individual system and
that an overall improvement in the system's
run the main load at a higher, and hence more
efficiency will be gained at high evaporating
energy efficient, evaporating pressure.
14
taken to ensure the liquid does not get significantly
2.7.1 MUL MULTIPLE TIPLE COMPRESSOR
warmer, so that it begins to evaporate, before it
SYSTEMS
enters the expansion device. In many systems the load is too great to be handled practically with one compressor. In these cases
2.7.1.2 INTERNALLY COMPOUNDED
compressors are connected in parallel, which has
COMPRESSORS
the added advantage that their use can be cycled in order to adjust the capacity to suit the load.
Two stage compression can be achieved within one, specially designed, compressor. The gas is compressed to the intermediate pressure in the
2.7.1.1 TWO STAGE SYSTEMS
first, low stage cylinder(s) and then compressed to the condensing pressure in the high stage
Two stage, or compound, systems are used when
cylinder(s). The intermediate condition is called the
there is a large difference between the evaporating
interstage pressure, and some form of cooling is
and condensing temperatures. This usually occurs
usually used to reduce the temperature of the
when process or product storage conditions
refrigerant before it enters the second stage of
require a low evaporating temperature, such as in
compression.
freeze drying or ice cream storage.
The selection and application of such a compressor
At these compression ratios two stage systems
is relatively simple; however, there are a limited
have to be used because a single stage system
number of compressor variations available.
would result in an unacceptably high discharge
Selecting a design that matches a specific system
temperature in the compressor. In addition, in
requirement usually results in a compromise which
some cases two stage compression can give more
is made at the expense of energy efficiency. The
efficient compressor operation.
fixed volume ratio of the two compression stages also means that efficiencies are lower than they
There is no easy rule to determine where two stage
could be where demand varies.
compression, with its additional design and installation complexity, becomes preferable to single stage compression. Generally, with
2.7.1.3 EXTERNALLY COMPOUNDING
refrigerants like R22, two stage compression may
COMPRESSORS
be used on systems using suction cooled compressors evaporating below about - 30ºC.
In this case two stage compression is achieved by There are two ways that two stage compression
using two separate compressors - one for the low
can be achieved and the method selected will affect
stage and another for the high. This more flexible
efficiency. In both cases, additional system capacity
approach enables the system designer to match a
can be obtained by first passing the refrigerant used
compressor combination to the load more
for interstage cooling through a liquid line
accurately and select the most economical
subcooler. If this method is used, care must be
interstage pressure.
15
The design and selection process is far more
system where the compression work is done by
complicated than with the internally compounded
either two positive displacement compressors or
variation, but the use of computer selection
by two stages of a multistage centrifugal unit. The
programs make it easier and quicker. To limit the
flash intercooler subcools the refrigerant liquid to
final discharge temperature interstage cooling is
the evaporator by vaporizing a portion of the
used, usually by injecting a small quantity of
refrigerant after the first throttling stage. The flash
refrigerant into the gas flow although other suitable
gas returns at an intermediate point in the
sources of cooling could be used.
compression process to improve the compression efficiency by cooling the superheated gas (Figure
A multistage system is used when large
13). 13).
temperature and pressure differences exist between the evaporator and the condenser. Figure
In large systems with a number of evaporators and
12 illustrates the basic arrangement for a two-stage
large compression compression (temperature) ratios, the
Figure 12: Schematic of 2-Stage Refrigeration System. (source: CEMET)
Figure 13: Diagram of a 2-Stage Vapour Compression Cycle. (source: CEMET)
16
number of flash intercoolers and compression
below the process or product storage temperature.
stag stages es is incr increa ease sed d to ma maxi ximi mize ze syst system em effi effici cien ency cy..
The The cond conden ense serr for for this this syst system em is also also the the evaporator of the high pressure system. The high stage system transfers the heat from this condenser evaporator to the external condenser. The low
2.7.2 CASCADE SYSTEMS
pressure system can therefore use a refrigerant Cascade systems are another method of
which has a suitably low boiling point for the
overcoming the problems in applications requiring
application, and its condensing pressure can be kept
low evaporating temperatures. Two separate
at a safe level by the high stage of the cascade.
refrigeration circuits are used, usually with different A cascade system cannot be as efficient as a well
refrigerants in each circuit.
designed externally compounded system, because The evaporator of the low pressure system is
there is a loss in efficiency due to the heat transfer
Figure 14: Three stage Cascade System. (source: CEMET)
Figure 15: Two stage cascade system with booster circuit. (source: CEMET)
17
between the two systems. It does, however, however, offer
In each case the first term refers to the heat source
more flexibility, as a small low temperature load
for heating applications, or the heat sink for cooling.
could be i nterface d with an e xisting h igh
The second term refers to the secondary
temperature system. In many cases cascading is the
refrigerant used for process or space heating and
only alternative if very low temperatures are
cooling. For example:
required.
An air-to-air heat pump (Figure 16)
Refrigerants used in each stage may be different and
provides heating or cooling. In the cooling
are selected for optimum performance at the given
mode, heat is removed from the air in the
evaporator and condenser temperatures. An
space and discharged to the outside air. In
alternative arrangement uses a common condenser
the heating mode, heat is removed from
with a booster circuit to obtain two separate
the outside air and discharged to air in the
evaporator temperatures (Figure 15).
space.
2.7.3 HEA HEAT T PUMP SYSTEMS
An air-to-water system extracts heat from ambient or exhaust air to heat or preheat water used for space or process heating.
A heat pump is a device used to transfer heat from a lower temperature to a higher temperature, for
A water-to-air system (Figure 17) provides
heating the warmer area or process. In many
heating and cooling of air with water as the
installations, reversible heat pumps are used, which
heat sink or source.
heat or cool the process, or space.
A water-to-water system extracts heat from
A four-way reversing valve is used to reverse the
a water source while simultaneously
refrigerant flow, to permit the use of the coils or
rejecting heat to a water heat sink, to either
exchangers in either the condenser or evaporator
heat or cool a space or process.
mode. With a fixed refrigerant circuit and no reversing valve, the secondary refrigerant flows can
Earth-to-air and earth to water systems have
be reversed through appropriate external valve or
limited use. Practical application is limited
damper arrangements.
to space heating where the total heating or cooling effect is small, and the ground coil
Various heat source and heat sink arrangements are
size is equally small.
possible, depending on heating and cooling requirements.
The COP for heat pump systems varies from 2 to 3 for small air-to-air space heating systems, to 5 or 6
Air-to-air.
for large systems that operate across small
Air-to-water.
temperature differences.
Water-to-air.
Water-to-water.
Most heat pump systems are provided with a
Earth-to-air.
backup heat source to offset reductions in heat
Earth-to-water.
output as the evaporator (heat source: outdoor
18
Figure 16: Typical schematic of an air-to-air heat pump system. (source: CEMET)
coil coil)) temp temper erat atur ure e fall falls. s. This This is part partic icul ular arly ly true true in airair-
resu result lt from from lack lack of prop proper er clea cleani ning ng.. Abso Absorp rpti tion on
to-air, space-heating systems where heat output
chillers face reductions in refrigerating capacity of
dec decrea reases ses as as the the out outd doortem tempera peratu turre lo lowers ers.
up to 24 per per cen cent, t, with pow power incr increa easses of 7.5 7.5 per cent, from poor maintenance.
2.7.3.1 EFFECTS OF MAINTENANCE ON SYSTEM EFFICIENCY Owners of refrigeration and heat pump equipment should follow the manufacturer's service and maintenance recommendations to maintain maximum system efficiency over the life of the equipment, leaking seals, poor lubrication and faulty controls will reduce system life and performance. A simple procedure, such as regular cleaning of the evaporator and condenser, has a marked effect on performance. Table 7 shows the effect of dirty heat transfer elements on an air-cooled reciprocating compressor system. Reductions in refrigerating capacity up to 25 per cent, with simultaneous
Figure 17: Typical schematic of water-to-air
increases in power input of up to 40 per cent, can
heat pump system. (source: CEMET)
19
3. EQUIPMENT
The following major components are required in
is contained in a common gas-tight housing.
vapour compression refrigeration systems.
Hermetic tic compressors are built into a welded shell, and there is no access to the internal parts for
Refrigerant compressors. compressors.
servicing or repair. Semi-hermetic compressors are
Evaporators.
assembled with removable covers, usually sealed by
Throttling devices.
gaskets, enabling a limited amount of access for on-
Condensers.
site maintenance.
Heat rejection equipment. Both types of compressor are designed and built
3.1
with specially selected motors. The motor's size
COMPRESSORS
and type is matched to the motion work of the compressor for specific applications and
The purpose of the compressor in a refrigeration
refrigerants. To obtain the maximum efficiency the
system is to draw the low pressure refrigerant gas
compressor must be closely matched to the system
from the evaporator and compress it to a higher
duty.
pressure. This enables the gas to be condensed back into liquid by some convenient low cost
Hermetic compressors and larger semi-hermetic
source of cooling, such as air or water.
compressors are usually suction-cooled, the refrigerant passing over the motor windings before entering the compressor cylinders. This helps to
3.1.1 TYPES OF COMPRESSOR
cool the motor windings, but reduces the capacity
HOUSING
of the compressor. Externally cooled types, where the gas passes directly into the t he cylinders, are usually
Most compressors are driven by an electric motor,
about 8% more efficient than the equivalent
sometimes built into a common casing. Other
suction-cooled models models but are only available up to
compressors have an external drive, the shaft
a motor size of about 5 kW.
passing through a rotating gas seal where it exits from the pressurised casing.
3.1.3 OPEN COMPRESSORS 3.1.2 HERMETIC AND SEMI-
This type of compressor has an external drive shaft
HERMETIC COMPRESSORS
allowing a suitably sized motor to be selected and connected to it, either with a direct coupling or via
These compressors have the motor directly
belts. It is important to size the motor accurately in
attached to the main shaft, and the whole assembly
relation to the compressor's duty. Running motors
20
at below their design duty reduces their power
factor and their efficiency.
improved flow through valves: o
less restricted gas flow path,
o
reduced pressure drop;
When comparing the input power requirements of open and semi-hermetic compressors, the motor's
efficiency and losses due to the drive have to be
minimised heat transfer from discharge to suction gas.
taken into account for open drive machines. Such modifications can improve efficiency by up to Where extended operation of the plant is
20%, although in many cases the capital cost of the
envisaged it could prove viable to invest in an
compressor will be higher because of the increased
energy efficient (high efficiency) motor. At present
complexity of manufacturing.
the cost will be higher than a standard motor but this could change as the price differential between
It is critical to the reliability of reciprocating
standard and high efficiency motors is decreasing.
compressors that liquid refrigerant or large
The payback time, derived by a simple cost analysis,
quantities of oil are not injected into the cylinders,
will usually be less than two years given the long
as this will cause mechanical failure in the
running hours and may show a better return on
compressor.
investment.
3.1.5 SCREW COMPRESSORS
3.1.4 RECIPROCATING COMPRESSORS
Screw compressors are available for duties from about 50 kW up to thousands of kilowatts and are
Reciprocating compressors are the most common
generally used on medium to high temperature
types of compressor and are available for a wide
applications. The geometry of the compressor
range of applications.
determines its optimum pressure ratio. Operation away from this ratio will significantly reduce its
The design of a compressor is optimised for
efficiency. For this reason manufacturers usually
operation within a designated application envelope
produce a range of machines with different
with specified refrigerants. Operating a compressor
operating characteristics.
at high temperature conditions with valves designed for low temperature operation could result in losses of up to 10% in the extraction rate.
A large quantity of oil is injected into screw
With many compressors it could also result in t he
compressors to seal the running clearances
motor being overloaded and tripping its protection
between the rotors and the casing. The oil has to be
device.
removed from the refrigerant in a suitable sized separator. A significant amount of the heat of
Compressors have been developed with improved
compression is absorbed by the oil, which must be
efficiencies. The main areas of improvement are:
removed by an oil cooler. It is preferable to cool the oil by using a supply of air or water. Using a supply of
refrigerant for cooling can reduce the system
clearance volume reduction;
21
capacity by up to 10%, with a corresponding loss of
compressors that the correct correct running speed of the
efficiency.
compressor has been used. With semi-hermetic compressors this speed is fixed by the design of the built-in motor.
3.1.6 SCROLL COMPRESSORS The scroll type of rotary compressor has been the
3.1. 3. 1.8 8 CA CAPA PACI CITY TY CO CONT NTRO ROL L
subject of extensive development in recent years, as improved machining techniques have made its
To maintain the maximum system efficiency in
production viable.
systems with widely varying loads, it is important to be able to vary the duty of the compressor. In a
Scroll compressors are being increasingly applied to
multi-compressor system this can either be
medium and small air-conditioning applications
achieved by switching a number of compressors off
because of their quiet, low vibration operation and
or by reducing their individual pumping capacities.
good efficiency. Their efficiency advantage over
The best way to save energy is always to switch off
reciprocating compressors at lower compression
any unnecessary machines.
ratios makes them ideal for high temperature refrigeration applications, such as beer cellar and milk tank cooling.
3.1.8.1 RECIPROCATING COMPRESSORS
Scroll compressors are also being developed for
There are a number of methods used to reduce the
lower temperature applications.
capacity of compressors:
3.1.7 COMPRESSOR PERFORMANCE DATA
blocked suction gas,
suction valve lifting;
discharge gas recirculation
The extraction rate and power input of a compressor depend principally on the evaporating and condensing temperatures. Compressor performance is usually presented in graphical (Fig) or in tabular format. These data are presented at specific rating conditions, and corrections have to be made to take into account actual site operating conditions for:
suction gas temperature;
liquid subcooling. Figure 18: Typical compressor performance
Care must be taken with data for open
data. (source: (sourc e: ETSU)
22
When selecting a compressor, it is important to
about 50% capacity, but below this it falls off very
check the manufacturer's data to ensure that the
quickly.
model chosen is of an energy efficient design. The reduction in input power should match, as closely as possible, the reduction in refrigeration duty.
3.2
It is also worthwhile checking whether
EVAPORATORS
There are two principal types of evaporator:
supplementary compressor cooling is required
while capacity control is in operation, as this will
direct expansion (sometimes called "dry expansion" or DX);
need additional energy.
flooded.
The number of stages of capacity reduction that can be obtained will depend on the design of the
3.2.1 DIRECT EXP EXPANSION ANSION
compressor, and is usually a function of the number of cylinders. On suction cooled compressors the
These are commonly used to cool either air or a
minimum capacity is often limited by the loss of
liquid. The expansion device used with this type of
cooling of the motor.
evaporator is an expansion valve. A direct expansion evaporator used for cooling air
3.1.8.2 SCREW COMPRESSORS
is shown in Figure 19. There are many different designs available using plain or finned tube, both
The capacity of a large screw compressor can be
with and without forced circulation of air or some
varied from 100% down to 10% by using a slide
process fluid. Certain tube designs incorporate
vane. The part load efficiency is acceptable down to
internal devices to maximise heat exchange and
Figure 19: Liquid distribution on a direct expansion circuit. (source: ETSU)
23
thus efficiency, by causing turbulence to keep the
evaporated before reaching the outlet.
liquid in full contact with the tube wall.
By monitoring the flow of refrigerant, the expansion device maintains a superheat of about 5ºC at the outlet of the evaporator. This ensures that the duty is as high as is practically possible while still
3.2.1.1 DESIGN FEATURES
protecting the compressor from liquid refrigerant A typical evaporator will have a number of parallel
returning down the suction line. This feature is
circuits designed to:
important for the reliability of reciprocating machines, but less so for rotary compressors.
maximise heat transfer;
ensure good oil return;
minimise pressure drop.
3.2.1.3 OPERATIONAL PROBLEMS
A distributor is used to ensure refrigerant flows evenly between the different parallel circuits.
The efficiency of an evaporator can be affected by an uneven distribution of refrigerant, and hence
To enhance the heat transfer in air-cooled designs, the surface of the refrigerant-carrying tubes is usually extended by using external fins. To maximise their surface the fins are spaced as closely together as possible without restricting the air flow.
cooling, between the different circuits. This can occur if the distributor is incorrectly positioned - it should always be vertical so that there is an even feed through each outlet - or if one
On low temperature systems, where ice can form
distributor line becomes damaged.
on the fin surfaces, a wider spacing has to be used to ensure adequate air flow when ice build-up occurs.
It is impossible for each circuit to be totally filled In the past few years compact plate heat
with saturated refrigerant, as there must be
exchangers have become increasingly popular for
sufficient superheat to enable the expansion device
direct expansion cooling of liquids. Due to their
to control the flow of refrigerant. This means that
design they have a very good heat transfer capability
the heat transfer efficiency will be reduced at the
and hence high efficiency. Some larger designs can
end of each circuit where superheated gas is
be disassembled for cleaning, whereas the smaller
present. Oil logging can also reduce the efficiency
type are brazed together as a sealed assembly. They
of an evaporator - more information on this subject
can be used with all halocarbon refrigerants, but
is given in Section 4.3.
because of the materials used for construction they arc not suitable for ammonia.
3.2.2 FLOODED 3.2.1.2 OPERATING FEATURES
There are two types of flooded evaporator:
Saturated refrigerant is fed through a distributor
shell and tube;
into the expansion tubes where it is totally
plate type.
24
4.3 for more information. Fouling on the
3.2.2.1 SHELL AND TUBE
external surfaces of the tubes, i.e. the These are commonly used in larger applications for
process fluid side, can be difficult to rectify.
cooling liquids. There are a number of different
This will also reduce heat transfer.
designs but they all have the same basic
characteristics.
Due to the internal volume of the shell, large quantities of refrigerant are required with the corresponding cost and
Design and operating features
environmental or safety issues if a leak
In a shell and tube evaporator, the fluid to
should occur.
be cooled is passed through the tubes with the evaporating refrigerant boiling off into
gas within the body of the shell.
3.2.2.2 PLATE TYPE
The refrigerant level in the shell is
Recently, the use of plate heat exchangers as
maintained so that the top tube is always
flooded evaporators in recirculation systems has
covered with liquid. In this way the most
become more common. They offer the following
efficient heat exchange, liquid to liquid, is
advantages over the shell and tube type:
achieved over the whole of the cooling interface. To ensure optimum efficiency, the liquid level is usually maintained by
higher heat transfer coefficients;
a smaller temperature difference between
using a low pressure float valve. The
the refrigerant and the cooled liquid,
operation of this type of device is
resulting in higher evaporating
explained in Section 7. Alternatively, an
temperatures and therefore improved
expansion device and level sensor can be
system efficiency;
used.
more compact units requiring less plant room space;
The space in the upper part of the shell
allows any droplets of liquid to be
smaller refrigerant charges; the ability to clean non-brazed assemblies,
separated from the gas returning to the
thus maintaining a good heat transfer
compressor. This separation is sometimes
capability.
achieved in a different vessel called a surge drum.
3.2.3 OIL CONTROL IN
Operational problems
EVAPORATORS
Flooded shell and tube evaporators are In order to maintain the optimum system efficiency
usually large and relatively expensive.
it is important that oil is not allowed to collect in the
Accumulation of oil can reduce the heat
evaporator, coating the tubes and thereby reducing
transfer and hence efficiency - see Section
their capability to transfer heat. Different actions
25
are required to control oil, depending on the type
system duty between a number of smaller
of evaporator and refrigerant.
evaporators, isolating some as the load diminishes.
3.2.3.1 DIRECT EXPANSION
3.2.3.2 FLOODED EVAPORATORS
EVAPORATORS Ammonia systems
The main rule with this type of evaporator, whether it is being used with halocarbons or ammonia, is to
Oil is almost totally insoluble in ammonia
maintain an adequate refrigerant velocity to carry
and will separate out, collecting in the
the oil through the tube assembly.
bottom of the evaporator and must be periodically drained, either manually or
Problems can occur if the evaporator has to
automatically. This is not a hazardous
operate over a wide range of loads, as the flow
operation providing proper safety
might not be sufficient at the lowest duty to achieve
precautions are taken. A careful log must
the minimum required velocity. Under these
be kept recording any oil added to or
conditions it may be necessary to split the total
removed from the system.
Figure 20:Typical oil rectification system diagram. (source: CEMET)
26
Any control connections made to the
the evaporator.
lower part of the evaporator's shell must be above the highest possible oil level. Oil
The size of evaporator should be decided at the
is very viscous at low temperatures and can
design stage by evaluating the additional evaporator evaporator
cause a restriction in small bore pipes.
capital cost and the resulting lower running costs, and comparing the simple paybacks obtained by
Halocarbon systems
each option.
Some refrigerants, for example R11 and
The heat transfer will be influenced by factors such
Rl2, are completely miscible with oil under
as:
all operating conditions and no special action is required to prevent oil logging.
oil logging;
fouling and corrosion of heat transfer surfaces;
Other refrigerants, for example R22 and
R502, are miscible at high temperatures
incorrect control of the refrigerant flow or level in the evaporator;
but, at low temperatures, an oil rich layer
·
will form on the top of the liquid
frost build up.
refrigerant. By carefully positioning tapping points ill the evaporator's shell, this oil rich
3.2.5 DEFROSTING
mixture can be removed from the evaporator and transferred into a rectifier. The rectifier is then heated to boil the
As noted before, allowance must be made in the fin
majority of the refrigerant out of the oil
spacing to allow for ice build-up on evaporators
before it is returned to the compressor.
operating with refrigerant temperatures below
The most energy efficient method of
0ºC. To maintain an adequate air flow through the
supplying this heat is to use the warm
fin block it has to be defrosted periodically,
refrigerant in the liquid line which incurs no
requiring the use of heat.
additional energy costs, and has the further advantage of increasing the liquid
Energy efficient defrosting depends on the
subcooling. A typical oil rectification
following factors:
arrangement is shown in Figure 20.
initiating a defrost operation only when it becomes necessary through loss of
3.2.4 ENERGY EFFICIENT
performance;
OPERATION OF
the necessary heat;
EVAPORATORS
The efficiency of a refrigeration system is increased
can be achieved by: ·maximising the size of the evaporator;
maintaining the peak heat transfer rate of
ensuring that the defrost heat is evenly distributed over the whole of the fin block;
when the evaporating temperature increases. This
using the most efficient method of applying
·stopping the defrost cycle as soon as the fin block is totally clear of ice;
·minimising the amount of defrost heat absorbed by the process fluid or product.
27
Table1: Types of Liquid Coolers
Type of cooler
U s u al R e f r i g e r a n t F e e d
U sual Range of
C o m m o nl y U s e d w i t h
Device
C a p ac i t y ( k W )
R e f r i g e r an t N u m be r s
Fl o o de d s h el l- a n d - ba r e - t u b e
Lo w pr e ss u r e f lo a t
1 75 - 17 5 0
71 7 ( A m m o ni a )
Floo Floode dedd shel shelll-an andd-fifinn nned ed-t -tub ubee
Low Low pres pressu sure re flo float at 1 75 - 35 0 00
11 , 12 , 2 2, 11 3
Hi g h pr e s su r e f lo a t , f i x e d o r i f i c e( s ) , w e i r ( s )
114, 134a, 500, 502 Spray-type-shell- an and-tube
Lo w pr e ss u r e f lo a t Hi g h pr e s su r e f lo a t
3 50 - 17 50
11 , 12 , 1 3B 1 , 2 2, 113, 114, 134a
Di re c t - e x p a n s i o n s he ll - a n d- t ub e
Th e rm a l ex p a n s i o n v a lv e
1 7. 5- 1 25 0
12 , 22 , 1 34 a , 5 00 , 50 2, 7 17
Fl o o de d B a u de l o t c o o l er
Lo w p r e ss u r e f lo a t
3 5- 3 50
71 7
Di re c t - e x p a n s i o n B a u de lo t c o o le r
Th e rm a l ex p a n s i o n v a lv e
1 7. 5- 8 5
12 , 22 , 1 34 a , 7 17
Fl o o de d d o ub l e- p i p e c o o le r
Lo w p r e ss u r e f lo a t
3 5- 8 5
71 7
Di re c t - e x p a n s i o n d ou b le - p i p e c o o le r Th e rm a l ex p a n s i o n v a lv e
1 7. 5- 8 5
12 , 22 , 1 34 a , 7 17
S h el l- a n d - c o i l c o o l er
Th e rm a l ex p a n s i o n v a lv e
7 - 35
12 , 22 , 1 34 a , 7 17
Fl o o de d t a n k - a n d- a g i t a t o r
Lo w p r e ss u r e f lo a t
1 75 - 70 0
71 7
3.3
EXPANSION EXP ANSION DEVICES
The purpose of an expansion valve is to:
thermostatic expansion valve;
high pressure float valve;
low pressure float valve.
reduce the pressure of the liquid
Capillary tubes (which just drop the refrigerant
refrigerant from the condensing pressure
pressure but cannot regulate flow) are used in
to the evaporating pressure;
domestic type systems. These are factory
modulate the flow of liquid refrigerant into
assembled and cannot be adjusted.
the evaporator Correct selection and installation of expansion
3.3.1 THERMOSTATIC
valves is very important, because their incorrect operation will reduce the efficiency and reliability of a system.
EXPANSION VALVES Thermostatic expansion valves are used on most
There are three types of expansion valve widely
commercial installations. A typical example shown
used in commercial and industrial refrigeration:
in Figure 21. The refrigerant pressure is dropped
28
through an orifice, and the flow of refrigerant is
across them varies varies widely, for example if the
regulated by a needle valve and diaphragm
condensing pressure 'floats' with ambient. To cope
arrangement. The diaphragm is moved by the
with such conditions other valves are now available.
pressure inside the controlling phial, which senses the temperature of the refrigerant leaving the
3.3.1.1 BALANCED PORT VALVES
evaporator which should be approximately 5º C higher than the evaporating temperature, to ensure there is no liquid refrigerant present which
Balanced port valves are very similar in design and
could damage the compressor. This temperature
operation to the conventional thermostatic valve
difference is the superheat setting of the valve and
apart from a special internal balanced port design.
can he set by adjusting the valve. Correct setting is
This allows the valve to control control inlet pressure
vital to the efficient and reliable operation of the
accurately over a much wider range. These valves
refrigeration system.
cost approximately 20% more than a conventional valve, but are currently available only in a limited
If the load on the evaporator changes, then the
range of sizes.
temperature of the refrigerant leaving the evaporator will also change. The controlling phial
3.3.1.2 ELECTRONIC EXPANSION
will sense this and automatically adjust the
VALVE
refrigerant flow to accommodate the load change. A major disadvantage of thermostatic valves is that
Electronic expansion valves work in a similar way to
they cannot work well if the pressure difference
thermostatic valves, except that the temperature is
Figure 21: Thermostatic Expansion Valve. (source: CEMET)
29
Figure 22: Electronic Electro nic expansion expans ion valve on direct direc t expansion expansi on air cooler. cooler. (source: (sour ce: ETSU)
sensed sensed electronic electronically ally and this signal signal opens opens and
the high hig h (recei ver) pre ssure or the
closes the orifice via a small electrical motor. The
(evaporator) pressure of the system.
low
valve can therefore operate with a wider difference in pressure across it. A further advantage is that they can be easily integrated into an electronic or microprocessor microprocessor control system. Figure 22 shows an
3.3.2.1 HIGH PRESSURE (HP) FLOAT VALVE
electronic expansion valve with a direct expansion air cooler.
A typical HP float valve is shown in Figure 23. This type of valve is used to maintain a liquid level in the
Electronic valves are much more expensive than
receiver and operates at receiver pressure.
conventional thermostatic valves, and will give a payback of less than a year only on systems with a
The receiver pressure controls the pilot line
capacity greater than 100kw.
pressure, and as this pressure varies the expansion valve opens and closes to supply liquid refrigerant from the receiver to the evaporator.
3.3.2 FLOA FLOAT T VAL ALVE VE SYSTEMS SYST EMS An HP float valve is used in large industrial systems A float valve system uses a float chamber with a
with single evaporators. As it provides no control of
separate modulating expansion valve, connected by
the level of refrigerant in the evaporator, the
a pilot line. The float chamber can either operate at
amount of refrigerant in the system must be
30
Figure 23: High pressure float valve. (source: ETSU)
Figure 24: Low pressure float expansion system. (source: ETSU)
correct, i.e. the system is said to be critically
evaporator and operates at evaporator pressure.
charged. To ensure correct operation, the
This liquid level affects the pressure in the pilot line,
evaporator must be fitted with a level gauge which
and as the pressure varies the expansion valve valve
is checked regularly.
modulates the supply of liquid from the receiver to the evaporator.
3.3.2.2 LOW PRESSURE (LP)
LP float valves are used on systems which have
FLOAT VALVE
more than one evaporator connected to one compressor or to several compressors in parallel.
A typical LP float system is shown in Figure 24. An It is important that the expansion valve is fitted at a
LP float valve is used to maintain a liquid level in the
31
level below the liquid liquid surface in the receiver, in
are used, such as electronic expansion valves.
order to prevent refrigerant gas going through the valve and hence reducing efficiency. A level gauge
3.4. 3. 4.1 1
AIRAI R-CO COOL OLED ED CO COND NDEN ENSE SERS RS
must be fitted to the receiver so that the liquid level can be checked to ensure adequate performance is
In an air-cooled condenser the refrigerant
maintained.
condenses inside tubes over which air is forced by fans. To improve the heat transfer, the tube surface is usually extended using corrugated metal fins.
3.4
CONDENSERS
A well designed plant should operate with a condensing temperature no higher than l4ºC above
There are three types of condenser in widespread
the ambient temperature. With larger condensers
use:
it is common practice to control the head pressure
air-cooled (using ambient air);
water-cooled (using mains, river or cooling
by switching off or slowing down fans, although this is inefficient.
tower water);
If air-cooled condensers are being used in a
evaporative cooled (using ambient air and
corrosive atmosphere (for example, near the sea or
recirculated water).
in polluted air) then a suitable tube/fin material
The two latter types take advantage of the lower wet bulb ambient temperature and the greater heat
combination or a coating should be used. Air-cooled condensers are susceptible to blockage
transfer affect of water, and therefore operate with
by air borne debris such as dust, feathers, packaging,
lower condensing temperatures. When comparing
and so on. They must be regularly cleaned (but not
different condenser types the power requirements
with refrigerant) to prevent a build up of
of associated fans, pumps and heaters should be
contamination, as this will reduce the air flow and
taken into account. In general, systems under 100
hence increase the condensing pressure.
kW capacity use air-cooled condensers unless there is a space or noise restriction.
3.4.2 WATER-COOLED
For a given capacity, a larger condenser will result in a lower condensing temperature and hence better
CONDENSERS
efficiency. Problems can be caused on installations which use thermostatic expansion valves if the
Water-cooled condensers condensers are of the shell and tube
condensing (head) pressure varies widely. Such
type. The cooling water flows in tubes inside the
valves are unable to control refrigerant flow reliably
shell, and refrigerant inside the shell condenses on
under such conditions, and reduced efficiency and
the outside of the cold tubes. Heat transfer is
reliability will result. Some form of head pressure
improved as the water velocity is increased. An
control may be used to raise the head pressure
efficient system will work with a temperature rise of
artificially, although this is inefficient and is not
5ºC for the water passing through the condenser,
necessary if more sophisticated expansion devices
and a difference of 5ºC between the condensing
32
temperature and that of the water leaving the
likely to cause a problem, cleanable condensers
condenser. On very small commercial installations
should be used.
mains water is often used directly, although this is becoming less common on new installations due to water metering.
3.4.3 EVAPORATIVE CONDENSERS
On larger installations the water will be cooled in a cooling tower, where the cooling effect is achieved
In an evaporative condenser, refrigerant is
by evaporating some of the cooling water into the
condensed in tubes which are wetted and over
air. Blockages in the air or water side will significantly
which air is forced. The water used to wet the
reduce the efficiency of the cooling tower. Such
outside of the tubes is recirculated, although a
blockages are common and are normally caused by
certain amount of make up water will be needed.
hard water deposits or algae growth. Water should
Evaporative condensers should operate with similar
be treated to prevent these and also to prevent
temperatures to the water-cooled conden-
bacteria growth. The cooling tower should cool the
ser/cooling tower combination above. The water
water to within 13 - l8ºC of the wet bulb ambient
used will require treatment, as described for water-
temperature (which can be up to 10ºC lower than
cooled condensers above.
the dry bulb temperature). The water side of the condenser is also liable to
3.4.4 LOSS OF CONDENSER
blockage caused by hard water deposits. If this is
EFFICIENCY DUE TO AIR IN SYSTEM Air and other non-condensable gases in a refrigeration system will increase the condensing temperature and hence reduce efficiency. For example, in a medium temperature ammonia system working with a condenser which contains 15% air, the running costs will increase by 12%. Air can remain in a system after installation or service, if the system has been inadequately evacuated prior to charging with refrigerant. While running, air can be drawn into a system operating at a suction condition lower than atmospheric pressure, if there is a leak on the low side of the system. It is possible to check for air and other non-
Figure 25: Draw-through-type of evaporative
condensable gases when the system is not working
condenser (source: CEMET)
33
and the temperatures have had a chance to
Any air should be safely purged from the system by
stabilise. If there is no air present, then the
a skilled refrigeration technician, with minimum
temperature in the condenser should be equivalent
refrigerant emission to the atmosphere. atmosphere.
to the temperature of the ambient air or of the water flowing through a water-cooled condenser. If air is in the system the temperature will be higher.
34
4. REFRIGERANTS
4.1
DESIRABLE
Large heat of vaporization to minimize
equipment size and refrigerant quantity.
CHARACTERISTICS
Low specific volume in the vapour phase
to minimize compressor size. This aspect is Refrigerants for Industrial, Commercial and
critical for reciprocating and screw type
Institutional refrigeration and heat pump systems
compressors.
are selected to provide the best refrigeration refrigeration effect
Low liquid phase specific heat to minimize
at a reasonable cost. The following characteristics
the heat transfer required when
are desirable.
subcooling the liquid below the condensing temperature.
Non-flammable to reduce the fire hazard.
Non-toxic to reduce potential health
desired condensing temperatures to
hazards.
eliminate requirement for heavy duty or
Low saturation pressure required at
Table 2: Physical Physica l Properties of some Refrigera Ref rigerants nts R e f r i g e r a nt
Chem.
Mo Mo l e c .
Bo i l i ng
Fr eez.
Crit ical
Crit ical
Cr i t i c a l
Formula
Mass
P o i nt
P o i nt
Temp
P r e s s ur e V o l um e
(NBP) ,
ºC
ºC
kPa
L/kg
°C
He li u m
He
4. 0 02 6
- 268 . 9
N o ne
- 2 67. 9
22 8. 8
14. 43
Hy dr o g en
H2
2. 0 15 9
- 252 . 8
- 2 59 . 2
- 23 9. 9
13 15
33. 21
28 . 97
- 194 . 3
---
- 14 0. 7
37 72
3. 0 48
Ai r O x y g en
O2
31 . 99 88
- 182 . 9
- 2 18 . 8
- 11 8. 4
50 77
2. 3 41
Me t h a ne
CH4
16 . 04
- 161 . 5
- 1 82 . 2
- 82 . 5
46 38
6. 1 81
Te t r a f lu o ro - m e t h a ne
CF4
88 . 01
- 127 . 9
- 1 84 . 9
- 45 . 7
37 41
1. 5 98
Et h yl en e
C 2H 4
28 . 05
- 103 . 7
- 1 69
9. 3
51 14
4. 3 7
Tr i f l uo r o m et ha n e
C HF 3
70 . 02
- 82. 1
- 1 55
25 . 6
48 33
1. 9 42
C hl o ro t r i f l u or o - m e t ha n e
C C l F3
10 4. 4 7
- 81. 4
- 1 81
28 . 8
38 65
1. 7 29
C a r bo n Di ox i de
C O2
44 . 01
- 78. 4
- 5 6. 6
31 . 1
73 72
2. 1 35
P r o py le ne
C 3H 6
42 . 09
- 47. 7
- 1 85
91 . 8
46 18
4. 4 95
P r o pa n e
C 3H 8
44 . 10
-4 2. 07
- 1 87 . 7
96 . 8
42 54
4. 5 45
C hl o ro d i f l uo r o - m et h a n e
C HC l F 2
86 . 48
-4 0. 76
- 1 60
96 . 0
49 74
1. 9 04
C hl o ro p en t a - f l uo r o et h a n e
C C l F 2C F 3
15 4. 4 8
-3 9. 1
- 1 06
79 . 9
31 53
1. 6 29
35
a
Ammonia
NH 3
1 7. 03
- 3 3. 3
- 77 . 7
13 3. 0
1 14 17
4 . 24 5
D i c h lo r o di -f lu or o m e t ha n e
C C l 2F 2
1 20 . 93
- 2 9. 7 9
- 15 8
11 2. 0
4 11 3
1 . 79 2
D i f l uo r o et h a n e
C H 3C H F 2
6 6. 05
- 2 5. 0
- 11 7
11 3. 5
4 49 2
2 . 74 1
S ul ph ur D i o x i d e
SO2
6 4. 07
- 1 0. 0
- 75 . 5
15 7. 5
7 87 5
1 . 91 0
C h l or o d i f lu o r o- e t h a ne
C H 3C C l F 2
1 00 . 5
-9.8
- 13 1
13 7. 1
4 12 0
2 . 29 7
M et hy l A m i n e
C H 3N H 2
3 1. 06
-6.7
- 92 . 5
15 6. 9
7 45 5
O c t a f lu o r oc yc l o- b u t a ne
C 4 F8
2 00 . 04
-5.8
- 41 . 4
11 5. 3
2 78 1
1 . 61 1
Butane
C 4H10
5 8. 13
-0.5
- 13 8. 5
15 2. 0
3 79 4
4 . 38 3
D i c h lo r o f lu o ro - m e t ha ne
C H C l 2F
1 02 . 92
8.8
- 13 5
17 8. 5
5 16 8
1 . 91 7
E t h y l A m i n e C 2 H 5N H 2
45 . 08
1 6. 6
- 8 0. 6
18 3. 0
56 19
T r i c h lo r of lu or o - m e t ha n e
C C I 3F
1 37 . 38
2 3. 8 2
- 11 1
19 8. 0
4 40 6
1 . 80 4
E t h yl Et h er C 4H 10O
74 . 12
3 4. 6
- 1 16. 3
19 4. 0
36 03
3 . 7 90
D i c h lo r o he x a - f l uo r o pr o pa n e
C 3C l 2 F 6
2 20 . 93
3 5. 6 9
- 12 5. 4
18 0. 0
2 75 3
T r i c h lo r oe t h yl en e
C HC l =C C l 2
1 31 . 39
8 7. 2
- 73
27 1. 1
5 01 6
Water
H 2O
1 8. 0 2
1 00
0
37 4. 2
2 21 03
1 . 74 2
3 . 12 8
high pressure equipment.
R11, R12, R502 and R22) are being phased out by
Low pressure portion of the cycle should
international agreement.
be above atmospheric pressure to prevent
inward leakage of air and water vapour into
The Montreal Protocol on substances suspected of
the refrigerant piping.
attacking ozone was first agreed in 1988, and has
High heat transfer coefficients.
now been signed by over 90 countries. HCFCs such as R22, which have much lower ozone depletion
Physical properties of various common refrigerants
potentials than CFCs, are termed transition
are listed in Table 2
substances and cannot be considered long term refrigerants. New HCFCs are being developed
The relative safety and hazard level of various
which, together with R22, are being used today to
refrigerants have been compiled and classified classified
replace CFCs in many applications.
under ANSI Code B9.l l97l and by Underwriter's Underwriter's laboratories. Table 2 provides a listing of these
New refrigerants which do not attack ozone are
properties for various refrigerants.
also being developed. R134a, the fifirst of of these to to become commercially available, has been
Many refrigerants widely used today belong to the
developed to replace R12 on mobile air-
family family of chemicals chemicals called CFCs (chloro(chloro-
cond condit itio ioni ning ng and and smal smalll refr refrig iger erat atio ion n appl applic icat atio ions ns.. It
fluorocarbons) fluorocarbons) which which are suspected of breaking breaking
is not a 'drop in' replacement for R12, although it
down ozone in the upper atmosphere. This
operates with very similar temperatures and
environmental concern is causing major changes in
pressures. It is not miscible with the mineral oils
refrigerant development and use. CFC and HCFC
currently used with CFCs and HCFCs, so new
(hydrochlorofluorocarbon) (hydrochlorofluorocarbon) type refrigerants (e.g.
synthetic oils have been developed. Systems
36
Table 3: Non-CFC refrigerants
Re f .
O DP
1
GW P
2
Avai labil it y
BP
3
at
Ef f ici ency
4
A pp l i c a t i o n
1Bar (ºC) R2 2
0 . 05
0. 3 4
No w
- 40 . 8
B et t er t h a n R1 2;
R1 R 1 2, R 50 2 re p la c e m en t
same as R502 MP 39
0 . 03
0. 2 2
No w
- 28 . 9
S i m i l a r t o R 12
Me di u m t em p r et a i l f o o d
MP 66
0 . 03 5
0. 0 24
No w
- 30 . 7
S i m i l a r t o R 12
R1 2 r e pl a c em e nt
HP 8 1
0 . 03
0. 5 2
No w
- 47 . 4
S a m e t o s li g ht l y
Me di u m t em p r et a i l f o o d,
bett better er than than R12 R12
ice ice mac machi hine nes, s, vend vendin ingg
S a m e t o s li g ht l y
Me di u m a n d h i g h t e m p
R1 34 a
0
0. 3 4
No w
- 26 . 1
w or or se se t ha ha n R 12 12 69 S 69 L
0 . 04 0 . 02 8
4. 0 4. 0 9
No w a p p ro x . No w
- 50 . 0 - 50 . 6
f oo oo d re ta ta ilil
S a m e t o s li g ht l y
Lo w t e m p c lo s e c o u pl ed
bett better er tha thann R502 R502
syst system emss
S a m e t o s li g ht l y
Lo w t e m p re m o t e s y st e m s
better than R502 HP 8 0 FX 10
0 . 02 0 . 02 3
0. 6 3 0. 7 6
N ow N ow
- 49 . 0 - 49 . 7
S li g ht l y wo r s e
Lo w t e m p re t a i l f o od ,
t h a n R50 2
t r a ns p or t
S li g ht l y be t t e r
Lo w t e m p
than R502 HP 6 2
0
0. 9 4
1 993 / 4
- 46 . 5
S i m i l a r t o R 50 2
Lo w t e m p
FX 40
0
0. 8 8
1 993 f o r t r i a l s
- 55 . 0
S li g ht l y wo r s e
Lo w t e m p
than R502 KL EA 60
0
0. 3 5
N ow
- 38 . 0 t o 4 5. 0
S i m i l a r t o R 50 2
Lo w t e m p
Notes: 1.
The The ODP ODP (ozo (ozone ne depl deplet etio ionn pot poten enti tial al)) isis rela relati tive ve to R11 R11 for for whi which ch OP=1 OP=1..
2.
The The GWP GWP (dir (direc ectt gree greenh nhou ouse se war warmi ming ng pot poten enti tial al)) is rel relat ativ ivee to R11 R11 for for which which GWP= GWP=1. 1.
3.
B P = b o i l i ng p o i n t
4
The effic efficien iency cy is based based on limi limited tedtest test data (not (not theore theoretica ticall calcul calculati ations ons)) in the case case ofnewly newly devel develope opedd refrig refrigera erants nts and and is theref therefore ore an indica tion of expecte d efficie ncy in actual in stallat ions. Much of this data is provisi onal - the actual ef fect on effici ency of any new refrig erant should be checked at the operati ng condit ions of the system .
running with R12 can be retrofilled with Rl34a if the
synthetic oils. Very few single single substances are totally
oil is also changed, providing that the components
suitable as refrigerants, and therefore blends of new
in the system can be used with the new refrigerant.
and existing substances are being developed.
Successful retrofills have been carried out, with
Blends have already been developed based on
minim inimu um disru isrup ptio tion to the the coo coolin ling appli ppliccatio ation. n.
HCF HCFCs and and are are curr curre ently ntly bei being ng use used d as tra tran nsiti sitio on substances. Care must be taken, however, to
Further Further ozone ozone benign benign refrigeran refrigerants ts are being
ensure t hat the blend re mains consistent co nsistent
developed which will also need to use the new
throughout a plants lifetime.
37
Table 3 gives information on non-CFC refrigerants
Water, is the most common refrigerant, and is used
that are available now and on those that will be
in combination with lithium bromide as the
available in the near future.
absorbent.
4.2
4.4
COMMON
BRINES AND
REFRIGERANTS REFRIG ERANTS -
SECONDARY SECONDAR Y
VAPOUR COMPRESS COMPRESSION ION
COOLANTS
CYCLES
Secondary refrigerants, brines and heat transfer fluids find common use in refrigeration applications. applications.
Freons: R22, used primarily in air conditioning; R-12,
These liquids are cooled or heated by the primary
used primarily in medium- and high-temperature
refrigerant and transfer heat energy without a
refrigeration (R-134a is now used as a replacement
change of state. Their use is common where:
for R-12); R-502, used primarily in low-temperature refrigeration. R-500 can still be found in older
equipment.
Large refrigerant quantities would otherwise be required.
Ammonia , refrigerant R-717, one of the earliest
Toxicity or flammability of the refrigerant is a concern.
refrigerants, is now limited to industrial applications
because of its high toxicity. High cycle efficiency,
Central refrigeration is used to produce cooling for a number of remote locations.
low specific volume, high latent heat and low cost
Many examples exist where brines and
led to its popularity, particularly in ice rink facilities
secondary coolants are used.
and other applications where large temperature
differences were required.
Chilled water or glycol-water solutions for air-conditioning and process cooling.
Carbon dioxide is a non-toxic, non-flammable,
Calcium chloride or sodium chloride in
odourless, colourless, colourless, and inert gas. Because of high
solution with water for ice production in
operating pressures and high horsepower
skating rink applications.
requirements its use as a refrigerant is limited to
Propylene glycol and water solutions for use in food and potable water refrigeration
specific industrial applications.
systems.
4.3
Hydrocarbon refrigerants in the liquid
COMMON
phase for extremely low temperature
REFRIGERANTS -
applications.
ABSORPTION CYCLE
Selection of the brine type and concentration is made on the basis of freezing point, crystallization
Ammonia is a refrigerant used with water as the
temperature, specific volume, viscosity, specific
absorbent (solvent). Use of ammonia is declining
heat and boiling point. Toxicity, flammability and
with the introduction of refrigerants that have low
corrosion characteristics are secondary factors, but
toxicity and operate at lower system pressures.
must be considered in the overall analysis.
38
5. ENERGY MANAGEMENT OPPORTUNITIES
'Energy 'Energy Ma Manage nagement ment Opportuni pportunities ties'' is a term term that
surfac sur faces es reduce red uce s the hea t transf tra nsfer er
represents the ways that energy energy can be used wisely sely to
efficiency, efficien cy, requiring requirin g higher temperature temperatu re
reduce operating operating costs. costs. A number number of Energy
differences differen ces to maintain the heat transfer
Management Manage ment Opportunitie Opport unities, s, subdivided subdiv ided into
rate. An increase in temperature difference
Housekeeping, Low Cost, and Retrofit categories are
reduces reduc es the COP.
outlined in this section with worked examples or written text to illustrate the potential energy savings.
Repair suction and liquid line insulation to
This is not a complete listing of the opportunities
reduce superheating of suction gas and loss
available for refrigeration and heat pump systems.
of subcooling. Refrigerant lines gain heat
However, it is intended to provide ideas for
when they are located in spaces that are
management, operating, and maintenance personnel
not air-conditioned, increasing the system
to allow them to identify other opportunities that are
load without producing useful cooling.
applicable to a particular facility. Other guides in this
series should be considered.
Calibrate controls and check operation on a regular basis to ensure that the refrigeration and heat pump systems operate efficiently.
5.1
HOUSEKEEPING
Maintain specified refrigerant charge in refrigeration and heat pump equipment.
OPPORTUNITIES
Insufficient refrigerant reduces system performance and capacity. Reduced mass flow rates of refrigerant causes excessive superheating of the refrigerant at the
5.1.1 GENERAL
evaporator which reduces the efficiency of
MAINTENANCE
the compressor, and increases the condensing temperatures.
Implemented housekeeping opportunities are Energy
Provide unrestricted air movement around
Management actions that are done on a regular basis
condensing units and cooling towers to
and never less than once a year. The following are
eliminate short circuiting or the airstreams
typical Energy Management Opportunities in this
which causes a higher condensing
category:
temperature and pressure.
Minimize the simultaneous operation of
Keep heat transfer surfaces of evaporators
heating and cooling systems. Strategically
and condensers clean, through regular
located thermometers will help identify
inspection and cleaning. Fouling of the
this problem.
39
instrumentation should be considered to
5.1.2 PLANT OPERA OPERATION TION
measure/monitor: Plant performance will be maintained if the system
is monitored and appropriate remedial action taken
when necessary. Adequate instrumentation is
necessary to enable a plant to be easily monitored.
pressures; temperatures; current and/or power.
The use of computers to analyse data will help to Figure 26 shows where such measurements should
highlight areas which should be investigated before
be taken on a water chilling system.
problems develop.
Many compressors can he used on part capacity,
5.1.3 5.1 .3
and the number of cylinders operating on a
INSTR IN STRUME UMENT NTA ATIO TION N
reciprocating compressor can be indicated by the signal to the solenoid valves which unload cylinders.
There should be sufficient instrumentation on a
On centrifugal or screw compressors an analogue
plant to enable the performance to be assessed and
indication of the control signal can be useful.
faults diagnosed. With smaller commercial systems pressure gauges, thermometers and amp probes of the type carried by service engineers are likely to be
Level gauges should be fitted to all vessels that contain liquid refrigerant, i.e. liquid receivers, shell
sufficient. With larger installations permanent
and tube evaporators and condensers, and
Figure 26 Simple direct expansion water chilling system (source: ETSU)
40
g n i t a r o p p a m v e E t
. t e e h s g o l a f o e l p m a x e n A : 4 e l b a T
. t p i m x e 5 E t P
0
t p e m l e 4 n t I P
5 . 0
n w e 0 i o t m a 1 5 / l F R F 5 l
7 4 5
. C p º t m i 5 x e 5 / E T T 2
6 . 3
4 . 8
R O T A R O P A V E
r e t a W
C . º t p 0 e l m 4 1 / e n I t T 7
R E S N E D N O C
d i u q i l g n i s n e d n o C
d i u e q n i i L l
e r u t a r e p m e t e g r a h c s i D
e r u t a r e p m e t
n o i t c u S
R O S S E R P M O C
C º 2 2 o P t
C º 5 2 P
0 . 8 2
d e t a r u t a 3 S P
0 . 0 3
. f f i D l i O
r a s s b 3 e r p >
2 . 4
. p m o C
g n i d a o L %
0 0 1
l a u t c 2 A T
2 . 7 5
d e t a r u t a 2 S P
0 . 0 3
o t C C º º l 3 7 a u + + t c 1 1 1 A T P P
1 . 6
d e t 5 a r o u t t a 1 S P 1
0 . 3
s p m A
5 . 2 9
s r u n o u H R
6 2 3 2
d e d n e m m o c e R
e m i T e u l a v
s e g u a g e l a c s l a u d m o r f s e r u t a r e p m e t d e t a r u t a s
o t r e f e r ) P ( s e g u a g e r u s s e r p
m o r f n e k a t s e r u t a r e p m e T : B N
0 0 4 1
2 9 . e t 7 . a 8 D 1
41
interstage vessels on two stage systems. The
5.1.5 HOUSEKEEPING
normal refrigerant level, and the acceptable
WORKED EXAMPLES
maximum and minimum levels should be marked oil the gauge.
Worked examples are used to illustrate potential energy and cost savings. The examples are considered typical of conditions found in refrigeration and heat pump systems.
5.1.3.1 PLANT MONITORING The instrumentation fitted to a system enables on-
5.1.5.1 REDUCE CONDENSING
site plant operators or off-site contractors to monitor performance and detect faults before they
TEMPERATURE
cause major decline in efficiency. Over time the performance of a 175 kW
refrigeration system, with an air-cooled, packaged
Log Sheets
condensing unit, deteriorated. Investigation Plant log sheets should be kept, containing
revealed that the space where the condensing unit
information on normal operation as well as
was located had been converted to a storage area
recording day to day operation. These logs
with stacked materials. Air flow to the condenser
allow performance to be assessed
was blocked, causing short circuiting of the cooling
providing that:
air stream.
-
On a day when the ambient temperature was
data is measured and recorded
35ºC, the air entering the condenser was 46.1ºC.
accurately
The actual refrigerating load was 120 kW.
-
information is correctly analysed
-
problems found are followed by appropriate action arid recorded.
Manufacturer's data for 120 kW cooling indicates that the compressor power is 42.3 kW at 35º C, and 49.76 kW at 46.1ºC. The system operates 2000 hours per year at the elevated temperature.
Table 4 shows an example log sheet for the plain
Removal of the stored materials from the
shown in Figure 26. The data recorded on a log
condenser vicinity would prevent short circuiting
sheet for a specific plant will depend on the
and lower the air temperature entering the
characteristics of that plant.
condenser to the ambient temperature. t emperature. Electricity cost is 0.10R/kWh Compressor energy required at 46.1ºC
5.1.4 TROUBLE SHOOTING From monitoring the refrigeration system, several irregularities can be linked directly to savings
=
2000 x 49.76
=
99 520 kWh
Compressor energy required at 35ºC
potential. Below table 5 gives a list of such potential
=
2000 x 42.3
symptoms.
=
84 600 kWh
42
Table 5: Common faults on refrigeration systems M ajor s ymptom
O t he r symptoms
F a ul t
S o l ut i o n
O p e r a t i on a l cos t penalty
Low cooling cooling duty c o m p are d wi th c o m p re s so r c ur ve s
Bubbles Bubbles in liquid liquid li n e an d l o w o rz e r o su b co o l i n g fr o m condenser
System System underchar undercharged ged LP fl o at o r T E V s y st em
Add refrigera refrigerant nt to co r r ec t le ve l
Up to 25% or more reduction reduction i n d u ty an d CO P
O n H P f lo lo at at s ys ys ttee ms ms :
H P f lo lo at at v alal ve ve s ttuu ck ck o pe pe n, n, b yp yp as as ssee d, d, g as as p as as ssii ng ng
D et et er er m mii ne ne w hy hy b yp yp as as s v alal ve ve w as as o pe pe ne ne d initially. Cor rect fault and close bypass valve
U p t o 5 0% 0% r ed ed uc uc titi on on i n d ut ut y a nd nd C OP OP
High actual compres sor di sc h ar ge te m pe r atu r e a nd nd l ow ow c om om pr pr es es ssoo r a bs o r be d p o we r
Broken or obstructed re c ip r o ca tin g c om om pr pr es es ssoo r s uc uc titi on on val ve
Repair valve and identify an d re c tif y c au se o f b lo lo ck ck ag ag e o r o bs bs ttrr uc uc titi on on
Loss of duty in proportio n to c y l in d e rs af fe c te d
High actual compres sor discha discharge rge temper temperat ature ure
Broken or obstructed recipr reciproca ocati ting ng comp compre ress ssor or disc discha harg rgee valve
Repair valve and identify and recti rectify fy cause cause of brea breaka kage ge or obst obstru ruct ctio ionn
Loss of duty and COP in propor proportio tionn to cylind cylinders ers affe affect cted ed
Low Low evap evapor orat atin ingg pres pressu sure re hi gh wat e r/ ai r s i de p r e ss ur e d ro p
Foul Foulin ingg of air/ air/wa wate terr side side o f e vap o r ato r
Clea Cleann evap evapor orat ator or and and l o c ate an d c ur e s o ur c e o f f o ul i n g
Up to 15% 15% loss loss of COP, COP, 25 % lo s s o f co o l i n g d ut y
Low evaporat evaporating ing pressure pressure h i gh ap p are n t s u pe r he a t
Blocked Blocked suction suction strainer strainer
Clean Clean suction suction strainer. strainer. I de n ti fy an d r ec ti fy s o u rc e of blockage
Up to 30% reduction reduction in COP
Loss of oil from c om om pr pr e ss ss o r c r an k ca se
Oil accumulat accumulation ion in f l oo oo de de d e va va po po ra ra t o r
Remove Remove excess excess oil, install install e ffff e ct ct i ve ve o ilil d r ai ai no r re c ti fi ca tio n s y ste m
Up to 25% reduction reduction in COP
Loss of oil from c o m p re s so r crankcase
Poor oil return from ex p an si o n val ve s y ste m
Re-design suction side pi p ew o rk
Up to 25% reductio n in du ty a nd CO P
In all systems, systems,possible possible subcooling, high
Obstruct Obstruction ion in liquid liquid line
Locate Locate and clear clear obstruction. Identify
Up to 15% loss in COP, 25% loss of cooling duty
Poor Poor evap evapor orat ator or e f fe c tiv en e s s
h i gh l iq u id l i n e suction superheat P oo oo r c on on de de ns ns er er e f fe c tive n e s s
Low Low suct suctio ionn supe superh rhea eatt
ca us e an d re c ti fy
H ig ig h c on on de de ns ns in in g t em p e ra tur e , hi gh liquid subcooling
V er er y h ig ig h o ve ve rc rc ha ha rg rg eo f L P fl o a t o r T EVs y st em
R em em ov ov e e xc xc es es s c ha ha rg rg e
U p t o 1 0% 0% l os os s o f d ut ut y,y, 15 % r e d uc ti o n in CO P
High condensing, high l i q ui d su b c oo l i n g
Air or non-condensable gas in s y st em
Purge non-condensable gas i n s y st e m
Up to 10% loss in COP
High water/air side p r e ss ur e d ro p
Fouling of air-water side o f c o nd e n se r
Clean condense r and l o cat e an d cu r e so u r ce of fouling
Up to 25% loss in COP, 10% l o ss i n du ty
LP floa floatt and and TEV: TEV: poss possib ible le
Inco Incorr rrec ectt or faul faulty ty expa expans nsii
Iden Identi tify fy and and rect rectififyy faul faultt
Up to 15% 15% redu reduct ctio ionn in duty duty..
l o w co m p re s so r d i sc ha rg e
on d e vi ce co n tr o l
Po te nt ia l co m p re s so r f ail u r e
t em p e ra tur e H ig ig h s uc uc titi on on s u pe pe rh rh ea ea t
H P f l oa oa t : l ow ow l iq iq ui ui d l ev ev e l i n
d ue t o l iq u id c ar ry o ve r S ys ys ttee m u n de de rc rc ha ha r g ed ed
e vap o rat o r
43
A d d r e frfr i ge ge ra ra n t t o c or or re re ct ct
U p t o 1 0% 0% l os os s o f d ut ut y.y.
l e ve l
7½ % re d u cti o n i n C OP
Energy Saved
Rand savings
=
99 520 - 84 600
=
14 920 kWh
=
14920 kWh x R0.10/kwh
=
R1492/yr
"Clean" refrigerant condensing temperature: 40.6ºC = 313.6 K "Dirty" COP =
0.25* x
= 0.25 x
274.7 = 1.55 319.1 - 274.7 T L (TH - TL )
"Clean" COP = 0.25* x
5.1.5.2 CLEAN EVAPORATORS
T L (TH - TL )
AND CONDENSERS = 0.25 x
An 880 kW centrifugal chiller with a forced draft cooling tower is used to produce chilled water for air conditioning. On a walk-through audit it was
280.2 = 2.10 313.6 - 280.2
*COP actual values estimated as .25 x COP (theoretical)
noticed that algae was growing on the wetted surfaces of the cooling tower. Water blowdown to control mineral deposits and chemical feed was
Change in COP =
performed by leaving the blowdown valve open. Chemical testing and treatment was neglected.
(2.10 - 1.55) 1.55
x 100
= 35% (improvement)
During a plant shutdown, the heat exchanger surfaces of the evaporator and condenser were
Power required for 880 kW cooling:
examined and found to be fouled. A contractor was hired to clean the equipment at a cost of R1,700 for each heat exchanger and Rl,400 for the cooling
"Dirty"
880 1.55
=
568 kW
"Clean"
880 2.10
=
419 kW
tower, for a total of R4.800. Electricity cost is 0.10/kWh. Performance of the system was evaluated, before and after the cleaning, using manufacturer's data The system operates at full load for an estimated
and estimated COP values.
900 hours per year. Savings because of cleaning are: "Dirty" refrigerant suction temperature: Savings
1.7ºC = 274.7 K
=
(568 - 419) kW x 900 hr x R0. 10/kWh
= R13410/yr
"Dirty" refrigerant condensing temperature: 46.lºC = 319.1 K
Simple Simple payback payback = =
"Clean" refrigerant suction temperature:
(Investme (Investment/Sa nt/Savings vings)) 0.36 years (4 months)
7.2ºC = 280.2 K
44
5.2
performance of the refrigeration system
LOW COST
will offset the increased power
OPPORTUNITIES
requirement of the cooling tower fan and make-up water costs.
Implemented low cost opportunities are Energy
Provide an automatic water treatment
Management actions that are done once and for
system to add chemicals, and control
which the cost in not considered great. The following
blowdown, to match the water losses of
are typical Energy Management Opportunities in
cooling tower and evaporative condenser
this category.
systems. Proper water treatment will maximize heat transfer effectiveness, and
Increase evaporator temperature to
keep condensing temperatures low.
increase system COP.
Benefits include reduced quantities of
Reset the temperature of the chilled water,
make-up and blowdown water, and lower
glycol solution or air as a function of the
operating and maintenance costs.
cooling required, to allow the evaporator
temperature to rise at part loads. For
peak electrical demand and make more
example, the setting of the air temperature
efficient use of available cooling or heating
leaving the evaporator of an air-
energy. Rescheduling may permit
conditioning system can be based on the
shutdown of some compressors in
latent load requirement. As the latent load
multiunit systems while running others at
falls, less dehumidification is required, and
optimum load and peak efficiency.
the controls adjust the evaporator
Operation at higher efficiency may delay
temperature upwards.
purchase of additional equipment when
Relocate the outdoor coil of an air-to-air heat
total load increases
pump to a clean exhaust airstream. A
Reschedule production cycles to reduce
Upgrade
automatic
controls
in
building's ventilation exhaust is warmer
refrigeration plants to provide accurate
than the outside ambient air during most of
and flexible operation. Solid state digital
the heating season.
control can optimize equipment and
Reduce condensing temperature to
system operation to meet load
increase system COP
requirements with minimum power
Relocate air cooled condensers and heat
consumption, and/or shed load to reduce
pump outdoor coils to clean exhaust
short term electrical peaks.
airstreams. Generally the building's
Replace high-maintenance, centrifugal
ventilation exhaust is cooler than the
compressors with compressors selected
outside ambient air when cooling is
for high efficiency when operating at part
required.
load conditions.
Reduce condenser water temperature by
resetting cooling tower temperature
Upgrade insulation on primary and secondary refrigerant piping circuits.
controls. Detailed analysis is required to
determine whether increased
Provide multispeed fan motors on cooling towers, evaporative coolers and air cooled
45
condensers. Normally, equipment is
5.2.1
selected to match the rarely attained peak
LOW COST WORKED EXAMPLES
design condition. Lower outdoor wet and
dry bulb temperatures, and lower indoor
Worked examples are used to illustrate potential
loads, predominate. Reducing condenser
cost savings. The examples are considered typical
air flow to match the capacity requirement
of the conditions found in building refrigeration and
reduces the fan power.
heat pump systems
Evaporative coolers and condensers operated in winter may provide adequate capacity when operated with dry coils.
5.2.1.1 WATER TREATMENT FOR
Maintenance, water and electrical costs
CONDENSER WATER
can be reduced. Heat tracing and pan heaters can be turned off. The detrimental
Maintain maximum heat transfer rates by
effect of icing on equipment and buildings
minimizing fouling. Consider the condenser water
is eliminated. Note that the reduced
system in Housekeeping Worked Example 2.
power requirements for fan and circulating
Assume that half the change in performance was
pumps in cooling towers and evaporative
because of condenser cleaning.
coolers may be offset by a COP decrease caused by higher condenser temperatures.
Reduce Reduced d elec electri trical cal costs costs = R 3 353 / 2
Detailed analysis is required.
R 1 676
Consider a new heat pump system instead of a new air conditioning system, if winter
An automatic water treatment system was
heating is required. The higher equipment
provided for the cooling tower, to optimize water
cost will be offset by reduced heating costs
make-up and blowdown, and automatically feed
during the winter season.
chemicals to control fouling. Capital cost was
Provide lockable covers on automatic
R3,000. Annual chemical costs are estimated at
controls and thermostats, to prevent
R800. Note that the system must be cleaned before
unauthorised tampering or adjustment.
automatic water treatment is initiated.
Use clean process cooling water that
Simple Sim ple paybac paybackk = R 3 000 /1 676
normally goes to drain for evaporative
=1.8 yrs
condenser or cooling tower make-up
water. While not conserving energy, this
At the end of the first year, the cost of cleaning the
will reduce operating costs.
exchangers, the cooling tower, and providing
Re-evaluate the use of hot gas bypass
condenser water treatment is negligible. See
when a refrigeration unit works at part-
Housekeeping Worked Example 2.
load for any significant period. It may be possible to eliminate the bypass feature
Other costs are reduced. Annual cleaning of
and cycle or turn off the refrigeration
exchangers is eliminated and controlled blowdown blowdown
system.
reduces make-up water requirements.
46
5.2.1.2 HEAT PUMP VERSUS
5.2.1.3. HOT GAS
ELECTRIC HEAT
BYPASS
A small office addition is planned for an industrial
A small manufacturing plant has a 90 kW capacity
facility in Cape Town. An economical means of
refrigeration plant operating at a COP of 3. The
heating and cooling the addition is desirable. The
compressor has six cylinders and operates at full-
plant rejects waste heat in the form of warm water.
load 24 hours per day, 5 days per week and 50
Loads for the proposed building, including
weeks per year. During weekends the refrigeration
ventilation, are 35.17 kW cooling, and 29.31 kW
load is less than 10 per cent of full-load, and the unit
heating. A rooftop packaged air conditioning
uses hot gas bypass to avoid low suction pressures
system with electric heating is proposed. The
and evaporator frosting. It is proposed to eliminate
estimated annual heating cost for the all-electric
hot gas bypass and cycle the unit on and off to meet
system is R2 45l.
the low loads. Controls will be modified to eliminate hot gas bypass and install anti-short cycle
A water-to-air heat pump was considered as an
timers at a cost of R1400. The hot gas bypass
alternative to the basic, air-conditioning with
imposes a cooling load of about 33 per cent on the
electric heat, rooftop package initially proposed.
unit at a cost of R1188 per year. In addition to the
The heat pump was selected to meet the design
cost of providing the 9 kW cooling load, by
heating and cooling loads, with electric duct heaters
eliminating hot gas bypass, this R1188 can be saved.
for 100 per cent backup. The COP for heating at the given water condition was 2.25 and similar to
Simple payback =
the air-conditioner performance in the summer.
=
The source of warm water was available 85 per
R1 400/R1 188 1.2 years
cent of the time during the heating season, and cooling water was available throughout the cooling
5.3
season.
OPPORTUNITIES
Annual heat pump energy costs = =
RETROFIT
(0.85xR2451)/2.25 +(0.15xR2451)
Implemented retrofit opportunities are defined as
1 294
energy management actions that are done once, and for which the cost is significant. Many of the
Annual savings = =
R2 451- R1 294
opportunities in this category will require detailed
R1 157
analysis by specialists and cannot be examined in detail in this guide. The following are typical Energy
The extra cost for a heat pump package over
Management Opportunities in the Retrofit
standard air conditioning with electric heat is
category.
estimated at R 3 000
Simple payback = =
Absorption equipment can provide low
R3 000/R1 157
cost cooling if dependable, high grade
2.6 years
waste heat is available.
47
Use a heat pump to upgrade the low
superheat can be used where lower
temperature waste heat to a temperature
suitable for building heating.
be taken in the design of the refrigerant
Provide a thermal storage system to
piping system to ensure proper return of
reduce compressor cycling, and allow
liquid liqui d refrigera refr igerant nt and oil fr om the
continuous operation at full-load and
desuperheater.
higher efficiency.
temperature latent heat cannot. Care must
Provide decentralized systems to match
temperature cooling medium to reduce
loads with specialized requirements. For
condensing temperatures. If an air-cooled
example, if a large system operates at a low
condenser requires
evap evapor orat ator or temp temper erat atur ure e when when only only a smal smalll
re pl ac em e nt , co n si de r us in g an
portion portion of the load requires requires low
eva pora tiv e cond ens er.
temper ature,
provid e a s mall, l ow
added expenditure.
evaporator temperature to improve COP. "p ig gy ba ck in g"
Impr oved
because of the higher COP may justify the
area. Operate the large system at a higher Co ns id er
major repair or
performanc performance e and reduced reduced energy energy cost
temperature system to serve the special
Use well, river or lake water as a lower
th e lo w
Use mechanical refrigeration equipment in facili facilitie ties, s, such such as indoo indoorr swimm swimming ing pools pools
temperature system onto the higher
where high ventilation rates are required
te m p e r a t u r e
sy s t e m t o r e d u c e
for for humi humidi dity ty cont contro rol.l. Wint Winter er heat heatin ingg cost costss
temperature differences and increase
for the ventilation air can be reduced by
COP.
reducing the ventilation rate. The total heat
Reclaim rejected condenser heat for space
of rejection can be used to preheat the
heatin hea tin g, proces pro cesss heatin hea tin g or water
ventilatio ventilation n supply supply air and preheat preheat the
preheating. In addition to reclaiming the
make-up water for the pool. Energy savings
otherwise wasted heat, the system COP
result.
may be increased when a lower temp erat ure cond ensi ng medi um is avai labl e. For exampl ex ampl e, prehea pr ehea ting ti ng
in many cases cases involve involve detailed detailed analysis. analysis. This booklet booklet
domestic water will reduce the energy
serves as a guide for the possible avenues to
required for water heating and reduce the
investigate and gives a feel for energy efficiency
conden con den sing sin g tem peratu per atu re. The cold col d
earning earning opportuni opportunities ties in refrigera refrigeration tion and cooling. cooling.
incoming water supply can often reduce the condensing water temperature by 5 to 10ºC, thereby increasing the system COP.
Calculations for 'retrofit' savings are site specific and
Desuperheat the refrigerant vapour (hot gas) leaving the compressor. The superheat can be recovered for process or make-up water preheating. Because the temperature of the hot gas is higher than the condensing temperature, the
48
APPENDIX 1: GLOSSARY OF TERMS
Glossary of terms used in commercial refrigeration refrigeration (Words in italics are other terms explained within the glossary.) Air cooled condenser:
A condenser cooled by natural or forced flow of air.
Ambient temperature:
The prevailing temperature of the atmosphere surrounding the component under consideration.
Atmospheric pressure:
The pressure exerted by the column of air in the atmosphere above the reference point.
Balanced port valve:
An expansion valve which gives good system stability despite widely varying operating conditions.
Boiling point:
The temperature at which evaporation of liquid takes place at a specific pressure.
Capacity control:
Variation in the quantity of refrigerant circulated in order to vary the refrigeration capacity .
Cascade system:
A refrigeration system composed of more than one circuit where the evaporation process of the higher temperature circuit cools the condenser of the lower temperature circuit.
CFC:
Chlorofluorocarbon Chlorofluorocarbon a derivative of a hydrocarbon hydrocarbon containing chlorine.
Changes of State:
When sufficient heat is added or removed, most substances undergo a change of state. The temperature remains constant until the change of state is complete. Change of state can be from solid to liquid, liquid to vapour or vice versa. Typical examples are ice melting and water boiling.
Condense:
The process of changing a vapour into a liquid by the extraction of heat.
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Condenser:
A heat exchanger in which a vapour is liquefied by the removal of heat.
Coefficient of performance:
(For a refrigerator:)The ratio of the refrigeration capacity to the power absorbed by the compressor.(For a heat pump:) The total heat delivered to the power absorbed by the compressor.
Compression ratio:
The ration of the absolute pressures before and after compression.
Compressor:
A machine for mechanically increasing the pressure of a gas.
Condensing pressure:
The pressure at which a vapour changes into a liquid at a specific temperature.
Condensing temperature:
The temperature of a fluid at which condensation occurs when at a known pressure.
Condensing unit:
A collection of components usually consisting of a compressor, condenser and receiver assembled onto a common base frame.
Cycle:
A cycle is a series of processes where the end point conditions or properties of the substance are identical to the initial conditions. In refrigeration, the processes required to produce a cooling effect are arranged to operate in a cyclic manner so that the refrigerant can be reused.
Defrost on demand:
An automatic defrost system which is initiated by an unacceptable build up of ice and terminated when the coil has cleared.
Defrost:
Elimination of an ice deposit from the surface of an evaporator .
Density of Saturated Liquid:
The density of liquid at saturation temperature and pressure is expressed in 3
kg/m . The specific volume o off the refrigerant liquid can be calculated calculated by taking the inverse of the density. Specif Specific ic Volu Volume me =
1 Density
Desuperheat:
Removal of part or all of the superheat in a gas.
Discharge:
The output side of the compressor .
Discharge temperature:
The temperature of the compressed fluid discharged from the compressor .
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Discharge pressure:
The pressure of the compressed fluid discharged from the compressor .
Energy in Liquids and Vapours:
When a liquid is heated, the temperature of the liquid rises to the boiling point . This is the highest temperature to which the liquid can rise at the measured pressure. The heat absorbed by the liquid in raising the temperature to the boiling point is called sensible heat . The heat required to convert the liquid to vapour at the same temperature and pressure is called latent heat .
Electronic expansion valve:
An electro-mechanical expansion valve controlled by a microprocessor microprocessor which has sensors attached to the evaporator and adjacent pipe work.
Enthalpy (h):
The total energy contained in a refrigerant is called the enthalpy. Most refrigerant tables assume, for convenience of calculations, that the saturated liquid at 40ºC has zero energy. kilogram of the Enthalpy of liquid (hf ) is the amount of energy contained in one kilogram liquid at a particular temperature, and is expressed in kJ/kg. contained in dry saturated vapour vapour at Enthalpy of vapour (hg) is the total energy contained a particular temperature and saturation pressure, and is expressed in kJ/kg. Latent heat of vaporization (hfg) is the amount of energy required to evaporate
one kilogram of liquid at a given temperature and pressure and is the difference between the enthalpy of the liquid and the vapour. It is expressed in kJ/kg. The enthal enthalpy py equatio equation n is: is: hfg = hg - hf
Enthalpy of a mixture is a value necessary in the calculation of most practical
applications because a refrigerant usually contains a mixture of both vapour and liquid. If the quality of the vapour is "x" then: h = hf + x(h x(hg - h)f Where, h
Entropy (s):
=
Enthalpy of "wet" vapour (kJ/kg)
hf
=
Enthalpy of the liquid (kJ/kg)
hg
=
Enthalpy of the vapour (kJ/kg)
x
=
Quality of the vapour (decimal fraction)
Entropy can be described as a measure of the molecular disorder of a substance, and is used to describe the refrigeration cycle. pressure condition is Entropy of saturated liquid (sf ) at a given temperature and pressure expressed in kJ/(kg·K).
51
temperature and pressure condition condition Entropy of saturated vapour (sg) at a given temperature s expressed in kJ/(kg·K). entropy between the saturated Entropy of vaporization (sfg), is the difference in entropy liquid and the saturated vapour. Evaporation and Condensation: Unlike freezing and melting, evaporation and condensation can take place at almost any temperature and pressure combination. Evaporation is the gaseous escape of molecules from the surface of a liquid and is accomplished by the absorption of a considerable quantity of heat without any change in temperature. The vapour that leaves the surface of a boiling liquid is called saturated vapour . The quantity of heat required to make the change of state is
called the latent heat of vaporization. Condensation occurs when the gaseous molecules return to the liquid state. Liquids including refrigerants, evaporate at all temperatures with increased rates of evaporation taking place at higher temperatures. The evaporated gases exert a pressure called the vapour pressure. As the temperature of the liquid rises, there is a greater loss of the liquid from the surface which increases the vapour pressure. Boiling occurs when the vapour pressure reaches the pressure of the surrounding space. During boiling, vapour is generated at a pressure equal to the gas pressure on the surface. If the pressure on the surface is increased, boiling takes place at a higher temperature and the boiling point is said to increase. Similarly, a reduction in the pressure will lower the boiling point. Evaporating temperature:
The temperature at which a fluid vaporises within an evaporator at a specific pressure.
Evaporating pressure:
The pressure at which a fluid vaporises within an evaporator at a specific temperature.
Evaporator:
A heat exchanger in which a liquid is vaporised to produce refrigeration.
Externally cooled:
A compressor which is cooled by air or water passing over the outside of its housing.
Extraction rate:
The quantity of heat which a refrigeration plant is capable of extracting under specified conditions of time and temperature.
Fin block:
A group of tubes which have been expanded into fins to form a heat exchanger .
52
HCFC:
Hydrochlorofluorocarbon.
Heat exchanger:
A device designed to ttransfer ransfer heat between two physically separated fluids.
Heat recovery:
The reclaim of heat from a refrigeration system for use in a heating process.
HeatTransfer Transfer::
Heat energy can flow only from a higher to a lower temperature level unless energy is added to reverse the process. Heat transfer will occur when a temperature difference exists within a medium or between different media . Higher heat transfer rates occur at higher temperature differences.
Hermetic compressor:
A compressor directly coupled to an electric motor and contained within a gas-tight welded casing.
High pressure switch:
A switch designed to stop the compressor motor should the discharge pressure reach a predetermined maximum valve.
Hot gas bypass:
A system whereby some or all of the discharge refrigerant is passed directly back into the compressor suction.
Immiscible:
A condition where oil and refrigerant are incapable of being mixed.
Latent Heat of Fusion:
For most pure substances there is a specific melting/freezing temperature relatively independent of the pressure. For example, ice begins to melt at 0ºC. The amount of heat necessary to melt one kilogram of ice at 0ºC to one kilogram of water at 0ºC is called the latent heat of fusion of water and equals 334.92 kJ/kg. The removal of the same amount of heat from one kilogram of water at 0ºC will change it back to ice.
Liquid refrigerant injection:
Introduction of liquid refrigerant into high temperature refrigerant gas to cool it.
Montreal Protocol:
International legislation to phase out production of CFCs and other substances suspected of depleting ozone.
Oil separator:
A device for separating oil from refrigerant vapour.
Open compressor:
A compressor driven by an external power unit, requiring a shaft seal.
tOperating conditions:
The conditionsunder which a refrigeration system works, including the evaporating pressure and condensing pressure .
Ozone depletion potential:
The potential of a substance to destroy stratospheric ozone.
53
Performance data:
The extraction rate and power input of a refrigeration system.
Plant room:
A secure room where most of the high pressure components of a refrigeration system are located along with the electrical panel.
Pressure:
Pressure is the force exerted on a surface, per unit area, and is expressed in kilopascals (kPa), megapascals (MPa), bar and pounds per square inch (psig).
Process:
A process is a physical or chemical change in the properties of matter, or the conversion of energy from one form to another. In refrigeration, a process is generally defined by the condition (or properties) of the refrigerant at the beginning and end of the process.
Quality of Vapour: Vapour:
Theoretically, when vapour leaves the surface of a liquid, it is pure and saturated at the particular temperature and pressure. In actual practice, tiny liquid droplets escape with the vapour. When a mixture of liquid and vapour exists, the ratio of the mass of the liquid to the total mass of the liquid and vapour mixture is called the 'quality' and is expressed as a percentage or decimal fraction.
Receiver:
A vessel permanently installed in the refrigeration system between the condenser and the expansion valve to provide a reservoir of liquid refrigerant.
Reciprocating:
A positive displacement compressor with piston(s) moving linearly and alternately in opposite directions in the cylinder(s).
Refrigerant:
The working fluid in a refrigeration system, which absorbs heat at a low temperature (by evaporation) and rejects heat at a high temperature (by condensation).
Refrigeration capacity:
The quantity of heat which a refrigeration plant is capable of extracting under specified conditions of time and temperature.
Refrigerant Refrig erant Tables: Tables:
Common properties of refrigerants are tabulated for both liquid and vapour phases, and at different temperature pressure conditions.
Rotary:
A compressor in which the rotation of the component varies the volume of the compression chamber.
Saturation:
A condition at which liquid and vapour may exist when in contact with each
54
other. Saturation Pressure:
Saturation pressure is normally the second column in a refrigerant table and is expressed as MPa (absolute). To obtain gauge pressure subtract 0.101325 MPa (101.325 kpa) from the absolute pressure.
Saturation Saturati on Temperatu Temperature: re: Saturation temperature, normally the first column in a refrigerant table, and given in K, is the temperature at which boiling will occur to produce vapour at the given saturation pressure. Semi-hermetic compressor:
A compressor directly coupled to an electric motor and contained within a gas-tight bolted housing.
Shut-off valve:
A valve used to isolate particular items of equipment.
Sight glass:
A device which allows a visual inspection of a liquid within a pressurised container.
Specific Volume Volume of Saturated Vapour: Vapour: The specific volume of saturated vapour is the volume occupied by one kilogram of dry saturated gas at the corresponding saturation temperature 3
and pressure, and is expressed in m /kg. Density of the vapour can be calculated by taking the inverse of the specific volume. Dens Densit ityy = Subcooled liquid:
1 Specific Volume
A liquid whose temperature is lower than the condensing temperature at its given pressure.
Suction (return) temperature:
The temperature at which refrigerant gas enters the compressor .
Suction cooled:
A compressor in which the motor is cooled by refrigerant gas passing over the motor windings.
Superheat:
The quantity of heat added to dry saturated vapour to raise it from it saturation temperature to a higher temperature.
Temperature:
Temperature is an indication of the heat energy stored in a substance. If the temperature of a substance was decreased to 273ºC or 0 K (Kelvin), known as absolute zero, the substance contains no heat energy and all molecular movement stops.
Temperature emperat ure difference:
The difference in temperature between two substances, surfaces or
55
environments involving transfer of heat. Thermostat:
An automatic switch which is responsive to temperature.
Thermostatic expansion valve:
A valve which automatically regulates the flow of liquid refrigerant into the evaporator to maintain within close limits the degree of superheat of the
vapour leaving the evaporator . Water-coole Water-cooled d condenser:
A condenser cooled by the circulation of water through it.
Work:
Work is the energy which is transferred by a difference in pressure or force of any kind. Work is subdivided into shaft work and flow work. Shaft work is mechanical energy used to drive a mechanism such as a pump,
condenser or turbine. Flow work is the energy transferred into a system by fluid flowing into, or out of, the system. Both forms of work are expressed in kilojoules, or on a mass basis, kJ/kg.
Since South Africa mainly uses metric units, these are the first choice in this guide. However, Imperial units are
56
APPENDIX 2: ENERGY, VOLUME AND MASS CONVERSION FACTORS
often given as well. The units used are given in the table below:
Table A1: Unit Conversion Con versionss
M e t r ic
I mp e r i a l
C onv e r s i on
P r e s s ur e a bs o l ut e
bar
psi
1 ba r g = 1 4 .7 p s ig
P r e s s ur e g au g e
barg
p s ig
1 ba r = 1 4 . 7 p s i
Flow, volumetric
l/ sec
c fm
1 l / s = 2 c f m ( ap pr o x )
Power
kW
hp
1 kW = 1 . 3 4 h p
Energy
kWh
Btu
1 kW h = 3 4 1 2 . 4 B t u
S p e c i f i c e ne r g y
J /l
Abbreviations: ps i : po u nd s p e r s q ua r e i nc h
kW : ki l o w a t t
ps i g : po po u n d s p e r s q u a r e i n c h g a u g e
hp : ho ho r s e p o w e r
l / s e c : l it re s p e r s e c on d
kW h: k i l o w a t t - h ou r
c f m : cu cu b ic f e e t p e r m in u t e
B t u : Br Bri t i s h t h e r m a l un i t s
J/l: Joules/litre
Pressur e absolut e = pressur e gauge + 1 bar 1 bar bar = 100 kPa Standard atmospheric pressure pressure = 1.013 bar
Example of measuring the COP of a refrigeration system directly:
57
APPENDIX 3: EXAMPLE OF MEASURING COP DIRECTLY.
COP is defined as the refrigeration affect (i.e. heat taken up in the evaporator) divided by the work (from the compressor) supplied supplied to the system. Supposing we have an 880 kW centrifugal refrigeration system. The liquid refrigerant (134a see relevant Pressure-Enthalpy diagram) condenses at 1Mpa (P3 from section 5.1.2.1) this corresponds to just over 40ºC. The The refrigerant is then expanded (at constant constant Enthalpy) to a pressure pressure of 0.22 MPa (from the PE diagram this corresponds to 10ºC) and a vapour fraction of about 35%. The outlet temperature and pressure (T6 and P6) are measured as 0ºC and 0.2 MPa. The temperature of the brine being 3
cooled is (T5 and T6) from 1 8'C at the inlet to 2ºC coming coming out. The flowrate of the brine is 0.0367 m /s (Fl). Calculate the COP of the system and the flowrate of the refrigerant. Cooling effect: Water flow
=
Temp change for water
36.67 kg/s =
18ºC-2ºC
Heat capacity (Cp) of water
=
16ºC
=
4.2 kJ/kg.ºC
(Cp is the amount of heat (in joules) that is given up (when the substance is cooled) or taken up (when the substance is heated), for a change in temperature of a degree C or K.Cp is generally given - as above - per kilogram of substance.) Thus the cooling effect
=
4.2 * 16 * 36.67 =
2464 kJ/s
=
cooling effect/ = compressor work
2462/880
After expansion valve
=
188 kJ/kg
(0.22 MPa & -10ºC)
After the evaporator
=
400 kJ/kg
(0.2 MPa & 0ºC)
Enthalpy difference
=
400 - 188
COP COP
= 2. 2.8(note: kJ/s = kW)
Refrigerant flow: Refrigerant enthalpies
=
212 kJ/kg
Assuming heat losses between the expansion valve and suction side of the compressor are negligible, Refrigerant flow required Note: Note:
2464/212
=
11.6 kg/s
given the the flow of refrige refrigerant, rant, the the actual COP COP may be estimate estimated d directly directly from the the refrigerant refrigerant enthalp enthalpy y
change (from the P-E graphs) over the evaporator, and the power drawn from the compressor. SOURCES OF FURTHER INFORMATION
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SOURCES OF FURTHER INFORMATION
For the latest news in energy efficiency technology: “Energy Management News” is a free newsletter issued by the ERI, which contains information on the latest developments in energy efficiency in Southern Africa and details of forthcoming energy efficiency events. Copies can be obtained from: The Energy Research Institute Department of Mechanical Engineering University of Cape Town Private Bag Rondebosch 7701 Cape Town South Africa Tel No: +27 (0) 21 650 3892 Fax No: +27 (0) 21 686 4838 E-mail:
[email protected] [email protected] www.eri.uct.ac.za
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