Refrigeration DMM

 

TPP4208 - Alat dan Mesin Pengolahan

4/3/2012

VAPOR COMPRESSION VAPOR REFRIGERATION REFRIGERA TION S YSTEMS

REFRIGERATION Dewi Maya Maharani, STP, M.Sc

EFFECTS OF REFRIGERATION ON FOODS

Instructional Objective: •

To learn the basic concepts of a vapor compression refrigeration system

ENERGY REMOVAL DURING REFRIGERATION

DESIRABLE EFFECTS a. Microbial growth rates decrease b. Chemical and biochemical reaction rates decrease

Removal of heat (Q) : Q = mCp  T

c. Shelf life increases (2-5 fold for every 10°C decrease in temperature)

Cp = specific heat of food above freezing

UNDESIRABLE EFFECTS a. Textural deterioration b. Chilling injury

m = mass/weight of food T = temperature temperature difference

Dewi Maya Maharani, STP, STP, M.Sc - Refrigeration

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TPP4208 - Alat dan Mesin Pengolahan

4/3/2012

VAPOR COMPRESSION REFRIGERA VAPOR REFRIGERATION TION SYSTEMS •

•

•

REFRIGERANT

A refrigeration system allows transfer of heat from a cooling chamber to a location where the heat can be easily discarded.

•

The transfer of heat is accomplished by using a refrigerant, which can change its state from liquid to gas.

•

However, unlike water the refrigerant has a much lower boiling point.

•

A fluid which, through phase changes from liquid to gas and back to liquid, facilitates heat transfer in a refrigeration system. Refrigerants have much lower boiling points than water and their boiling points can be varied by changing the pressure of the system. A good example of a common refrigerant is ammonia (NH3).

COMPONENT OF A REFRIGERATION REFRIGERATION SYSTEM  SYSTEM VAPOR COMPRESSION VAPOR REFRIGERATION REFRIGERA TION S YSTEMS •

•

•

•

Ammonia boils at -33.3C, compared to 100 oC for water at atmospheric pressure. Similar to water, ammonia needs latent heat of vaporization to change from liquid to vapor, and it discharges latent heat of condensation to change from vapor to liquid. The boiling point of a refrigerant can be varied by changing the pressure. Thus, to increase boiling point of ammonia to 0 oC, its pressure must be raised to 428.5 kPa (62.1 psia)

Major component of a vapor-compression refrigeration system are shown in the following diagram

CONDENSOR

d

b COMPRESSOR a

EXPANSION VALVE

e

c

EVAPORATOR

Dewi Maya Maharani, STP, STP, M.Sc - Refrigeration

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TPP4208 - Alat dan Mesin Pengolahan

4/3/2012

MECHANISM

COMPONENT OF A REFRIGERATION REFRIGERATION SYSTEM  SYSTEM c

CONDENSOR

CONDENSOR

d

d

b

EXPANSION VALVE

e

COMPRESSOR   COMPRESSOR

EVAPORATOR

e d

P2 

e

c

b

   P

Diagram P-H

a

H1 

H2  H3 

Enthalpy (H; kJ/kg)

COMPONENT OF A REFRIGERATION SYSTEM CONDENSOR   CONDENSOR

COMPRESSOR   COMPRESSOR

a

EVAPORATOR

B. Compressor Where the T and P of C. Condenser the refrigerant vapor (1) Where the heat is transferred is increased from the refrigerant to another medium (air or water) (2) When this happens, the refrigerant decreases decreases in T and condenses

a EVAPORATOR

A. Evaporator (1) Where the liquid refrigerant refrigerant vaporizes  vaporizes into a gas (2) When this happens, heat from the stored food is "extracted"

COMPONENT OF A REFRIGERATION SYSTEM CONDENSOR

b

e

COMPRESSOR   COMPRESSOR

Function as heat pumps and pumps and contain four essential mechanical components

c

d EXPANSION VALVE

b

EXPANSION VALVE

a

   )   a    PP1     k    (

c

d

c b

EXPANSION VALVE   VALVE

e

COMPRESSOR

a

EVAPORATOR

D. Expansion valve (1) Where the flow of liquid refrigerant is controlled (2) When this happens, the evaporator receives a constant supply of refrigerant

Dewi Maya Maharani, STP, STP, M.Sc - Refrigeration

3

 

TPP4208 - Alat dan Mesin Pengolahan

4/3/2012

MECHANISM

MECHANISM

CONDENSOR

c

d

CONDENSOR

b

EXPANSION VALVE

e

COMPRESSOR   COMPRESSOR

a

exits the compressor

MECHANISM CONDENSOR

COMPRESSOR   COMPRESSOR

a EVAPORATOR

Location c :  :  - compressed gas enters the condenser - the condensing temperature must be higher than that of an easily available heat sink, e.g., ambient air, water, etc. - the refrigerant gas discharges latent heat of condensation the heat sink and changes phase to liquid  liquid 

MECHANISM c

d

CONDENSOR

b

EXPANSION VALVE

b

e

Location b : : - superheated compressed gas

e

d EXPANSION VALVE

EVAPORATOR

Location a : : - refrigerant gas enters compressor and compressed to a high pressure

c

COMPRESSOR   COMPRESSOR

a

EVAPORATOR

d

c b

EXPANSION VALVE

e

COMPRESSOR   COMPRESSOR

a

EVAPORATOR

Location d :  :  - refrigerant in a saturated liquid state - expansion valve separates high as refrigerant passes through the expansion valve the sudden decrease in pressure causes some of the

refrigerant to change into gas

Location e : : - the refrigerant absorbs heat, equivalent to its latent heat of vaporization, and completely converts into gas  gas  

Dewi Maya Maharani, STP, STP, M.Sc - Refrigeration

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MATHEMA THEMATICAL TICAL EX PRESSIONS USEFUL IN THE MA ANALYSIS OF VAPOR-COMPRESSION REFRIGERATION

MATHEMATICAL MATHEMA TICAL EX PRESSIONS USEFUL IN THE ANALYSIS OF VAPOR-COMPRESSION REFRIGERATION

COOLING LOAD:

REFRIGERANT FLOW RATE

•

•

The cooling load is total heat energy that must be removed from a given space in order to lower the temperature to a desired level. A common unit of cooling load is “ton of refrigeration”   refrigeration”

1 ton of refrigeration = 288,000 Btu/24 hr = 303,852 kJ/24 hr

•

The refrigerant flow rate depends upon the total cooling load on the system and the amount of heat that refrigerant can absorb

Refrigerant flow rate = (Cooling Load) / (H2 - H1)

MATHEMATICAL MATHEMA TICAL EX PRESSIONS USEFUL IN THE ANALYSIS OF VAPOR-COMPRESSION REFRIGERATION

MATHEMATICAL MATHEMA TICAL EX PRESSIONS USEFUL IN THE ANALYSIS OF VAPOR-COMPRESSION REFRIGERATION

COMPRESSOR

CONDENSER

•

The work done on the refrigerant during the compression step is the product on the enthalpy increase of the refrigerant inside the compressor and the refrigerant flow rate

rate of work done on the compressor = (refrigerant flow rate) (H 3 - H2)

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The heat rejected to the environment in the condenser depends upon the refrigerant flow rate and the latent heat of condensation of the refrigerant

heat rejected in the condenser = (refrigerant flow rate) (H3 - H1)

Dewi Maya Maharani, STP, STP, M.Sc - Refrigeration

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TPP4208 - Alat dan Mesin Pengolahan

4/3/2012

MATHEMATICAL MATHEMA TICAL EX PRESSIONS USEFUL IN THE ANALYSIS OF VAPOR-COMPRESSION REFRIGERATION

MATHEMATICAL MATHEMA TICAL EX PRESSIONS USEFUL IN THE ANALYSIS OF VAPOR-COMPRESSION REFRIGERATION

EVAPORATOR

COEFFICIENT PERFORMANCE

•

The heat absorbed by the evaporator depends upon the refrigerant flow rate and the latent heat of evaporation of the refrigerant.

heat absorbed by the evaporator = (refrigerant flow rate) (H2 - H1)

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The coefficient performance is a ratio between the heat absorbed by the refrigerant as it flows through the evaporator to the heat equivalent of the energy supplied to the compressor.

COP = (H2 - H1) / (H3- H2)

Power requirement in horsepower/ton horsepower/to n refrigerant (for F12)

Example •

HP/(ton)r = 12,000 BTU 1 HP (COP) h (ton) (2545 BTU/h)

=

4.715 (COP)

A refrigeration system is to be operated at an evaporator coil temperature of -30oF (-34oC) and a condenser temperature of 100oF (37.8oC) for the liquid refrigerant. For Freon 12, determine: (a) the high-side pressure; (b) the low-side pressure; (c) the refrigeration capacity per unit weight of refrigerant; (d) COP; (e) HP of compressor per ton of refrigerant; (f) quantity of refrigerant circulated through the system per ton of refrigeration

Dewi Maya Maharani, STP, STP, M.Sc - Refrigeration

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TPP4208 - Alat dan Mesin Pengolahan

4/3/2012

PRESSURE ENTHALPY (P-H) DIAGRAMS •

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P-H diagrams are useful in designing and analyzing vapor compression r efrigeration systems These diagrams are available for all type of refrigerants Saturated liquid line

133

Saturated vapor line Liquid

   ) P     a 1    P    k    (

d

P2 

e

Liquid & vapor

Vapor

c

b Constant temperature line

   P

H1 

Constant entropy line

a

H2 

12.3

H3 

o

-30 100

Enthalpy (H; kJ/kg)

133

  e   n    i    l   p   m   e    t    t   n   a    t   s   n   o    C

e

a

12.3

32

74

oF

 

Dewi Maya Maharani, STP, STP, M.Sc - Refrigeration

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TPP4208 - Alat dan Mesin Pengolahan

4/3/2012

For Freon 12 Lihat Gambar 10.1, p. 400 b

d 133

   ) P1    a    i   s   p    (

Condenser

   P

  n   o    i   s   e   v   n   l   a   a   p   v   x    E

e

P2 

Evaporator

133

c

d

b

P= 12.3 psia (85 kPa)

T = 100oF (37.8oC) 12.3

32

74

P=133 psia (917 kPa)

Lihat Gambar 10.5, p. 406

a

e

a

12.3

T = -30oF (-34oC)

94

H2

P= 12.3 psia

74 btu/lb (172 kJ/kg)

H1

P= 133 psia

32 btu/lb (74 kJ/kg)

H3

P=133 psia

94 btu/lb (218 kJ/kg)  

Enthalpy (H; btu/lb)

Refrigerant capacity (heat/kg refrigerant): H2 – H2  – H1  H1 = ((172 – ((172 – 74)  74) x 103) J/kg = 98,000 J/kg 32

74

94

For Freon 12 COP = (H2-H1)/(H3-H2)    ) P1    a    i   s   p    (

133

d

c

b

= (172-74)/(218-172)

One ton refrigerant for F12 = 12,000 BTU/h or 3517 W

= 2.1

   P

P2 

12.3

HP per ton refrigerant:

a

e

Weight =

Lihat Gambar 10.1, p 400: Cp/cv F-12 = 1.14 32 Enthalpy (H; btu/lb)

74

94

HP/(ton) = 4.715/ (COP) = 4.715/(1.14)(2.1) = 1,97

=

Cooling capacity/ton refrigerant

Cooling capacity/unit weight of refrigerant

12,000 BTU/h 42 BTU/lb

= 286 lb refrigerant/h = 0.0359 kg refrigerant/s