Basic Design Vapor-Compression Refrigeration System

CLB 20703 - Chemical Engineering Thermodynamics 1 Vapor-Compression Refrigeration System

1.0 PROBLEM STATEMENT. Design a vapor-compression refrigeration system to cool a system to -5 oC with the capability for up to 20 kW of cooling. You have a reservoir at 20 oC to reject heat.

2.0 INTRODUCTION.

2.1 Refrigeration Refrigeration is a process in which work is done to move heat from one location to another. The work of heat transport is traditionally driven by mechanical work, but can also be driven by magnetism, laser or other means. Refrigeration has many applications, including, but not limited to: household refrigerators, industrial freezers, cryogenics, air conditioning, and heat pumps. The job of a refrigeration plant is to cool articles or substances down to, and maintain them at a temperature lower than the ambient temperature. Refrigeration can be defined as a process that removes heat. The oldest and most well-known among refrigerants are ice, water, and air. In the beginning, the sole purpose was to conserve food. The Chinese were the first to find out that ice increased the life and improved the taste of drinks and for centuries Eskimos have conserved food by freezing it. The first mechanical refrigerators for the production of ice appeared around the year 1860. In 1880 the first ammonia compressors and insulated cold stores were put into use in the USA. Electricity began to play a part at the beginning of this century and mechanical refrigeration plants became common in some fields like breweries, slaughter-houses, fishery, and ice production. After the Second World War the development of small hermetic refrigeration compressors evolved and refrigerators and freezers began to take their place in the home. Today, these appliances are regarded as normal household necessities.

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CLB 20703 - Chemical Engineering Thermodynamics 2 Vapor-Compression Refrigeration System

There are countless applications for refrigeration plants now. Examples are: 1. Foodstuff conservation 2. Process refrigeration 3. Air conditioning plants 4. Drying plants 5. Fresh water installations 6. Refrigerated containers 7. Heat pumps 8. Ice production 9. Freeze-drying 10. Transport refrigeration

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CLB 20703 - Chemical Engineering Thermodynamics 3 Vapor-Compression Refrigeration System

2.2 Basic Refrigeration Cycle

Figure 2.1: The Basic Refrigeration Cycle

Mechanical refrigeration is accomplished by continuously circulating, evaporating, and condensing a fixed supply of refrigerant in a closed system. Evaporation occurs at a low temperature and low pressure while condensation occurs at a high temperature and high pressure. Thus, it is possible to transfer heat from an area of low temperature to an area of high temperature.

Referring to the illustration below, beginning the cycle at the evaporator inlet (1), the low-pressure liquid expands, absorbs heat, and evaporates, changing to a low-pressure gas at the evaporator outlet (2). The compressor (4) pumps this gas from the evaporator through the accumulator (3), increases its pressure, and discharges the high-pressure gas to the condenser (5). The accumulator is designed to protect the compressor by preventing slugs of liquid 3

CLB 20703 - Chemical Engineering Thermodynamics 4 Vapor-Compression Refrigeration System

refrigerant from passing directly into the compressor. An accumulator should be included on all systems subjected to varying load conditions or frequent compressor cycling. In the condenser, heat is removed from the gas, which then condenses and becomes a high-pressure liquid. In some systems, this high-pressure liquid drains from the condenser into a liquid storage or receiver tank (6). On other systems, both the receiver and the liquid line valve (7) are omitted

A heat exchanger (8) between the liquid line and the suction line is also an optional item, which may or may not be included in a given system design. Between the condenser and the evaporator an expansion device (10) is located. Immediately preceding this device is a liquid line strainer/drier (9), which prevents plugging of the valve or tube by retaining scale, dirt, and moisture. The flow of refrigerant into the evaporator is controlled by the pressure differential across the expansion device or, in the case of a thermal expansion valve, by the degree of superheat of the suction gas. Thus, the thermal expansion valve shown requires a sensor bulb located at the evaporator outlet. In any case, the flow of refrigerant into the evaporator normally increases as the evaporator load increases.

As the high-pressure liquid refrigerant enters the evaporator, it is subjected to a much lower pressure due to the suction of the compressor and the pressure drop across the expansion device. Thus, the refrigerant tends to expand and evaporate. In order to evaporate, the liquid must absorb heat from the air passing over the evaporator. Eventually, the desired air temperature is reached and the thermostat or cold control (11) will break the electrical circuit to the compressor motor and stop the compressor. As the temperature of the air through the evaporator rises, the thermostat or cold control remakes the electrical circuit. The compressor starts, and the cycle continues. In addition to the accumulator, a compressor crankcase heater (12) is included on many systems. This heater prevents accumulation of 4

CLB 20703 - Chemical Engineering Thermodynamics 5 Vapor-Compression Refrigeration System

refrigerant in the compressor crankcase during the non-operating periods and prevents liquid slugging or oil pump out on start-up.

Additional protection to the compressor and system is afforded by a high- and lowpressure cutout (13). This control is set to stop the compressor in the event that the system pressures rise above or fall below the design operating range. Other controls not indicated on the basic cycle which may be part of a system include: evaporator pressure regulators, hot gas bypass regulators, electric solenoid valves, suction pressure regulators, condenser pressure regulators, low-side or high-side float refrigerant controllers, oil separators, etc. It is extremely important to analyze completely every system and understand the intended function of each component before attempting to determine the cause of a malfunction or failure.

Figure 2.2: T-s Diagram refrigeration cycle

It has an area on the left (grey), in which the operating medium is liquid and supercooled. In the centre (blue) there is a mixture of steam and liquid, the wet steam. On the right of it (orange) the operating medium is in pure steam form and superheated. The real refrigeration cycle with its typical phase transitions can also be represented in this T-s 5

CLB 20703 - Chemical Engineering Thermodynamics 6 Vapor-Compression Refrigeration System

diagram. The cycle has many similarities to the familiar steam power cycle. The major difference is that the cycle is anticlockwise. Thus the processes of evaporation and condensation and expansion and compression (pumping) swap places. The enclosed area (green) corresponds to the compressor work added to the cycle.

2.3 Vapor Compression Refrigeration Cycle System

Figure 2.3: Vapor Compression Refrigeration Cycle System

A simple vapor compression refrigeration system consists of the equipment such as: 1. Compressor 2. Condenser 3. Expansion valve 4. Evaporator.

Compressor The low pressure and temperature vapour refrigerant from evaporator is drawn into the compressor through the inlet or suction valve A, where it is compressed to a high pressure and temperature. This high pressure and temperature vapour refrigerant is discharged into the condenser through the delivery.

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CLB 20703 - Chemical Engineering Thermodynamics 7 Vapor-Compression Refrigeration System

Condenser The condenser or cooler consists of coils of pipe in which the high pressure and temperature vapour refrigerant is cooled and condensed. Expansion Valve It is also called throttle valve or refrigerant control valve. The function of the expansion valve is to allow the liquid refrigerant under high pressure and temperature to pass at a controlled rate after reducing its pressure and temperature. Some of the liquid refrigerant evaporates as it passes through the expansion valve, but the greater portion is vaporized in the evaporator at the low pressure and temperature Evaporator An evaporator consists of coils of pipe in which the liquid-vapour. Refrigerant at low pressure and temperature is evaporated and changed into vapour refrigerant at low pressure and temperature. In evaporating, the liquid vapour refrigerant absorbs its latent heat of vaporization from the medium (air, water or brine) which is to be cooled.

Vapor Compression Cycle (T-s diagram cycle)

Figure 2.4: T-s Diagram of Vapor Compression Refrigeration Cycle

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CLB 20703 - Chemical Engineering Thermodynamics 8 Vapor-Compression Refrigeration System

The cycle works between temperatures T1 and T2 representing the condenser and evaporator temperatures respectively. The various process of the cycle A-B-C-D (A-B’-C’-D and A-B”-C”-D) are as given: i) Process B-C (B’-C’ or B”-C”): Isentropic compression of the vapor from state B to C. If vapor state is saturated (B), or superheated (B”), the compression is called dry compression. If initial state is wet (B’), the compression is called wet compression as represented by B’-C’. ii) Process C-D (C’-D or C”-D): Heat rejection in condenser at constant pressure. iii) Process D-A: An irreversible adiabatic expansion of vapor through the expansion value. The pressure and temperature of the liquid are reduced. The process is accompanied by partial evaporation of some liquid. The process is shown by dotted line. iv) Process A-B (A-B’ or A-B”): Heat absorption in evaporator at constant pressure. The final state depends on the quantity of heat absorbed and same may be wet (B’) dry (B) or superheated (B”). Coefficient of Performance (COP)

Heat extracted at low temperature = Heat transfer during the process A-B = refrigerating effect q 2 = (hb – ha) Work of compression = w = (hc - hb) (adiabatic compression)

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CLB 20703 - Chemical Engineering Thermodynamics 9 Vapor-Compression Refrigeration System

So,

Now, heat rejected to the condenser = q1 = w +q2

Work compression refrigerant cycles

The Basic component of a compression based refrigeration cycles are a compressor, a condenser an expansion valve an evaporator and a refrigerant (a volatile liquid). Compression based refrigeration cycles work because of a combination of physical laws common to all liquids. First, the temperature at which a liquid boils decreases as the ambient pressures decreases. Seconds its takes heat to boil (vaporize a liquid). A liquid vaporizing because of a reduction in ambient pressure absorbs heat from its surrounding. A compressor the active element in the cycles forces refrigerant to circulate. A compressor pulls, cools, low pressure refrigerant vapour out of the evaporator and compressed it, raising both the pressure and temperature of the refrigerant vapour. This hot compressed refrigerant vapor then flows into a condenser. A condenser the high pressure side of the cycles contains both hot vapor and liquid refrigerant. Because of its high pressure, the vapour refrigerant condensed at a high temperature, expelling heat to the air around the condenser. The resulting warm, pressurized liquid refrigerant then flows to and through an expansion valve.

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CLB 20703 - Chemical Engineering Thermodynamics 10 Vapor-Compression Refrigeration System

Rate circulation refrigeration

A refrigeration device is provided that has a refrigeration chamber for refrigerating a heat load. A refrigerant supply means is provided for supplying a mass flow rate of refrigerant to refrigerate the heat load and a venture-like device is provided for circulation the refrigerant within a circulation path located within the refrigerant chamber so that heat is transferred from the heat load to the refrigerant. The venture-like device has a high pressure inlet for receiving the mass flow rate of refrigerant from the refrigerant supply means. A low pressure inlet is provided for receiving a re-circulation mass flow rate of the refrigerant from the circulation path after heat is transferred to the refrigerant. Also a high pressure outlet is provided for discharging a combined mass flowrate of the refrigerant, comprising the mass flowrate of the refrigerant received within the high pressure inlet and re-circulation mass flowrate of the refrigerant. Heat absorbed compression cycle The absorption cycle is a process by which refrigeration effect is produced through the use of two fluids and some quantity of heat input, rather than electrical input as in the more familiar vapor compression cycle. Both vapor compression and absorption refrigeration cycles accomplish the removal of heat through the evaporation of a refrigerant at a low pressure and the rejection of heat through the condensation of the refrigerant at a higher pressure. The method of creating the pressure difference and circulating the refrigerant is the primary difference between the two cycles. The vapor compression cycle employs a mechanical compressor to create the pressure differences necessary to circulate the refrigerant. In the absorption system, a secondary fluid or absorbent is used to circulate the refrigerant.

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CLB 20703 - Chemical Engineering Thermodynamics 11 Vapor-Compression Refrigeration System

3.0 RESULT CALCULATION.

Table 3.1: Data for point 1 until point 4 for refrigeration.

Process

Point

T(°C)

Isentropic

2 3

4 Isobaric Heat Rejection Throttling 1 2 Isobaric Heat Absorption

-5 34.1

P(KPa) H (kJ/KG) 243.68 247.5 782.03 271.13

S (KJ/KG.K) 0.934345 0.934345

20

275.48

79.32

0.30063

-5 -5

243.68 243.68

79.32 247.5

0.30063 0.934345

Energy Flow

Win=m(H3H2)=2.4622KW QH=m(H3-H4) m= 0.1042 kg/s

QC=m(H2-H1) = 17.524KW

Figure 3.1: Phase Diagram for Vapour Compression Refrigeration Cycle

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CLB 20703 - Chemical Engineering Thermodynamics 12 Vapor-Compression Refrigeration System

At point 2 By using saturated refrigerant-134a Pressure Table: a) Find Pressure at temperature -5 By Interpolation (

)

b) Find Enthalpy By Interpolation (

)

c) Find S By Interpolation (

At point 3 By using superheated refrigerant-134a pressure table: a) Find Pressure By interpolation,

b) Find Temperature `

Step 1 Find T at 0.7MPa By interpolation using S value,

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)

CLB 20703 - Chemical Engineering Thermodynamics 13 Vapor-Compression Refrigeration System

Step 2 Find T at pressure 0.8MPa By interpolation using S value,

Step 3 Find T at pressure 0.782MPa By interpolation, (

)

c) Find Enthalpy Step 1 Find H at 0.7MPa By interpolation using S value,

Step 2 Find H at pressure 0.8MPa By interpolation using S value,

Step 3 Find H at pressure 0.816MPa By interpolation, (

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)

CLB 20703 - Chemical Engineering Thermodynamics 14 Vapor-Compression Refrigeration System

At point 4 By using saturated refrigerant-134a Table a) Find Enthalpy at Temperature (20oC) By interpolation,

b) Find S at Temperature (20oC)

Isobaric heat rejection Find ̇ ̇(

)

̇(

)

̇

Isentropic ̇(

)

(

)

Isobaric heat absorption ̇( (

) )

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CLB 20703 - Chemical Engineering Thermodynamics 15 Vapor-Compression Refrigeration System

Determine The Coefficient Of Performance (C.O.P.)

To Check the QH .

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CLB 20703 - Chemical Engineering Thermodynamics 16 Vapor-Compression Refrigeration System

4.0 DISCUSSION.

The challenge in refrigeration and air conditioning is to remove heat from a low temperature source and dump it at a higher temperature sink. Compression refrigeration cycles in general take advantage of the idea that highly compressed fluids at one temperature will tend to get colder when they are allowed to expand. If the pressure change is high enough, then the compressed gas will be hotter than our source of cooling (outside air, for instance) and the expanded gas will be cooler than our desired cold temperature. In this case, we can use it to cool at a low temperature and reject the heat to a high temperature.

Vapour-compression refrigeration cycles specifically have two additional advantages. First, they exploit the large thermal energy required to change a liquid to a vapour so we can remove lots of heat out of our air-conditioned space. Second, the isothermal nature of the vaporization allows extraction of heat without raising the temperature of the working fluid to the temperature of whatever is being cooled. This is a benefit because the closer the working fluid temperature approaches that of the surroundings, the lower the rate of heat transfer. The isothermal process allows the fastest rate of heat transfer Vapour compression refrigeration is the primary method to provide mechanical cooling. All vapor compression systems consist of the following four basic components along with the interconnecting piping. These are the evaporator, condenser, compressor and the expansion valve

Many of the impracticalities associated with the reversed Carnot cycle can be eliminated by vaporizing the refrigerant completely before it is compressed and by replacing the turbine with a throttling device, such as expansion valve or capillary valve, this is called the ideal vapor-compression refrigeration cycle. It consists of four processes which is isentropic 16

CLB 20703 - Chemical Engineering Thermodynamics 17 Vapor-Compression Refrigeration System

compression in a compressor, constant-pressure heat rejection in a condenser, throttling in an expansion device and constant-pressure heat absorption in an evaporator.

In this Project the refrigerant enters the evaporators at state 1 as a low quality saturated mixture, and it completely evaporates by absorbing heat from the refrigerated space. The refrigerant leaves the evaporator as saturated vapor and enters the compressor. The enthalpy at state 1-2 is 243.68kJ/kg and entropy is 0.94391kJ/kg.K. The refrigerant enters the compressor at state 2 as saturated vapor and is compressed isentropic to the condenser pressure. The temperature of the refrigerant increased to 307.25K during this isentropic compression process to well above the temperature of the surrounding medium. The work in, Win

at compressor is 2.4622KW. The value entropy for state 2-3 is same with 1-2 but the

enthalpy is differ which is 247.5 kJ/kg.

The refrigerant then enters the condenser as superheated vapor at state 3 and leaves as saturated liquid at state 4 as a result of heat rejection to the surroundings. The heat rejection, QH is 20 kW with mass flowrate 0.1042 kg/s. Moreover, the value of enthalpy is 79.32kJ/kg and entropy is 0.3006kJ/Kg.K.

The saturated liquid refrigerant at state 4 is throttled to the evaporator pressure by passing it through an expansion valve or capillary tube. The temperature of the refrigerant drops below the temperature of the refrigerated space during this process is 268.15 K. The enthalpy and entropy value for this state is same at state 3. The refrigerant leaves the throttled and reenters the evaporator, completing the cycle.

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CLB 20703 - Chemical Engineering Thermodynamics 18 Vapor-Compression Refrigeration System

The condenser and the evaporator do not involve any work, and the compressor can be approximated as adiabatic. The Coefficient of Performance (COP) is determined by dividing the enthalpy, H2 and H1 with H3 and H2 and COP for this cycle is 6.5618.

When designing a refrigeration system, there are several refrigerants from which to choice. The irreversibility in the vapor-compression cycle causes the coefficient of performance of practical refrigerators to depend to some extent on the refrigerant. Nevertheless, such characteristics as its toxicity, flammability, cost, corrosion properties and vapor pressure in relation to temperature are of greater importance in the choice of refrigerant. So that air cannot leak into the refrigeration system, the vapor pressure of the refrigerant at the evaporator temperature should be greater than atmospheric pressure. On the other hand, the vapor pressure at the condenser temperature should be unduly high, because of the initial cost and operating expense of high-pressure equipment.

These two requirements limit the choice of refrigerant to relatively few fluids. This project, the refrigerant was choice is Tetrafluorethane (HFC-134a) also known as 1,1,1,2Tetrafluoroethane that used usually in an ordinary household refrigerator.

HFC-13a is

replacement to the hydrochloroflurocarbons, less than fully halogenated hydrocarbons which cause relatively little ozone depletion. But, HFC-134a gives effect to the people especially of health.

Advantage use vapor compression refrigeration cycle is they are quite safe thus ensuring long life of the refrigeration system compare with the other cycle. Besides that, the compression refrigeration system the heat is given up only from the condenser, so it heat rejection factor is small. The cost for running this refrigeration cycle is low. Moreover, the

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CLB 20703 - Chemical Engineering Thermodynamics 19 Vapor-Compression Refrigeration System

refrigeration cycle can be employed over a large range of temperatures and very important in these cycles is the coefficient of performance is quite high.

The refrigerant enters the compressor Vapor-compression refrigeration cycles specifically have two additional advantages. First, they exploit the large thermal energy required to change a liquid to a vapor so can remove lots of heat out of our air-conditioned space. Second, the isothermal nature of the vaporization allows extraction of heat without raising the temperature of the working fluid to the temperature of whatever is being cooled. This is a benefit because the closer the working fluid temperature approaches that of the surroundings, the lower the rate of heat transfer. The isothermal process allows the fastest rate of heat transfer. The cycle operates at two pressures, P-high and P-low, and the state points are determined by the cooling requirements and the properties of the working fluid. Most coolants are designed so have relatively high vapor pressures at typical application temperatures to avoid the need to maintain a significant vacuum in the refrigeration cycle.

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5.0 CONCLUSION. As the conclusion, HFC 134-a is the best choosing refrigerant of the vapour compression system. It was quite safe thus ensuring long life of the refrigeration system compare with the other cycle. The cost for running this refrigeration cycle is low. Furthermore, a higher COP heat pump will consume less purchased energy than one with a lower COP. The overall environmental impact of a heating or air conditioning installation depends on the source of energy used as well as the COP of the equipment. The operating cost to the consumer depends on the cost of energy as well as the COP or efficiency of the unit.

6.0 REFFERENCES.

1. Smith. Introduction to Chemical Engineering Thermodynamic (SI Unit). 2008. Mc Graw Hill. 2. Yunus A. Cengel. Michael A. Boles. Thermodynamic An Engineering Approach (SI Unit). 2007. New York. Edition Sixth. 3. Second-Law- based Thermodynamic Analysis of Two Stages and Mechanical Sub cooling Refrigeration Cycle by SM Zubair, M.Yaakub, and SH Khan from Department of Mechanical Engineering, King Fahd University of Petroleum and Mineral, Dhahron 31261, Saudi Arabia. 13 February 1995. 4. Earl Logan, Jr. “Thermodynamics Processes and Applications,”Columbus Division, Ohio State University Columbus, Ohio. 5. Performance Improvement of Air Cooled Refrigeration System by Using Evaporative Cooled Air Condenser by E. Hajidavalloo, H. Eghtedari. 25 August 2009.

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