Chapter 4REFRIGERATION CYCLES

Siti Mariam Basharie @JKT, FKMP, UTHM 2015

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Objectives

1. Introduce the concepts of refrigerators and heat pumps and the measure of their performance.

2. Analyze the ideal and actual vapor-compression refrigeration cycles.

3. Discuss the operation of refrigeration and heat pump systems.

4. Evaluate the performance of innovative vapor-compression refrigeration systems.

Refrigerators and heat pumps The transfer of heat from a low-temperature region to a high-temperature one requires special devices called refrigerators.

Refrigerators and heat pumps are essentially the same devices; they differ in their objectives only.The objective of a refrigerator is to remove heat (QL) from the cold medium while the objective of a heat pump is to supply heat (QH) to a warm medium.The performance of the refrigerators and heat pump is measured by using the Coefficient of Performance (COP) where,

The Current applications of RefrigerationProbably the most widely used current applications of refrigeration are for air conditioning of private homes and public buildings, and refrigerating foodstuffs in homes, restaurants and large storage warehouses. The use of refrigerators in kitchens for storing fruits and vegetables has allowed adding fresh salads to the modern diet year round, and storing fish and meats safely for long periods. Optimum temperature range for perishable food storage is 3 to 5 °C.In commerce and manufacturing, there are many uses for refrigeration. Refrigeration is used to liquify gases - oxygen, nitrogen, propane and methane, for example. In compressed air purification, it is used to condense water vapor from compressed air to reduce its moisture content.

The Current applications of RefrigerationIn oil refineries, chemical plants, and petrochemical plants, refrigeration is used to maintain certain processes at their needed low temperatures (for example, in alkylation of butenes and butane to produce a high octane gasoline component). Metal workers use refrigeration to temper steel and cutlery. In transporting temperature-sensitive foodstuffs and other materials by trucks, trains, airplanes and seagoing vessels, refrigeration is a necessity.Dairy products are constantly in need of refrigeration, and it was only discovered in the past few decades that eggs needed to be refrigerated during shipment rather than waiting to be refrigerated after arrival at the grocery store. Meats, poultry and fish all must be kept in climate-controlled environments before being sold. Refrigeration also helps keep fruits and vegetables edible longer.

Refrigerant : The working fluid of Refrigeration System

A refrigerant is a substance or mixture, usually a fluid, used in a heat pump and refrigeration cycle. In most cycles it undergoes phase transitions from a liquid to a gas and back again. Many working fluids have been used for such purposes. Fluorocarbons, especially chlorofluorocarbons, became commonplace in the 20th century, but they are being phased out because of their ozone depletion effects. Other common refrigerants used in various applications are ammonia, sulfur dioxide, and non-halogenated hydrocarbonssuch as propane.[1]

The ideal refrigerant would have favorable thermodynamic properties, be noncorrosive to mechanical components, and be safe, including free from toxicity and flammability. It would not cause ozone depletion or climate change..

Refrigerant : The working fluid of Refrigeration System

The inert nature of many halons, chlorofluorocarbons (CFC) and hydrochlorofluorocarbons (HCFC), with the benefits of their being nonflammable and nontoxic, made them good choices as refrigerants, but their stability in the atmosphere and their corresponding global warming potential and ozone depletion potential raised concerns about their usage. In order from the highest to the lowest potential of ozone depletion are Bromochlorofluorocarbon, CFC then HCFC. Though HFC and PFC are non-ozone depleting, many have global warming potentials that are thousands of times greater than CO2. Some other refrigerants such as propane and ammonia are not inert, and are flammable or toxic if released.New refrigerants were developed in the early 21st century that are safer for the environment, but their application has been held up due to concerns over toxicity and flammability.[2]

Refrigerant : The working fluid of Refrigeration System

The R-400 series is made up of zeotropic blends (those where the boiling point of constituent compounds differs enough to lead to changes in relative concentration because of fractional distillation) and the R-500 series is made up of so-called azeotropic blends. The R-700 series is made up of non-organic refrigerants, also designated by ASHRAE.Below are some notable blended HFC mixtures. There exist many more (see list of refrigerants). All R-400 (R-4xx) and R-500 (R-5xx) hydroflurocarbons are blends, as noted above.R-401A is a HCFC zeotropic blend of R-22, R-152a, and R-124. It is designed as a replacement for R-12.[8]

R-404A is a HFC "nearly azeotropic" blend of 52 wt.% R-143a, 44 wt.% R-125, and 4 wt.% R-134a. It is designed as a replacement of R-22 and R-502 CFC. Its boiling point at normal pressure is -46.5 °C, its liquid density is 0.485 g/cm3.[9]

R-406A is a zeotropic blend of 55 wt.% R-22, 4 wt.% R-600a, and 41 wt.% R-142b.R-407A is a HFC zeotropic blend of 20 wt.% R-32, 40 wt.% R-125, and 40 wt.% R-134a.[10]

R-407C is a zeotropic hydrofluorocarbon blend of R-32, R-125, and R-134a. The R-32 serves to provide the heat capacity, R-125 decreases flammability, R-134a reduces pressure.[11]

R-408A is a zeotropic HCFC blend of R-22, R-125, and R-143a. It is a substitute for R-502. Its boiling point is -44.4 °C.[12]

R-409A is a zeotropic HCFC blend of R-22, R-124, and R-142b. Its boiling point is -35.3 °C. Its critical temperatiure is 109.4 °C.[13]

R-410A is a near-azeotropic blend of R-32 and R-125. The US Environmental Protection Agency recognizes it as an acceptable substitute for R-22 in household and light commercial air conditioning systems.[14] It appears to have gained widespread market acceptance under several trade names.[15]

R-438A another HFC blended replacement for R-22, with five components: R-32, R-125/R-134a, R-600, and R-601a, blended in respective ratios 8.5+.5,-1.5%; 45±1.5%; 44.2±1.5%; 1.7+.1,-.2%; 0.6+.1,-.2%. The mean ‘’mo’’lecular weight of this mix is 99, resulting in the tradename ISCEON MO99 from manufacturer DuPont (a line of blended HFC products developed initially by Rhodia, and sold to DuPont).[16][17]

R-500 is an azeotropic blend of 73.8 wt.% R-12 and 26.2 wt.% of R-152a.R-502 is an azeotropic blend of R-22 and R-115.

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The Reversed Carnot CycleSequence of Processes:1-2 Reversible and isothermal heat

absorption in an evaporator;2-3 Isentropic compression (in a

compressor);3-4 Reversible and isothermal

condensation (in a condenser); and4-1 Isentropic expansion (in a turbine).

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The Reversed Carnot Cycle

Both COPs increase as the difference between the two temperatures decreases, i.e. as TL rises or TH falls.

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The Reversed Carnot Cycle

The reversed Carnot cycle is the most efficient refrigeration cycle operating between TL and TH. But not a suitable model for refrigeration cycles because: (i) process 2-3 involves compression of a liquid–vapor mixture - requires a

compressor that will handle two phases, (ii) (ii) process 4-1 involves expansion of high-moisture-content refrigerant

in a turbine.

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Example The Reversed Carnot Cycle

A steady-flow Carnot refrigeration cycle uses refrigerant-134a as the working fluid. The refrigerant changes from saturated vapor to saturated liquid at 30°C in the condenser as it rejects heat. The evaporator pressure is 160 kPa. Show the cycle on a T-s diagram relative to saturation lines, and determine:

(a) the coefficient of performance, (b) the amount of heat absorbed from the refrigerated space, and (c) the net work input.

Answers: (a) 5.64, (b) 147 kJ/kg, (c) 26.1 kJ/kg

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The Ideal Vapor-compression Refrigeration CycleThe Ideal Vapor-compression Refrigeration Cycle is the ideal model for refrigeration systems. This model cycle is the most widely used cycle for refrigerators, A-C systems, and heat pumps.

The refrigerant is vaporized completely before it is compressed and the turbine is replaced with a throttling device.

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The Ideal Vapor-compression Refrigeration CycleSequence of Processes:1-2 Isentropic compression in a compressor;2-3 Constant pressure heat rejection in a

condenser;3-4 Throttling process in an expansion device; 4-1 Constant pressure heat absorption in an

evaporator.

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State of Processes4-1 Constant Pressure EvaporationHeat from a cold space is absorbed by the refrigerant. As a result, the refrigerant evaporates at a constant evaporator pressure, from state 4 to become a drier saturated vapor at state 1.

1-2 Isentropic compressionThe saturated vapor is compressed from the evaporator pressure to the condenser pressure, in a reversible adiabatic manner. The refrigerant exits the compressor as a superheated vapor at state 2.

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State of Processes2-3 Constant Pressure CondensationHeat is rejected from the refrigerant to a warm space. As a result, the refrigerant condenses at a constant condenser pressure until it becomes a saturated liquid at state 3.

3-4 Constant Enthalpy ExpansionThe refrigerant expands through the throttle valve adiabatically. As a result, it’s pressure drops from the condenser to the evaporator pressure. The enthalpy is constant during the process, i.e. h3 = h4.

Note: The expansion process is highly irreversible, thus making the vapor-compression cycle an irreversible cycle.

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Energy AnalysisAll four processes can be analyzed as steady-flow processes.The kinetic and potential energy changes of the steam are usually small. Thus the Steady-flow Energy Equation per unit mass of steam reduces to:

The P-h diagram of an ideal vapor-compression refrigeration cycle.

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Energy Analysis• Refrigeration Capacity,

– Is defined as the amount of heat that has to be transferred from a cold space per unit time

– Used to determine the mass flow rate of refrigerant

• Mass flow rate of refrigerant

massunitpereffectingrefrigeratcapacityorrefrigeratm

LQ

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Cycle Analysis2 methods can be used for cycle analysis.• Using property table for refrigerants• Using the p-h diagram

P

h

1

23

4

s co

nsta

nt

v constant

x co

nsta

nt

qH = h2 – h3

qL= h1 – h4

win = h2 – h1

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p-h diagram for R-134a

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In class practiceThe ideal vapor-compression refrigeration Cycle

A refrigerator uses refrigerant-134a as the working fluid and operates on an ideal vapor-compression refrigeration cycle between 0.12 and 0.7 MPa. The mass flow rate of the refrigerant is 0.05 kg/s. Show the cycle on a T-s diagram with respect to saturation lines. Determine:

a) the rate of heat removal from the refrigerated space,b) the power input to the compressor, c) the rate of heat rejection to the environment, and d) the coefficient of performance.

Answers: (a) 7.41 kW, 1.83 kW, (b) 9.23 kW, (c) 4.06

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The Actual Vapor-compression Refrigeration CycleAn actual vapor-compression refrigeration cycle involves irreversibilities in various components - mainly due to fluid friction (causes pressure drops) and heat transfer to or from the surroundings. As a result, the COP decreases.

Differences :• Non-isentropic

compression;• Superheated vapor at

evaporator exit;• Sub-cooled liquid at

condenser exit;• Pressure drops in

condenser and evaporator.

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Subcooling And Its Effects

• In the condenser, the vapor can be

further cooled at constant pressure to

a temperature that is lower than

temperature in condenser.• Subcooling (undercooling) increases

the refrigerating effect from (h1 – h4) to

(h1 – h4’) where h4 is enthalpy with

subcooling and h4’ is initial enthalpy

• Subcooling is limited by temperature of

cooling water and temperature

difference of cycle..

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In class practiceThe Ideal?@ Actual Cycle

10–?Consider a 300 kJ/min refrigeration system that operates on an ideal vapor-compression refrigeration cycle with refrigerant-134a as the working fluid. The refrigerant enters the compressor as saturated vapor at 140 kPa and is compressed to 800 kPa. Show the cycle on a T-s diagram with respect to saturation lines, and determine the:

a) quality of the refrigerant at evaporator inlet,b) coefficient of performance, and c) power input to the compressor.

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In class practiceThe Actual Cycle

10–?Refrigerant-134a enters the compressor of a refrigerator as superheated vapor at 0.14 MPa and 10°C at a rate of 0.12 kg/s, and it leaves at 0.7 MPa and 50°C. The refrigerant is cooled in the condenser to 24°C and 0.65 MPa, and it is throttled to 0.15 MPa. Disregarding any heat transfer and pressure drops in the connecting lines between the components, show the cycle on a T-s diagram with respect to saturation lines, and determine:

a) the rate of heat removal from the refrigerated space, b) the power input to the compressor, c) the isentropic efficiency of the compressor, and d) the COP of the refrigerator.

Answers: (a) 19.4 kW, 5.06 kW, (b) 82.5 percent, (c) 3.83

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In class practiceThe Actual Cycle

10–17 Edition ?A commercial refrigerator with refrigerant-134a as the working fluid is used to keep the refrigerated space at 30°C by rejecting its waste heat to cooling water that enters the condenser at 18°C at a rate of 0.25 kg/s and leaves at 26°C. The refrigerant enters the condenser at 1.2 MPa and 65°C and leaves at 42°C. The inlet state of the compressor is 60 kPa and -34°C and the compressor is estimated to gain a net heat of 450 W from the surroundings. Determine

(a) the quality of the refrigerant at the evaporator inlet, (b) the refrigeration load, (c) the COP of the refrigerator, and (d) the theoretical maximum refrigeration load for the same power input to

the compressor.

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Innovative Vapor-Compression Refrigeration System The simple vapor-compression refrigeration cycle is the most

widely used refrigeration cycle, and is adequate for most refrigeration applications.

The ordinary vapor-compression refrigeration systems are simple, inexpensive, reliable, and practically maintenance-free.

However, for large industrial applications, efficiency (not simplicity) is the major concern.

For some applications the simple vapor-compression refrigeration cycle is inadequate and needs to be modified.

For moderately and very low temperature applications, some innovative refrigeration systems are used. The following cycles will be discussed:• Cascade refrigeration systems• Multistage compression refrigeration systems

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Cascade Refrigeration System

• Some industrial applications require moderately low temperatures, and the temperature range they involve may be too large for a single vapor compression refrigeration cycle to be practical.

• A large temperature range also means a large pressure range in the cycle and a poor performance for a reciprocating compressor.

• One way of dealing with such situations is to perform the refrigeration process in stages, that is, to have two or more refrigeration cycles that operate in series.

• Such refrigeration cycles are called cascade refrigeration cycles.

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Cascade Refrigeration SystemA two-stage cascade refrigeration cycle is as shown in fig. The two cycles are connected through the heat exchanger in the middle, which serves as the evaporator for the top cycle and the condenser for the bottom cycle.

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Cascade Refrigeration System

• From the T-s diagram, the compressor work decreases and the amount of heat absorbed from the refrigerated space increases as a result of cascading.

• Therefore, cascading improves the COP of a refrigeration system.

• Some refrigeration systems use three or four stages of cascading.

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Cascade Refrigeration SystemThe refrigerants in both cycles are assumed to be the same. This is not necessary, however, since there is no mixing taking place in the heat exchanger. Therefore, refrigerants with more desirable characteristics can be used in each cycle. In this case, there would be a separate saturation dome for each fluid, and the T-s diagram for one of the cycles would be different. Also, in actual cascade refrigeration systems, the two cycles would overlap somewhat since a temperature difference between the two fluids is needed for any heat transfer to take place

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Thermodynamics Analysis

Assuming the heat exchanger is well insulated and the kinetic and potential energies are negligible, the heat transfer from the fluid in the bottoming cycle should be equal to the heat transfer to the fluid in the topping cycle.

Thus, the ratio of mass flow rates through each cycle should be

The coefficient of performance of the cascade system is

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Consider a two-stage cascade refrigeration system operating between pressure limits of 0.8 and 0.14 MPa. Each stage operates on the ideal vapor-compression refrigeration cycle with refrigerant-134a as the working fluid. Heat rejection from the lower cycle to the upper cycle takes place in an adiabatic counter-flow heat exchanger where both streams enter at about 0.4 MPa. If the mass flow rate of the refrigerant through the upper cycle is 0.24 kg/s, determine the:

a) mass flow rate of the refrigerant through the lower cycle, b) rate of heat removal from the refrigerated space, c) power input to the compressor, and d) coefficient of performance of this cascade refrigerator.

Answers: (a) 0.195 kg/s, (b) 34.2 kW, 7.63 kW, (c) 4.49

Problem 11-42Cascade Refrigeration System

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Consider a two-stage cascade refrigeration system operating between pressure limits of 1.2 MPa and 200 kPa with refrigerant-134a as the working fluid. Heat rejection from the lower cycle to the upper cycle takes place in an adiabatic counter-flow heat exchanger where the pressure in the upper and lower cycles are 0.4 and 0.5 MPa, respectively. In both cycles, the refrigerant is a saturated liquid at the condenser exit and a saturated vapor at the compressor inlet, and the isentropic efficiency of the compressor is 80 percent. If the mass flow rate of the refrigerant through the lower cycle is 0.15 kg/s, determine the:

a) mass flow rate of the refrigerant through the upper cycle, b) rate of heat removal from the refrigerated space, and c) coefficient of performance of the system.

Answers: (a) 0.212 kg/s, (b) 25.7 kW, (c) 2.68

Problem 11-47Cascade Refrigeration System

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When the fluid used throughout the cascade refrigeration system is the same, the heat exchanger between the stages can be replaced by a mixing chamber (called a flash chamber) since it has better heat transfer characteristics. This system can be carried out with the use of one or more compressors.

Multistage Compression Refrigeration System

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• Flash chamber used in a multi-stage refrigeration system is to separate vapor and liquid refrigerant during the throttling process

• The purpose is to avoid vapor refrigerants from entering evaporator.• The vapor developed during throttling (flash vapor) is bled out of the

throttling device and fed back to the compressor.

Multistage Compression Refrigeration System

44The p-h diagram of the two-stage refrigeration system

Multistage Compression Refrigeration System

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In this system, the liquid refrigerant expands in the first expansion valveto the flash chamber pressure, which is the same as the compressor interstagepressure. Part of the liquid vaporizes during this process. This saturated vapor (x fraction at state 3) is mixed with the superheated vapor from the low-pressure compressor (1-x fraction at state 2), and the mixture enters the high-pressure compressor at state 9 (=1). This is, in essence, a regeneration process.

The Process

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The saturated liquid (state 7) expands through the second expansion valve into theevaporator, where it picks up heat from the refrigerated space.The compression process in this system resembles a two-stage compressionwith intercooling, and the compressor work decreases.

The Process

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• 1 kg refrigerant moves through condenser• 1kg liquid enters 1st throttle valve• 1kg (mostly liquid) enters flash chamber and

starts to evaporate and becomes mixture of gas (x)kg and liquid (1–x)kg

• (x) moves towards 2nd stage compressor at Pi • (1–x)kg liquid make its way through the 2nd

throttle valve into the evaporator• (1–x)kg vapor enters the 1st stage

compressor where it is compressed to Pi• At Pi (state 3) (1-x)kg vapor mixes with (x)kg

vapor adiabatically and becomes 1kg vapor• 1kg vapor is compressed in 2nd stage

compressor• 1kg vapor enters condenser to be condensed

and becomes 1kg liquid

Multistage Compression Refrigeration System

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• Fraction of refrigerant which evaporates in the flash chamber is given as x6= (h6 – hf) /hfg

• Refrigerating Effect, QL = Q81 = (1 – x6)(h1 – h8)

• Total work input, Win = Wcomp1 + Wcomp2 = W12 + W94

= (1 – x6)(h2 – h1) + (1)(h4 – h9)

• Coefficient of Performance, COP = QL / Win

Multistage Compression Refrigeration System

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A two-stage compression refrigeration system operates with refrigerant-134a between the pressure limits of 1 and 0.14 MPa. The refrigerant leaves the condenser as a saturated liquid and is throttled to a flash chamber operating at 0.5 MPa. The refrigerant leaving the low-pressure compressor at 0.5 MPa is also routed to the flash chamber. The vapor in the flash chamber is then compressed to the condenser pressure by the high-pressure compressor, and the liquid is throttled to the evaporator pressure. Assuming the refrigerant leaves the evaporator as saturated vapor at a rate of 0.25 kg/s and that both compressors are isentropic, determine the:

a) fraction of the refrigerant that evaporates in the flash chamber, b) rate of heat removed from the refrigerated space, and c) coefficient of performance.

Problem 11-44 Two-Stage Compression Refrigeration Cycle

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A two-stage cascade refrigeration system operates between pressure limits of 1.2 MPa and 200 kPa with refrigerant-134a as the working fluid. Saturated liquid refrigerant leaving the condenser is throttled to a flash chamber operating at 0.45 MPa. The vapor from the flash chamber is mixed with the refrigerant leaving the low-pressure compressor. The mixture is then compressed to the condenser pressure by the high-pressure compressor. The liquid in the flash chamber is throttled to the evaporator pressure. The mass flow rate of the refrigerant is 0.15 kg/s. Assuming saturated vapor refrigerant leaves the evaporator and the isentropic efficiency is 80 percent for both compressors, determine the: a) mass flow rate of refrigerant in the high-pressure compressor, b) rate of heat removal from the refrigerated space, and c) coefficient of performance of the system.d) rate of heat removal and the COP if this refrigerator operated on a single-

stage cycle between the same pressure limits with the same compressor efficiency and flow rate as in part (a).

Problem 11-48 or 11.57 in 8th Edition Two-Stage Compression Refrigeration Cycle

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In Home Practices

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Refrigerant-134a enters the compressor of a refrigerator at 100 kPa and -20°C at a rate of 0.5 m3/min and leaves at 0.8 MPa. The isentropic efficiency of the compressor is 78 percent. The refrigerant enters the throttling valve at 0.75 Mpa and 26°C and leaves the evaporator as saturated vapor at -26°C. Show the cycle on a T-s diagram with respect to saturation lines and determine ;

(a) the power input to the compressor, and(b) the rate of heat removal from the refrigerated space.

ExerciseActual Compression Refrigeration Cycle

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Refrigerant-134a enters the compressor of a refrigerator at 140 kPa and -10°C at a rate of 0.3 m3/min and leaves at 1 MPa. The isentropic efficiency of the compressor is 78%. The refrigerant enters the throttling valve at 0.95 MPa and 30°C and leaves the evaporator as saturated vapor at -18.5°C. Show the cycle on a T-s diagram with respect to saturation lines and determine,

i. the power input to the compressor; andii. the rate of heat removal from the refrigerated space.

Vapor-Compression Refrigeration Cycle Test 2 Sem 2 - 2014/2015

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A two-stage cascade compression refrigeration system provides cooling at -40 °C while operating the high temperature condenser at 1.6 MPa. Each stage operates in the ideal vapor-compression cycle. The upper vapor compression refrigeration system uses water as its working fluid and operates its evaporator at 5°C. The lower cycle uses refrigerant-134a as its working fluid and operates its condenser at 400 kPa. This system produces a cooling effect of 20 kJ/s. Sketch the T-s diagram and determine the mass flow rate of R-134a and water in their respective cycles, and their overall COP of this cascaded system.

Vapor-Compression Refrigeration Cycle Test 2 Sem 1 - 2014/2015

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Consider a two-stage cascade refrigeration system operating between the pressure limits of 1.2 MPa and 200 kPa with refrigerant-134a as the working fluid. The refrigerant leaves the condenser as a saturated liquid and is throttled to a flash chamber operating at 0.45 MPa. Part of the refrigerant evaporates during this flashing process, and this vapor is mixed with the refrigerant leaving the low-pressure compressor. The mixture is then compressed to the condenser pressure by the high-pressure compressor. The liquid in the flash chamber is throttled to the evaporator pressure and cools the refrigerated space as it vaporizes in the evaporator. The mass flow rate of the refrigerant through the low-pressure compressor is 0.15 kg/s. Assuming the refrigerant leaves the evaporator as a saturated vapor and the isentropic efficiency is 80% for both compressors, determine: i. the mass flow rate of the refrigerant through the high-pressure compressor;

andii. the rate of heat removal from the refrigerated space.

Vapor-Compression Refrigeration Cycle Final Exam Sem 2 - 2014/2015

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In Batu Pahat, there is an ice producing plant that operates on the vapour-compression refrigeration cycle with R-134a as the working fluid. The cycle evaporator and condenser pressure are 140 kPa and 1200 kPa respectively. Cooling water with a rate of 200 kg/s and temperature rise of 10oC is used to remove heat from the condenser. Potable water is supplied to the chiller section of the refrigeration cycle to produce ice. For each kg of ice produced, 333 kJ of energy must be removed from the potable water supply. Assuming ideal conditions, i. sketch the process flow diagram;ii. sketch the T-s diagram;iii. determine the mass flow rate of the refrigerant in kg/s; andiv. determine the mass flow rate of the potable water supply in kg/s.

Vapor-Compression Refrigeration Cycle Final Exam Sem 1 - 2014/2015