Chapter 11

Refrigeration Cycles

1

OUTLINE

• Refrigerators And Heat Pumps• The Reversed Carnot Cycle• The Ideal Vapor-compression Refrigeration Cycle• Actual Vapor-compression Refrigeration Cycle• Selecting The Right Refrigerant• Heat Pump Systems• Gas Refrigeration Cycles• Absorption Refrigeration Systems

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OBJECTIVE• Introduce the concepts of refrigerators and heat pumps and the

measure of their performance.

• Analyze the ideal vapor-compression refrigeration cycle.

• Analyze the actual vapor-compression refrigeration cycle.

• Review the factors involved in selecting the right refrigerant for an application.

• Discuss the operation of refrigeration and heat pump systems.

• Analyze gas refrigeration systems.

• Introduce the concepts of absorption-refrigeration systems.

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When we are interested in the heat energy removed from a low-temperature space, the device is called a refrigerator.

When we are interested in the heat energy supplied to the high-temperature space, the device is called a heat pump.

Heat Pump Purpose:

to transfer heat to a

high-temperature

medium, called the

heating load, QH.

Heat Pump Purpose:

to transfer heat to a

high-temperature

medium, called the

heating load, QH.

Refrigerator Purpose:

To remove heat, called the cooling

load, QL from a low-

temperature medium.

Refrigerator Purpose:

To remove heat, called the cooling

load, QL from a low-

temperature medium.

1. REFRIGERATORS & HEAT PUMPS

The performance of refrigerators and heat pumps is expressed in terms of coefficient of performance (COP), defined as

COPQ

W

COPQ

W

RL

net in

HPH

net in

Desired output

Required input

Cooling effect

Work input

Desired output

Required input

Heating effect

Work input

,

,

Both COPR and COPHP can be larger than 1. Under the same operating conditions, the COPs are related by

COP COPHP R 1

6

Refrigerators, air conditioners, and heat pumps are rated with a SEER number or seasonal adjusted energy efficiency ratio. The SEER is defined as the Btu/hr of heat transferred per watt of work energy input. The Btu is the British thermal unit and is equivalent to 778 ft-lbf of work (1 W = 3.4122 Btu/hr). An EER of 10 yields a COP of 2.9.

Refrigeration systems are also rated in terms of tons of refrigeration. One ton of refrigeration is equivalent to 12,000 Btu/hr or 211 kJ/min.

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Reversed Carnot Refrigerator and Heat Pump:

is the most efficient refrigeration cycle operating between TL and TH

NOT a suitable model for refrigeration cycles since processes 2-3 and 4-1 are not practical Process 2-3 Compression of a liquid–vapor mixture, requires a compressor that will handle two phasesProcess 4-1 Expansion of high-moisture-content refrigerant in a turbine.

2. THE REVERSED CARNOT CYCLE

Schematic of a Carnot refrigerator and T-s diagram of the reversed Carnot cycle.Schematic of a Carnot refrigerator and T-s diagram of the reversed Carnot cycle.

COPT T

T

T T

COPT T

T

T T

R CarnotH L

L

H L

HP CarnotL H

H

H L

,

,

/

/

1

1

1

1

COP ↑ as TL ↑ or TH ↓

8

Why not use the reversed Carnot refrigeration cycle?

•Easier to compress vapor only and not liquid-vapor mixture.•Cheaper to have irreversible expansion through an expansion valve, not turbine

9

Eliminated the impracticalities of Reversed Carnot:a.Refrigerant is vaporized completely before compressionb.Turbine is replaced by throttling deviceConsists of 4 main processesThe ideal vapor-compression refrigeration cycle involves an irreversible (throttling) process to make it a more realistic model for the actual systems, thus it is not internally reversible cycle.

Ideal Vapor-Compression Refrigeration Cycle

Process Description 1-2 Isentropic compression 2-3 Constant pressure heat rejection in the condenser3-4 Throttling in an expansion valve4-1 Constant pressure heat addition in the evaporator

3. THE IDEAL VAPOR-COMPRESSION REFRIGERATION CYCLE

T-s diagram

The P-h diagram is another convenient diagram often used to illustrate the

refrigeration cycle.

In P-h diagram,Process 2-3 & 4-1:constant pressureProcess 3-4, Throttling Δh = 0

COPQ

W

h h

h h

COPQ

W

h h

h h

RL

net in

HPH

net in

,

,

1 4

2 1

2 3

2 1

*Rule of thumb: COP improves by 2 to 4% for each °C the evaporating temperature is raised or the condensing temperature is lowered.

All 4 components in vapor-compression refrigeration cycle are steady flow devices.Steady-flow energy balance on a unit-mass basis:

Condenser & evaporator do not involve any work, compressor can be approximated as adiabatic.COP of refrigerators & heat pumps operating on the vapor-compression cycle can be expressed as follows:

All 4 components in vapor-compression refrigeration cycle are steady flow devices.Steady-flow energy balance on a unit-mass basis:

Condenser & evaporator do not involve any work, compressor can be approximated as adiabatic.COP of refrigerators & heat pumps operating on the vapor-compression cycle can be expressed as follows:

ieoutinoutin hhwwqq

1@1 Pghh 3@3 Pfhh

Where: and for ideal case

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The ordinary household refrigerator is a good example of the application of this cycle.

Example 11-1 A refrigerator uses refrigerant-134a as the working fluid and operates on anideal vapor-compression refrigeration cycle between 0.14 and 0.8 MPa. Ifthe mass flow rate of the refrigerant is 0.05 kg/s, determine (a) the rate of heat removal from the refrigerated space and the power input to the compressor,(b) the rate of heat rejection to the environment, and (c) the COP ofthe refrigerator.

Example 11-1 A refrigerator uses refrigerant-134a as the working fluid and operates on an ideal vapor-compression refrigeration cycle between 0.14 and 0.8 MPa. If the mass flow rate of the refrigerant is 0.05 kg/s, determine (a) the rate of heat removal from the refrigerated space and the power input to the compressor,(b) the rate of heat rejection to the environment, and (c) the COP of the refrigerator.

Using the Refrigerant-134a Tables, we have:

kgkJhhMPaP MPag /19.23914.0 14.0@11 KkgkJss MPag /94467.014.0@1

kgkJhss

MPaP/40.275

8.02

12

2

kgkJhhMPaP MPaf /48.958.0 8.0@33 kgkJhthrottlinghh /48.95434

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

kgkJhhMPaP MPag /19.23914.0 14.0@11 KkgkJss MPag /94467.014.0@1

kgkJhss

MPaP/40.275

8.02

12

2

kgkJhhMPaP MPaf /48.958.0 8.0@33 kgkJhthrottlinghh /48.95434

41 hhmQL

kWkgkJskg 19.7/48.9519.239/05.0

12 hhmWin

kWkgkJskg 81.1/19.23940.275/05.0

(b) the rate of heat rejection to the environment, QH

kgkJhhMPaP MPag /19.23914.0 14.0@11 KkgkJss MPag /94467.014.0@1

kgkJhss

MPaP/40.275

8.02

12

2

kgkJhhMPaP MPaf /48.958.0 8.0@33 kgkJhthrottlinghh /48.95434

32 hhmQH

kWkgkJskg 00.9/48.9540.275/05.0

(c) the COP of the refrigerator.

97.381.1

19.7

kW

kW

W

QCOP

in

LR

From part (a)

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4. ACTUAL VAPOR-COMPRESSION REFRIGERATION CYCLE

An actual vapor-compression refrigeration cycle differs from the ideal onein several ways, owing mostly to the irreversibilities that occur in variouscomponents. Two common sources of irreversibilities are fluid friction(causes pressure drops) and heat transfer to or from the surroundings.

T-s diagram of an actual vapor-compression refrigeration cycle

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In the ideal cycle, the refrigerant leaves the evaporator and enters the compressor as saturated vapor. In practice, it may not be possible to control the state of the refrigerant so precisely.

Instead, it is easier to design the system so that the refrigerant is slightly superheated at the compressor inlet. This slight overdesign ensures that the refrigerant is completely vaporized when it enters the compressor. The result of superheating, heat gain in the connecting line, and pressure drops in the evaporator and the connecting line is an increase in the specific volume, thus an increase in the power input requirements to the compressor since steady-flow work is proportional to the specific volume.

1

21

The compression process in the ideal cycle is internally reversible andadiabatic, and thus isentropic.

The actual compression process, however, 1)involves frictional effects, which increase the entropy, and 2)involves heat transfer, which may increase or decrease the entropy, depending on the direction.

Therefore, the entropy of the refrigerant may increase (process 1-2) or decrease (process 1-2’) during an actual compression process, depending on which effects dominate. The compression process 1-2’ may be even more desirable than the isentropic compression process since the specific volume of the refrigerant and thus the work input requirement are smaller in this case. Therefore, the refrigerant should be cooled during the compression process whenever it is practical and economical to do so.

2

22

In the ideal case, the refrigerant is assumed to leave the condenser as saturated liquid at the compressor exit pressure.

In reality, however, it is unavoidable to have some pressure drop in the condenser as well as in the lines connecting the condenser to the compressor and to the throttling valve.Also, it is not easy to execute the condensation process with such precision that the refrigerant is a saturated liquid at the end, and it is undesirable to route the refrigerant to the throttling valve before the refrigerant is completely condensed. Therefore, the refrigerant is subcooled somewhat before it enters the throttling valve. We do not mind this at all, however, since the refrigerant in this case enters the evaporator with a lower enthalpy and thus can absorb more heat from the refrigerated space. The throttling valve and the evaporator are usually located very close to each other, so the pressure drop in the connecting line is small.

3

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Example 11-2

Refrigerant-134a enters the compressor of a refrigerator as superheated vapor at 0.14 MPa and -10°C at a rate of 0.05 kg/s and leaves at 0.8 MPa and 50°C. The refrigerant is cooled in the condenser to 26°C and 0.72 MPa and is throttled to 0.15 MPa. Disregarding any heat transfer and pressure drops in the connecting lines between the components, determine (a)the rate of heat removal from the refrigerated space and the power input to the compressor, (b)the isentropic efficiency of the compressor, and (c)the coefficient of performance of the refrigerator.

kgkJhthrottlinghh

kgkJhhCT

MPaP

kgkJhCT

MPaP

kgkJhCT

MPaP

Cf

/83.87)(

/83.8726

72.0

/71.28650

8.0

/37.24610

14.0

434

26@33

3

22

2

11

1

24

kgkJhthrottlinghh

kgkJhhCT

MPaP

kgkJhCT

MPaP

kgkJhCT

MPaP

Cf

/83.87)(

/83.8726

72.0

/71.28650

8.0

/37.24610

14.0

434

26@33

3

22

2

11

1

a) The rate of heat removal from the refrigerated space, QL and the power input to the compressor, Win are determined from their definitions:

41 hhmQL

kWkgkJskg 93.7/83.8737.246/05.0

12 hhmWin

kWkgkJskg 02.2/37.24671.286/05.0

25

kgkJhthrottlinghh

kgkJhhCT

MPaP

kgkJhCT

MPaP

kgkJhCT

MPaP

Cf

/83.87)(

/83.8726

72.0

/71.28650

8.0

/37.24610

14.0

434

26@33

3

22

2

11

1

b) the isentropic efficiency of the compressor,

12

12

hh

hh

w

w s

a

sC

%8.93938.037.24672.286

37.24620.284

, At P2s = 0.8 MPa s2s = s1 =0.9724 kJ/kg.K)

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c) The coefficient of performance of the refrigerator, (COP)

93.302.2

93.7

kW

kW

W

QCOP

in

LR

,

From part (a)

Assignment: Can you compare & evaluate the COP obtained above with the COP of ideal vapor compression refrigeration in Example 11-1 ??

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5. SELECTING THE RIGHT REFRIGERANT

CFCsAMMONIA

HYDROCARBONCO2, AIR, H2O

Industrial & heavy sectorsADV low cost, high COP, low E cost, high heat transfer, no effect on ozone layerDISADV toxicity

Low cost & versatileR-11: large capacity water chillersR-12: domestic refrigerators & freezersR-22: NH3 competitorsR-502: R-115 & R-22 blends, commercial refrigerators (supermarket)Fully haloganated CFCs damage ozone layerDeveloped R-134a chlorine-free

PARAMETERS TO CONSIDER

1) Temperatures of the 2 media which the refrigerant exchanges heat

2) Toxicity, flammability, chemical stability

3) Availability at low costFor heat pumps: Tmin & Pmin may be

considerably higher

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6. HEAT PUMP SYSTEMS

A heat pump can be used to heat a house in winter and to cool it in summer by adding a reversed valve

A heat pump can be used to heat a house in winter and to cool it in summer by adding a reversed valve

The most energy source for heat pumps is atmospheric air (air-to- air systems).

Water-source systems usually use well water and ground-source (geothermal) heat

pumps use earth as the energy source. They typically have higher COPs but are more complex and more expensive to install.

Both the capacity and the efficiency of a heat pump fall significantly at low

temperatures. Therefore, most air-source heat pumps require a supplementary

heating system such as electric resistance heaters or a gas furnace.

Heat pumps are most competitive in areas that have a large cooling load during the

cooling season and a relatively small heating load during the heating season. In these areas,

the heat pump can meet the entire cooling and heating needs of residential or

commercial buildings.

The most energy source for heat pumps is atmospheric air (air-to- air systems).

Water-source systems usually use well water and ground-source (geothermal) heat

pumps use earth as the energy source. They typically have higher COPs but are more complex and more expensive to install.

Both the capacity and the efficiency of a heat pump fall significantly at low

temperatures. Therefore, most air-source heat pumps require a supplementary

heating system such as electric resistance heaters or a gas furnace.

Heat pumps are most competitive in areas that have a large cooling load during the

cooling season and a relatively small heating load during the heating season. In these areas,

the heat pump can meet the entire cooling and heating needs of residential or

commercial buildings.

The power cycles can be used as refrigeration cycles by simply reversing them. Of these, the reversed Brayton cycle, which is also known as the gas refrigeration cycle, is used to cool aircraft and to obtain very low (cryogenic) temperatures after it is modified with regeneration.

7. GAS REFRIGERATION CYCLES

Internally reversible, executed in ideal gas refrigeration cycle

T-s diagramArea under 4-1 = QL

Area enclosed by 12341 = Win

Heat transfer not isothermal, low COP

2 desirable characteristics: simpler, lighter component & incorporated with regeneration

Internally reversible, executed in ideal gas refrigeration cycle

T-s diagramArea under 4-1 = QL

Area enclosed by 12341 = Win

Heat transfer not isothermal, low COP

2 desirable characteristics: simpler, lighter component & incorporated with regeneration

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The work output of the turbine can be used to reduce the work input requirements to the compressor. Thus, the COP of a gas refrigeration cycle is

where;

COPq

w

q

w wRL

net in

L

comp in turb out

, , ,

7. GAS REFRIGERATION CYCLES

31

With regenerator (heat exchanger), the high pressure gas is further cooled to T4 before expanding in the turbine. Lowering the turbine inlet temperature will automatically lowers the turbine exit temperature.

7. GAS REFRIGERATION CYCLES

32

Example 11-6An ideal gas refrigeration cycle using air as the working medium is to maintain a refrigerated space at -18°C while rejecting heat to the surrounding medium at 27°C. The pressure ratio of the compressor is 4. Determine(a)the maximum and minimum temperatures in the cycle, (b)the coefficient of performance, and (c) the rate of refrigeration for a mass flow rate of 0.05 kg/s.

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Example 11-6An ideal gas refrigeration cycle using air as the working medium is to maintain a refrigerated space at -18°C while rejecting heat to the surrounding medium at 27°C. The pressure ratio of the compressor is 4. Determine(a)the maximum and minimum temperatures in the cycle, (b)the coefficient of performance, and (c) the rate of refrigeration for a mass flow rate of 0.05 kg/s.

kgkJhKT /07.255255 11

11

22 rr P

P

PP

7867.01 rP

Assumption: 1 Steady operating conditions exist. 2 Air is an ideal gas withvariable specific heats. 3 Kinetic and potential energy changes are negligible.

kgkJhKT /19.300300 33 3860.13 rP

Through interpolation!Isentropic relations of ideal gas:

34

(a)the maximum and minimum temperatures in the cycle,

kgkJhKT /07.255255 11

147.37867.0411

22 rr P

P

PP

7867.01 rP

kgkJhKT /19.300300 33 3860.13 rP

kgkJh /74.3792 CKT 1063792

3465.0386.125.033

44 rr P

P

PP

kgkJh /60.2014 CKT 712024

35

(b) the coefficient of performance, COP

kgkJhKT /07.255255 11 7867.01 rP

kgkJhKT /19.300300 33 3860.13 rP

outturbincomp

L

innet

LR ww

q

w

qCOP

,,,

kgkJhhqL /47.5360.20107.25541

kgkJhhw outturb /59.9860.20119.30043,

kgkJhhw incomp /67.12407.25574.37912,

147.37867.0411

22 rr P

P

PP

kgkJh /74.3792

CKT 1063792

3465.0386.125.033

44 rr P

P

PP kgkJh /60.2014

CKT 712024

05.259.9867.124

47.53

RCOP

36

(c) the rate of refrigeration for a mass flow rate of 0.05 kg/s.

kWkgkJskgqmQ Lionrefrigerat 67.2/47.5305.0

37

Refrigerant is absorbed by a transport medium and compressed in liquid form. The most widely used absorption refrigeration system is the ammonia-water system, where am monia serves as the refrigerant and water as the transport medium. The work input to the pump is usually very small.

8. ABSORPTION REFRIGERATION SYSTEMS

PRINCIPLE OF ARSThe system is much like the vapor-compression cycle, except for the compressor which has been replaced by a complex absorption mechanism

Absorption unit absorber, pump, generator, rectifier, regenerator, valvePURPOSE increased the Pressure of NH3

PRINCIPLE OF ARSThe system is much like the vapor-compression cycle, except for the compressor which has been replaced by a complex absorption mechanism

Absorption unit absorber, pump, generator, rectifier, regenerator, valvePURPOSE increased the Pressure of NH3

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8. ABSORPTION REFRIGERATION SYSTEMS

MECHANISM:1.NH3 vapor leaves evaporator enters the absorber, dissolves & reacts with H2O (exo rxn, heat released)2.The NH3+H2O solution pumped to generator, vaporize some solution by heat transfer to the solution3.The vapor passes through rectifier to separate the water4.High P NH3 vapor enter condenser & the rest of the cycle5.The hot NH3+H2O solution passes through the regenerator, transfer some heat to rich solution from the pump6.The solution is throttled to the absorber P.

MECHANISM:1.NH3 vapor leaves evaporator enters the absorber, dissolves & reacts with H2O (exo rxn, heat released)2.The NH3+H2O solution pumped to generator, vaporize some solution by heat transfer to the solution3.The vapor passes through rectifier to separate the water4.High P NH3 vapor enter condenser & the rest of the cycle5.The hot NH3+H2O solution passes through the regenerator, transfer some heat to rich solution from the pump6.The solution is throttled to the absorber P.

1

2

2

3

56

4

39

The COP of ARS is defined as

The MAX COP is determined by assuming that the entire cycle is totally reversible.Heat from the source (Qgen) transferred to Carnot HE & Woutput of this HE is supplied to Carnot Refrigerator

Determining the maximum COP of an absorption refrigeration system. Determining the maximum COP of an absorption refrigeration system.

The COP of actual absorption refrigeration systems is usually less than 1.

The COP of actual absorption refrigeration systems is usually less than 1.

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