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CHAPTER 5

REFRIGERATION CYCLE

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CHAPTER 5REFRIGERATION CYCLE

5.1 THERMODYNAMIC APPLICATIONS

5.2 INTRODUCTION: REFRIGERATION

5.3 REFRIGERATOR & HEAT PUMP

5.4 THE REVERSE CARNOT CYCLE

5.5 IDEAL VAPOR-COMPRESSIONREFRIGERATION CYCLE

5.6 IDEAL VAPOR-COMPRESSIONREFRIGERATION CYCLE: PROCESS &

ANALYSIS5.7 ACTUAL VAPOR-COMPRESSION

REFRIGERATION CYCLE

5.8 PRESSURE ENTHALPY DIAGRAM

(P-h Diagram)

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CHAPTER 5REFRIGERATION CYCLE

5.9 INNOVATIVE VAPOR-COMPRESSION

REFRIGERATION SYSTEM

5.10 ABSORPTION REFRIGERATION SYSTEMS

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THERMODYNAMIC APPLICATIONS

THERMODYNAMICCYCLE

POWERGENERATION

REFRIGERATION

POWER PLANT ENGINE REFRIGERATOR AIRCONDITIONING

CH 3: GAS CH 2CH 5 CH 6

CH 4: VAPOR

5.1 THERMODYNAMIC APPLICATIONS

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A majorapplications of

thermodynamics

HOTAREA

Transfer heat

RCOLDAREA

Device

REFRIGERATOR

Refrigerationcycles

Vapor-compression cycle:Phases change

Gas refrigeration cycle:Remain gaseous phase

5.2 INTRODUCTION: REFRIGERATION

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CARNOT HEAT PUMPCARNOTREFRIGERATOR

Not Practical forrefrigeration cycle

Process (1-2):Compressionliquid-vapor

mixtureCompressorhandle 2 phases

Process (3-4):Expansionof high

moisturecontent

Solution

Ideal Vapor-CompressionRefrigeration Cycle

5.4 THE REVERSE CARNOT CYCLE

CompressorTurbine

CondenserTH

EvaporatorTL

4

3

Cold medium at TL

Warm medium at TH

1

2

T

s

4

QH

QL 1

23

REVERSE CARNOTCYCLE

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T 2

3

4 QL

QH

Win

1

4s

Saturatedliquid

Saturatedvapor

s

Solve impractically in Carnot Refrigeration cycle

100%vaporized

beforecompression

Turbine

Throttling device:Expansion valveCapilary tube

T

s

4

QH

QL

1

23CompressorTurbine

CondenserTH

EvaporatorT

L

4

3

Cold medium at TL

Warm medium at TH

1

2

QH

QL

Compressor

Expansionvalve

Condenser

Evaporator

4

3

Cold refrigerated space

Warm environment

1

2 Win

Evaporator coilCapillary tube

Condensercoil

Compressor

QHQL

Isentropicturbine

5.5 IDEAL VAPOR-COMPRESSION REFRIGERATION CYCLE

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QH

QL

Compressor

Expansionvalve

Condenser

Evaporator

4

3

Cold refrigerated space

Warm environment

1

2 Win

T

s

2

3

4 QL

QH

Win

1

4s

Saturatedliquid

Saturatedvapor

P

h

23

4QL

QH

Win

1

REFRIGERATION CYCLE

IsentropicCompression

(1-2)

ConstantPressure

Heat

Rejection(2-3)

Throttlingin

Expansion

Device(3-4)

ConstantPressure Heat

Absorption

(4-1)

PERFORMANCE

Refrigerator:

Heat pump:

Analysis

Steady flow deviceSteady flow energyequation

5.6 IDEAL VAPOR-COMPRESSION REFRIGERATIONCYCLE: PROCESS & ANALYSIS

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EXAMPLE 5.1A refrigerator uses refrigerant-134a as the

working fluid and operates on an ideal vapor-compression refrigeration cycle between 0.14and 0.8 MPa. If the mass flow rate of the

refrigerant is 0.05 kg/s, determine:

a) The rate of heat removal from therefrigerated space and the power input to thecompressor.

b) The rate of heat rejection to the environmentc) The COP of the refrigerator.

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IRREVERSIBILITIES:Connecting line, DevicesHeat transfer

Fluid friction: Pressure drop

T

s

2

3

4 QL

QH

Win

1

4s

EVAPORATORCOMPRESSOR

CONDENSER

Superheating:refrigerant completelyvaporizedHeat gain: Connecting linePressure drop: evaporator

Specific volume :Work input , power input

Compression process:Inlet: superheated vaporFriction effect: Entropy Heat tranfer:

Entropy : (1-2)Entropy : (1-2)

Heatdirection

Pressure drop:in condenserin connecting lineOutlet:hard to maintain saturatedliquid

Sub-cooled: beforethrottling device

QH

QL

Compressor

Expansionvalve

Condenser

Evaporator

4 3

Cold refrigerated space

Warm environment

1

2 Win

5

7

6

8

T

s

23

4

1

5

6 7 8

2

5.7 ACTUAL VAPOR-COMPRESSION REFRIGERATIONCYCLE

IDEAL VAPOR-COMPRESSIONREFRIGERATION

CYCLE

ACTUAL CYCLE

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EXAMPLE 5.2

Refrigerant-134a enters the compressor of a

refrigerator as superheated vapor at 0.14 Mpa and -10C at a rate of 0.06 kg/s and leaves at 0.8 MPa and50C. The refrigerant is cooled in the condenser to 26C and 0.72 MPa and is throttled to 0.15 MPa.

Disregarding any heat transfer and pressure drops inthe connecting line between the components,determine:

a) The rate of heat removal from the refrigeratedspace and the power input to the compressor

b) The isentropic efficiency of the compressor.

c) The coefficient of performance of the refrigerator

5 8 PRESSURE ENTHALPY DIAGRAM

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P

h

x

s

T

P

h

23

4 1

T, P =constant s =constant

h =constant

T, P =constant

qL= h1-h4 win= h2-h1

qH= h2-h3

h3=h4

Sub-cooling

5.8 PRESSURE ENTHALPY DIAGRAM(P-h Diagram)

5 9 INNOVATIVE VAPOR COMPRESSION REFRIGERATION

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SolutionIndustrial ApplicationLarge pressure rangeLarge temperaturerangeModerate lowtemperature

Poor performance ofreciprocating compressor

Refrigeration process in stages

2 @ more refrigeration cycle in series

Cascade Refrigeration System (2 stages)

Ratio of mass flow rate:

QH

Compressor

Expansionvalve

Condenser

4

3

Warm environment

5

6

A

Expansionvalve

1

2

B

Evaporator

QL

Cold refrigerated space

Compressor

Heat

exchanger

Evaporator

Condenser

8

7

Heat

T

s

2

3

4 QL

QH

1

A

B

Increase inrefrigeration

capacity

Decrease incompressor work

5

6

7

8COPR:

5.9 INNOVATIVE VAPOR-COMPRESSION REFRIGERATIONSYSTEM

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EXAMPLE 5.3

Consider a two-stage cascade refrigeration system operating

between the pressure limits of 0.8 and 0.14 MPa. Each stageoperates on an ideal vapor-compression refrigeration cycle withrefrigerant-134a as the working fluid. Heat rejection from thelower cycle to the upper cycle takes place in an adiabatic counter

flow heat exchanger where both streams enter at about 0.32MPa. (In practice, the working fluid of the lower cycle is at ahigher pressure and temperature in the heat exchanger foreffective heat transfer. If the mass flow rate of the refrigerantthrough the upper cycle is 0.05 kg/s, determine:

a) The mass flow rate of the refrigerant through the lowercycle,

b) The rate of heat removal from the refrigerated space and thepower input to the compressor, and

c) The coefficient of performance of this cascade refrigerator

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Solution 5.3

T

s

2

3

4

1

A

B

5

6

7

8

mB?.

QL?

.

Win?

.

COPR?

h1=239.16

h2=255.93

h3=55.16

h7=95.47

h4=55.16

h5=251.88

h6=270.92

h8=95.47

0.8 MPa

0.32 MPa

0.14 MPa

5 10 ABSORPTION REFRIGERATION SYSTEMS

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QH

QL

Expansionvalve

Condenser

Evaporator

Cold refrigerated space

Warm environment

Wpump

Rectifier

Expansionvalve

Pump

Regenerator

QgenGenerator

Qcool

Cooling water

Absorber

PureNH3

PureNH3

H2O

NH3+H2O

NH3+H2O

Q

Solarenergy

Economical factor:inexpensive thermal energy (100-200C)geothermal, solar, waste heat,Heat transfer from external source heat-driven systemAbsorption of refrigerant by a transport medium NH3(ref.) + H2O (trans. med.)Similar with VCRC except: complex absorption mechanismLarge commercial and industrial installation

Cycles of operation

Absorption

systemNH3pressure

Condensercooled &condensedheat rejection

Expansion

valveThrottling

EvaporatorAbsorbheat

5.10 ABSORPTION REFRIGERATION SYSTEMS

5 10 ABSORPTION REFRIGERATION SYSTEMS

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QH

QL

Expansionvalve

Condenser

Evaporator

Cold refrigerated space

Warm environment

Wpump

Rectifier

Expansion

valve

Pump

Regenerator

QgenGenerator

Qcool

Cooling water

Absorber

PureNH3

Pure

NH3

H2O

NH3+H2O

NH3+H2O

Q

Solarenergy

Absorber:NH3(vapor) dissolvesand react with H2ONH3. H2O (Exotermic)

amount of NH3dissolves in H2Odepend on temperatureControl temperature:Lower as possiblemax. NH3dissolved

Pump:NH3(rich) + H2O

solution pump togenerator

Generator:Heat transfer to NH3+H2O solution from asource: vaporized the

solution.

Rectifier:The vapor (rich inNH3) pass throughthe rectifier

Separate H2O andreturn it to thegeneratorThe high pressurepure NH3 (vapor)pass through the restof the cycle.

Regenerator:The hot NH3+H2Osolution (weak inNH3 ) pass through aregenerator.Transfer heat to therich solution leaving

the pump.

Absorption Refrigeration VSVapor-Compression Refrigeration

Small work inputA liquid is compressed instead of a vaporSteady-flow work specific volume

More complex, occupy more space.More difficult to service: Less common

Absorption Refrigeration SystemConsideration

Cost of thermal energy < electricity

5.10 ABSORPTION REFRIGERATION SYSTEMS

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END OF CHAPTER 5

REFRIGERATION CYCLE