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8/12/2019 Pp Chapter 5 Refrigeration Cycle Sem 2 2011-2012
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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