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Chapter 4
Refrigeration cycles
DEPARTMENT OF MARINE ENGINEERINGFACULTY OF OCEAN TECHNOLOGY
INSTITUTE TECHNOLOGI SEPULUH NOPEMBERME091307 (THERMODYNAMICS)
Aguk Zuhdi Muhammad Fathallah
4.1 Refrigerators and heat pumps
Refrigerator/heat pump– Transfer heat from a low temperature region to high temperature region.
Objective of refrigerator :Maintain the refrigerated space at low temperatureby removing heatfrom it
Objective of heat pump :Maintain the heated space at a high temperatureby transfer heatto it
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4.1 Refrigerators and heat pumps
4.1 Refrigerators and heat pumps
eperformanc oft coefficien COP where
inputWork
effect Heating
input required
output DesiredCOP
inputWork
effect Cooling
input required
output DesiredCOP
innet
HHP
innet
LR
=
===
===
W
Q
W
Q
1COPCOP RHP +=
Performance of refrigerators and heat pumps
1COPHP >
3
4.2 The reversed Carnot cycle
High Temperature Heat Source, TH
Low Temperature Heat Sink, TL
Net Work Output, Wnet
Heat Supplied, qs
Heat Rejected, qr
Forward Heat Engine
rsnet
rnets
qqw
qwq
−=+=
4.2 The reversed Carnot cycle
High Temperature Heat Sink, TH
Low Temperature Heat Source, TL
Net Work Output, Wnet
Heat Rejected, qr
Heat Supplied, qs
Reversed Heat Engine
srnet
rnets
qqw
qwq
−==++
Carnot refrigerator
Carnot heat pump
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4.2 The reversed Carnot cycle
Operation of the Carnot refrigerator1-2 Isentropic
compression (Compressor)
2-3 Isothermal heat rejection (Condenser)
3-4 Isentropic expansion (Turbine)
4-1 Isothermal heat absorption (Evaporator)
4.2 The reversed Carnot cycle
H
LCarnot HP,
L
HCarnot R,
1
1COP
1
1COP
TT
TT
−=
−=
Performance of Carnot refrigerators and Carnot heat pumps
COPs increase as the difference between the two temperatures decrease, where
TL rises or TH falls
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4.2 The reversed Carnot cycle
Impractical approximation especially for
Process 1-2 (Compressor)
• Involve compression of liquid-vapor mixture (two phases)
Process 3-4 (Turbine)
• Expansion of high moisture content refrigerant
Refrigeration load and effect
Is the rate which heat must be removed from the refrigerated space in order to produce and maintain the desired temperature
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3.516
evaporatorat effect ingRefrigerat
load ingRefrigerat rate flow Mass
hh
kWmr −
=
=
&
Power required to drive the compressor ( )12 Power, hhmw r −= &&
Quantity of cooling water in condenser
)()( 32inout hhmTTcm rpcc −=− &&
4.2 The reversed Carnot cycle
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4.3 The ideal vapor-compression refrigeration cycle
Elimination of impracticalities with the reversed Carnot cycle by
Vaporizing the refrigerant completely before compression process
Replacing the turbine with a throttling device (expansion valve or capillary tube)
4.3 The ideal vapor-compression refrigeration cycle
1-2 Isentropic compression (Compressor)
2-3 Isobaric heat rejection (Condenser)
3-4 Throttling (Expansion device)
4-1 Isobaric heat absorption (Evaporator)
Not an internally reversible cycle since it involves an irreversibible (throttling) process
Higher efficiency but high cost and complexity
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4.3 The ideal vapor-compression refrigeration cycle
COP improved by 2 to 4 percent for each oC the evaporating temperature is raised or the condensing temperature is lowered
4.3 The ideal vapor-compression refrigeration cycle
( ) ( )
case idealfor @
and @ where
COP
and
COP
equation,energy flowsteady The
33
1g1
12
32
innet
HHP
12
41
innet
LR
ieoutinoutin
Phh
Phh
hh
hh
w
q
hh
hh
w
q
hhwwqq
f=
=
−−==
−−==
−=−+−
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Refrigerant R134a enters the compressor of a refrigerator as superheated vapor at 0.14MPa and -10oC at a rate of 0.12kg/s, and it leaves at 0.7MPa and 50oC. The refrigerant is cooled in the condenser to 24oC and 0.65MPa, and it is throttled to 0.15MPa. Disregrading 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 and the power input to the compressor
b. The isentropic efficiency of the compressor
c. The COP of the refrigerator
4.4 Actual vapor-compression refrigeration cycle
Irreversibilities in actual cycle
•Fluid friction and piping losses (cause pressure drops)
•Heat transfer to or from the surroundings
•Mechanical inefficiencies
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4.4 Actual vapor-compression refrigeration cycle
4.4 Actual vapor-compression refrigeration cycleActual compression with frictional
effects such as fluid friction and
heat transfer
(s increase 1-2 or s decrease 1-2’)
Over designed Superheated
vapor (ensure 100% vapor at
compressor inlet)
More desirable cause lower
work input requirement
Pressure drop at condenser
until throttling valve
Refrigerant is subcooled
before enteres the throttling
valve
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4.5 Selecting the right refrigerant
Choice of refrigerants
Chlorofluorocarbons (CFCs)
Ammonia
Hydrocarbons (propane, ethane, ethylene, etc)
Carbon dioxide
Air
Water
Chlorine free R-134a
Refrigerants
4.5 Selecting the right refrigerant
Selection of the refrigerant
1. Temperatures of the two media (the refrigerated space and the environment)
2. Non toxic
3. Non corrosive
4. Non flammable
5. Chemically stable
6. High enthalpy of vaporization (minimise mass flow rate)
7. Low cost
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4.6 Heat pump systems
Different between heat pump and air conditioner
1.Condenser of the heat pump (located indoors) function as the evaporator of the air conditioner.
2.Evaporator of the heat pump (located outdoors) serves as the condenser of the air conditioner
HEAT PUMP OPERATION – HEATING MODE
Fan
Fan
Compressor
IndoorCoil
Reversing valve
Expansion valve
Hot Air
Cold Air
Outdoor coil
High – Pressure liquid
Low – Pressure liquid vapor
Low – pressure vapor
High – Pressure vapor
4.6 Heat pump systems
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Fan
Fan
Compressor
Expansion valve
Out Hot Air
Cold Air
HEAT PUMP OPERATION – COOLING MODE
IndoorCoil
Reversing valveOutdoor coil
4.6 Heat pump systems
High – Pressure liquid
Low – Pressure liquid vapor
Low – pressure vapor
High – Pressure vapor
4.7 Innovative vapor-compression refrigeration systems
Ordinary vapor compression refrigeration
a. Simple
b. Inexpensive
c. Reliable
d. Practically maintenance freeNot efficient , not simplicity for large industrial application
Need modification and refinement to innovative system
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4.7 Innovative vapor-compression refrigeration systems
Cascade Refrigeration Systems
win
win
Closed type
Could be different working fluid for both compressor
4.7 Innovative vapor-compression refrigeration systems
Cascade Refrigeration Systems
Increase refrigeration capacity and decrease in compressor work
Refrigerants for both cycles not necessary to be same cause no mixing
)()(
)(
)(
)(
)()(
1256
41
innet
LCascade R
85
32
3285
hhmhhm
hhm
w
QCOP
hh
hh
m
m
hhmhhm
BA
B
B
A
BA
−+−−==
−−=
−=−
&&
&
&
&
&&
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4.7 Innovative vapor-compression refrigeration systems
Multistage Compression Refrigeration Systems
A
A
m=x
m=1-x
m=1
winA
winB
Direct contact heat exchanger
Two phase, closed vessel
m=x
m=1-x
winA
winB
QH
QL
4.7 Innovative vapor-compression refrigeration systems
Multistage Compression Refrigeration Systems
Heat exchanger replaced by a mixing chamber/flash chamber
Saturated vapor, state 3Superheated vapor from Low pressure compressor, state 2
High pressure compressor, state 9
Saturated liquid expand through second expansion process, state 7
Regeneration process
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4.7 Innovative vapor-compression refrigeration systems
Multipurpose Refrigeration Systems with a Single Compressor
For application with more than one temperature : Refrigerator - Freezer
Study case : Requirement for refrigerator - freezer
1.Refrigerator : maintained above the ice point due to high water content goods
2.Freezer: maintain at about -18oC for ice making or solid good storage
QL
QL
QH
Win
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4.7 Innovative vapor-compression refrigeration systems
Multipurpose Refrigeration Systems with a Single CompressorHigh expanding
pressure, T
above ice point
Low expanding pressure, T below ice point
4.7 Innovative vapor-compression refrigeration systems
Liquefaction of Gases
How to liquefy the Oxygen gas? Nitrogen gas?
What is the usage of the liquefied gases?
• Preparation of liquid propellants for Rockets
• Study of material at low temperatures
• Exciting phenomena such as superconductivity
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4.7 Innovative vapor-compression refrigeration systems
Liquefaction of Gases
W
4.8 Gas refrigeration cycles
Also known as reversed Brayton cycle
For the surrounding temperature, To
1-2 Gas compression process
2-3 Heat rejection of refrigeration gas to To
3-4 Gas expansion process to temperature T4
4-1 Heat absorption from the refrigerated space
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4.8 Gas refrigeration cycles
Also known as reversed Brayton cycle
[kJ/kg]
[kJ/kg]
[kJ/kg]
12in comp
43out turb
41L
out turbin comp
L
innet
LR
hhw
hhw
hhq
ww
q
w
qCOP
−=
−=
−=
−==
4.8 Gas refrigeration cycles
Use a gas refrigeration cycle with a regenerator.
This is the idealized cycle with a regenerator. This is the idealized cycle with a regenerator. Other system configurations are possibleOther system configurations are possible
QH
QL
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4.9 Absorption refrigeration systems
4.9 Absorption refrigeration systems
After the evaporator,1. NH3 dissolved in water NH3.H2O (exothermic reaction –
release heat to cooling water)
2. Pump transfer the liquid NH3.H2O to regenerator to vaporized some of the solution NH3.H2O.
3. The vapor rich in NH3 is then passes through the rectifier to separated the NH3 from water.
4. High pressure NH3 vapor is use for the rest of the refrigeration process.
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4.9 Absorption refrigeration systems
gen
Labsorption
in pumpgen
Labsorption
absorption output Required
output Desired
Q
QCOP
WQ
QCOP
COP
≅
+=
=
4.9 Absorption refrigeration systems
For maximum
COP (totally reversible
cycle)