Chapter 10: Refrigeration Cycles - Saylor
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Chapter 10-1 Chapter 10: Refrigeration Cycles The vapor compression refrigeration cycle is a common method for transferring heat from a low temperature to a high temperature. The above figure shows the objectives of refrigerators and heat pumps. The purpose of a refrigerator is the removal of heat, called the cooling load, from a low-temperature medium. The purpose of a heat pump is the transfer of heat to a high-temperature medium, called the heating load. 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. In general, the term heat pump is used to describe the cycle as heat energy is removed from the low-temperature space and rejected to the high- temperature space. The performance of refrigerators and heat pumps is expressed in terms of coefficient of performance (COP), defined as
Transcript of Chapter 10: Refrigeration Cycles - Saylor
Chapter 10-1
Chapter 10: Refrigeration Cycles
The vapor compression refrigeration cycle is a common method fortransferring heat from a low temperature to a high temperature.
The above figure shows the objectives of refrigerators and heat pumps. Thepurpose of a refrigerator is the removal of heat, called the cooling load, froma low-temperature medium. The purpose of a heat pump is the transfer ofheat to a high-temperature medium, called the heating load. When we areinterested in the heat energy removed from a low-temperature space, thedevice is called a refrigerator. When we are interested in the heat energysupplied to the high-temperature space, the device is called a heat pump. Ingeneral, the term heat pump is used to describe the cycle as heat energy isremoved from the low-temperature space and rejected to the high-temperature space.
The performance of refrigerators and heat pumps is expressed in terms ofcoefficient of performance (COP), defined as
Chapter 10-2
COP QW
COP QW
RL
net in
HPH
net in
= = =
= = =
Desired outputRequired input
Cooling effectWork input
Desired outputRequired input
Heating effectWork input
,
,
Both COPR and COPHP can be larger than 1. Under the same operatingconditions, the COPs are related by
COP COPHP R= +1
Can you show this to be true?
Refrigerators, air conditioners, and heat pumps are rated with a SEERnumber or seasonal adjusted energy efficiency ratio. The SEER is defined asthe Btu/hr of heat transferred per watt of work energy input. The Btu is theBritish thermal unit and is equivalent to 778 ft-lbf of work (1 W = 3.4122Btu/hr). An EER of 10 yields a COP of 2.9.
Refrigeration systems are also rated in terms of tons of refrigeration. Oneton of refrigeration is equivalent to 12,000 Btu/hr or 211 kJ/min. How didthe term “ton of cooling” originate?
Reversed Carnot Refrigerator and Heat Pump
Shown below are the cyclic refrigeration device operating between twoconstant temperature reservoirs and the T-s diagram for the working fluidwhen the reversed Carnot cycle is used. Recall that in the Carnot cycle heattransfers take place at constant temperature. If our interest is the coolingload, the cycle is called the Carnot refrigerator. If our interest is the heatload, the cycle is called the Carnot heat pump.
Chapter 10-3
The standard of comparison for refrigeration cycles is the reversed Carnotcycle. A refrigerator or heat pump that operates on the reversed Carnot cycleis called a Carnot refrigerator or a Carnot heat pump, and their COPs are
COPT T
TT T
COPT T
TT T
R CarnotH L
L
H L
HP CarnotL H
H
H L
,
,
/
/
=−
=−
=−
=−
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Notice that a turbine is used for the expansion process between the high andlow-temperatures. While the work interactions for the cycle are notindicated on the figure, the work produced by the turbine helps supply someof the work required by the compressor from external sources.
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.
What problems result from using the turbine?
Chapter 10-4
The Vapor-Compression Refrigeration Cycle
The vapor-compression refrigeration cycle has four components: evaporator,compressor, condenser, and expansion (or throttle) valve. The most widelyused refrigeration cycle is the vapor-compression refrigeration cycle. In anideal vapor-compression refrigeration cycle, the refrigerant enters thecompressor as a saturated vapor and is cooled to the saturated liquid state inthe condenser. It is then throttled to the evaporator pressure and vaporizes asit absorbs heat from the refrigerated space.
The ideal vapor-compression cycle consists of four processes.
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
Chapter 10-5
The P-h diagram is another convenient diagram often used to illustrate therefrigeration cycle.
The ordinary household refrigerator is a good example of the application ofthis cycle.
Results of First and Second Law Analysis for Steady-FlowComponent Process First Law Result Compressor s = const. ( )W m h hin = −2 1
Condenser P = const. ( )Q m h hH = −2 3
Throttle Valve ∆s > 0 h h4 3= Wnet = 0
Qnet = 0 Evaporator P = const. ( )Q m h hL = −1 4
Chapter 10-6
COP QW
h hh h
COP QW
h hh h
RL
net in
HPH
net in
= =−−
= =−−
,
,
1 4
2 1
2 3
2 1
Example 10-1
Refrigerant-134a is the working fluid in an ideal compression refrigerationcycle. The refrigerant leaves the evaporator at -20oC and has a condenserpressure of 0.9 MPa. The mass flow rate is 3 kg/min. Find COPR andCOPR, Carnot for the same Tmax and Tmin , and the tons of refrigeration.
Using the Refrigerant-134a Tables, we have
StateCompressor inletT Cx
h kJkg
s kJkg K
StateCompressor exitP P kPa
s s kJkg K
h kJkg
T C
StateCondenser exitP kPax
h kJkg
s kJkg K
StateThrottle exitT T Ch
o s
s
s
so
o
1
2010
23531
0 9332
2
900
0 9332
2751
44 74
3
9000 0
99 56
0 3656
4
20
1
1
1
1
2 2
2 1
2
2
3
3
3
34 1
4
= −=
UV||
W||
=
=⋅
RS||
T||
= =
= =⋅
U
V|||
W|||
=
=
RS|T|
==
UV||
W||
=
=⋅
RS||
T||
= = −=
.
.
..
.
.
.
.
.h
x
s kJkg K
3
4
4
0 357
0 3670
UV||
W||
=
=⋅
RS|T|
.
.
Chapter 10-7
COP QW
m h hm h h
h hh h
kJkgkJkg
RL
net in
= =−−
=−−
=−
−
=
( )( )
( . . )
( . . )
.
,
1 4
2 1
1 4
2 1
23531 99 56
2751 235 31
3 41
The tons of refrigeration, often called the cooling load or refrigeration effect,are
( )
min( . . )
min.
Q m h hkg kJ
kgTon
kJ
Ton
L = −
= −
=
1 4
3 23531 99 56 1
211
193
COP TT T
KK
R CarnotL
H L,
( )( . ( )).
=−
=− +
− −=
20 27344 74 20
391
Another measure of the effectiveness of the refrigeration cycle is how muchinput power to the compressor, in horsepower, is required for each ton ofcooling.
The unit conversion is 4.715 hp per ton of cooling.
Chapter 10-8
.
..
.
,WQ COP
hpTon
hpTon
net in
L R
=
=
=
4 715
4 715341
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Actual Vapor-Compression Refrigeration Cycle
Chapter 10-9
Heat Pump Systems
Other Refrigeration Cycles
Cascade refrigeration systems
Very low temperatures can be achieved by operating two or more vapor-compression systems in series, called cascading. The COP of a refrigerationsystem also increases as a result of cascading.
Chapter 10-10
Multistage compression refrigeration systems
Chapter 10-11
Multipurpose refrigeration systems
A refrigerator with a single compressor can provide refrigeration at severaltemperatures by throttling the refrigerant in stages.
Liquefaction of gases
Another way of improving the performance of a vapor-compressionrefrigeration system is by using multistage compression with regenerativecooling. The vapor-compression refrigeration cycle can also be used toliquefy gases after some modifications.
Chapter 10-12
Gas Refrigeration Systems
The power cycles can be used as refrigeration cycles by simply reversingthem. Of these, the reversed Brayton cycle, which is also known as the gasrefrigeration cycle, is used to cool aircraft and to obtain very low (cryogenic)temperatures after it is modified with regeneration. The work output of theturbine can be used to reduce the work input requirements to the compressor.Thus, the COP of a gas refrigeration cycle is
COP qw
qw wR
L
net in
L
comp in turb out
= =−, , ,
Chapter 10-13
Chapter 10-14
Absorption Refrigeration Systems
Another form of refrigeration that becomes economically attractive whenthere is a source of inexpensive heat energy at a temperature of 100 to 200oCis absorption refrigeration, where the refrigerant is absorbed by a transportmedium and compressed in liquid form. The most widely used absorptionrefrigeration system is the ammonia-water system, where ammonia serves asthe refrigerant and water as the transport medium. The work input to thepump is usually very small, and the COP of absorption refrigeration systemsis defined as
COP QQ W
QQR
L
gen pump in
L
gen
= = =+
≅Desired outputRequired input
Cooling effectWork input ,
Chapter 10-15
Thermoelectric Refrigeration Systems
A refrigeration effect can also be achieved without using any moving partsby simply passing a small current through a closed circuit made up of twodissimilar materials. This effect is called the Peltier effect, and a refrigeratorthat works on this principle is called a thermoelectric refrigerator.
