REFRIGERATORS AND HEAT

PUMPS
The transfer of heat from a low-temperature
region to a high-temperature one requires special
devices called refrigerators.
Another device that transfers heat from a low-
temperature medium to a high-temperature one
is the heat pump.
Refrigerators and heat pumps are essentially the
same devices; they differ in their objectives only.

The objective of a refrigerator is to remove heat (QL)

for fixed values of
from the cold medium; the objective of a heat pump is
QL and QH
to supply heat (QH) to a warm medium.
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THE REVERSED CARNOT CYCLE
The reversed Carnot cycle is the most efficient refrig. cycle operating between TL and TH.
It is not a suitable model for refrigeration cycles since processes 2-3 and 4-1 are not practical because
Process 2-3 involves the compression of a liquid–vapor mixture, which requires a compressor that will
handle two phases, and process 4-1 involves the expansion of high-moisture-content refrigerant in a
turbine.

Both COPs increase as the

difference between the two
temperatures
decreases, that is, as TL rises
or TH falls.

Schematic of a
Carnot refrigerator
and T-s diagram of
the reversed Carnot
cycle. 2
The standard of comparison for refrigeration cycles is the reversed Carnot cycle. A
refrigerator or heat pump that operates on the reversed Carnot cycle is called a
Carnot refrigerator or a Carnot heat pump, and their COPs are
1 TL
COPR , Carnot  
TH / TL  1 TH  TL
1 TH
COPHP , Carnot  
1  TL / TH TH  TL
Notice that a turbine is used for the expansion process between the high and low-
temperatures. While the work interactions for the cycle are not indicated on the
figure, the work produced by the turbine helps supply some of 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 instead of the expansion valve?

3
THE IDEAL VAPOR-COMPRESSION REFRIGERATION
CYCLE
The vapor-compression refrigeration cycle is the ideal model for refrigeration systems.
Unlike the reversed Carnot cycle, the refrigerant is vaporized completely before it is
compressed and the turbine is replaced with a throttling device.

This is the most

widely used cycle
for refrigerators, A-
C systems, and heat
pumps.

Schematic and T-s

diagram for the ideal
vapor-compression
refrigeration cycle.
4
The ideal vapor-compression refrigeration cycle involves an irreversible (throttling)
process to make it a more realistic model for the actual systems.
Replacing the expansion valve by a turbine is not practical since the added benefits
cannot justify the added cost and complexity.
Steady-flow
energy balance

An ordinary
household
refrigerator.

The P-h diagram of an ideal vapor-

compression refrigeration cycle. 5
The performance of refrigerators and heat pumps is expressed in terms of coefficient
of performance (COP), defined as
Desired output Cooling effect QL
COPR   
Required input Work input Wnet ,in
Desired output Heating effect Q
COPHP    H
Required input Work input Wnet ,in

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

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Example 11-1

Refrigerant-134a is the working fluid in an ideal compression refrigeration cycle. The

refrigerant leaves the evaporator at -20oC and has a condenser pressure of 0.9 MPa.
The mass flow rate is 3 kg/min. Find COPR and COPR, Carnot for the same Tmax and
Tmin , and the tons of refrigeration.

Using the Refrigerant-134a Tables, we have

State 2 
State1  kJ 
 h  238.41 Compressor exit kJ
Compressor inlet   1
kg  h2 s  278.23
  P2 s  P2  900 kPa  kg
T1  20o C   s  0.9456 kJ kJ  T2 s  43.79 C
o

x1  1.0   1 kg  K s2 s  s1  0.9456 
kg  K 

State 3  kJ State 4 
   x4  0.358
Condenser exit   3
h 101.61
kg Throttle exit 
  kJ
P3  900 kPa   kJ T4  T1  20o C   s4  0.4053
s3  0.3738  kg  K
x3  0.0   kg  K h4  h3 


7
 Q&L m&(h1  h4 ) h1  h4
COPR   
Wnet , in m&(h2  h1 ) h2  h1
&
kJ
(238.41  101.61)
kg

kJ
(278.23  238.41)
kg
 3.44
The tons of refrigeration, often called the cooling load or refrigeration effect, are
Q&L  m&(h1  h4 )
kg kJ 1Ton
3 (238.41  101.61)
min kg 211 kJ
min
 1.94 Ton
TL
COPR , Carnot 
TH  TL
(20  273) K

(43.79  (20)) K
 3.97 8
Another measure of the effectiveness of the refrigeration cycle is how much input
power to the compressor, in horsepower, is required for each ton of cooling.

The unit conversion is 4.715 hp per ton of cooling.

W&net , in 4.715
& 
QL COPR
4.715 hp

3.44 Ton
hp
 1.37
Ton

9
Heat Pump Systems

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