11-13
11-20 A commercial refrigerator with refrigerant-134a as the working fluid is considered. The quality of the refrigerant at
the evaporator inlet, the refrigeration load, the COP of the refrigerator, and the theoretical maximum refrigeration load for
the same power input to the compressor are to be determined.
Assumptions 1 Steady operating conditions exist. 2 Kinetic and potential energy changes are negligible.
Analysis (a) From refrigerant-134a tables (Tables A-11 through A-13)
P1  60 kPa 
 h1  230.03 kJ/kg
T1  34C 
P2  1200 kPa 
 h2  295.16 kJ/kg
T2  65C

P3  1200 kPa 
 h3  111.23 kJ/kg
T3  42C

h4  h3  111.23 kJ/kg
Water
18C
26C
QH
1.2 MPa
65C
42C
P4  60 kPa

 x 4  0.4795
h4  111.23 kJ/kg 
Using saturated liquid enthalpy at the given temperature,
for water we have (Table A-4)
Condenser
3
Qin
2
Expansion
valve
Win
Compressor
4
60 kPa
-34C
1
Evaporator
hw1  h f @ 18C  75.47 kJ/kg
QL
hw 2  h f @ 26C  108.94 kJ/kg
(b) The mass flow rate of the refrigerant may be determined from an energy balance on the compressor
m R (h2  h3 )  m w (hw2  hw1 )
m R (295.16  111.23)kJ/kg  (0.25 kg/s)(108.94  75.47)kJ/kg

 m R  0.0455 kg/s
The waste heat transferred from the refrigerant, the compressor power input, and the refrigeration load are
Q H  m R (h2  h3 )  (0.0455 kg/s)(295.16  111.23)kJ/kg  8.367 kW
W in  m R ( h2  h1 )  Q in  (0.0455 kg/s)(295.16  230.03) kJ/kg  0.450 kW  2.513 kW
Q L  Q H  W in  Q in  8.367  2.513  0.450  5.404 kW
(c) The COP of the refrigerator is determined from its definition
COP 
Q L 5.404

 2.15
W in 2.513
(d) The reversible COP of the refrigerator for the same
temperature limits is
COPmax 
1
1

 5.063
T H / T L  1 (18  273) /(30  273)  1
Then, the maximum refrigeration load becomes
Q L,max  COPmaxW in  (5.063)(2.513 kW)  12.72 kW
T
2
·
QH
2
·
Win
3
4
·
QL
1
s
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