7-22
7-58 A commercial refrigerator with R-134a as the working fluid is considered. The evaporator inlet and
exit states are specified. The mass flow rate of the refrigerant and the rate of heat rejected are to be
determined.
QH
Assumptions 1 The refrigerator operates steadily. 2
The kinetic and potential energy changes are zero.
Condenser
Properties The properties of R-134a at the
evaporator inlet and exit states are (Tables A-11
through A-13)
P1
x1
P2
T2
Expansion
valve
120 kPa ½
¾h1 65.38 kJ/kg
0.2
¿
120 kPa ½
¾h2 238.84 kJ/kg
20qC ¿
Evaporator
120 kPa
x=0.2
Analysis (a) The refrigeration load is
Q L
(COP)W in
Win
Compressor
QL
120 kPa
-20qC
(1.2)(0.45 kW) 0.54 kW
The mass flow rate of the refrigerant is determined from
m R
Q L
h2 h1
0.54 kW
(238.84 65.38) kJ/kg
0.0031 kg/s
(b) The rate of heat rejected from the refrigerator is
Q H
Q L W in
0.54 0.45 0.99 kW
PROPRIETARY MATERIAL. © 2008 The McGraw-Hill Companies, Inc. Limited distribution permitted only to teachers and
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7-22 exit states are specified. The mass flow rate of the... 7-58