11-13
An ideal vapor-compression refrigeration cycle with refrigerant-134a as the working fluid is considered. The rate of heat removal from the refrigerated space, the power input to the compressor, the rate of heat rejection to the environment, and the COP are to be determined.
Assumptions 1 Steady operating conditions exist. 2 Kinetic and potential energy changes are negligible.
Analysis (a ) In an ideal vapor-compression refrigeration cycle, the compression process is isentropic, the refrigerant enters the compressor as a saturated vapor at the evaporator pressure, and leaves the condenser as saturated liquid at the condenser pressure. From the refrigerant tables (Tables A-12 and A-13),
()()throttling
kJ/kg 82.88kJ/kg
82.88liquid sat.MP a 7.0C 95.34kJ/kg 50.273MP a 7.0K kJ/kg 94779.0kJ/kg
97.236 vapor sat.kP a 12034MPa 7.0 @ 3322122kPa 120 @ 1kPa 120 @ 11=≅==⎭
⎬⎫
=︒==⎭⎬⎫
==⋅====⎭⎬⎫=h h h h P T h s s P s s h h P f g g
Then the rate of heat removal from the
refrigerated space and the power input to the compressor are determined from
and
()()()()()()kW 1.83kW 7.41=-=-==-=-=kJ/kg 236.97273.50kg/s 0.05kJ/kg 82.8897.236kg/s 0.0512in
41h h m W h h m Q L
(b ) The rate of heat rejection to the environment is determined from
kW 9.23=+=+=83.141.7in
W Q Q L H (c ) The COP of the refrigerator is determined from its definition,
4.06===kW 1.83kW
7.41COP in
R W Q L
11-15
An ideal vapor-compression refrigeration cycle with refrigerant-134a as the working fluid is considered. The throttling valve in the cycle is replaced by an isentropic turbine. The percentage increase in the COP and in the rate of heat removal from the refrigerated space due to this replacement are to be determined.
s
Assumptions 1 Steady operating conditions exist. 2 Kinetic and potential energy changes are negligible.
Analysis If the throttling valve in the previous problem is replaced by an isentropic turbine, we would have s 4s = s 3 = s f @ 0.7 MPa = 0.33230 kJ/kg·K and the enthalpy at the turbine exit would be
(
)()()kJ/kg
58.8248.2142802.049.222802.085503.009275.033230.0kPa
120 @44kPa
120 @34=+=+==-=⎪⎪⎭⎫
⎝
⎛-=fg
s f s fg f
s h x h h s s s x
Then,
()()()kW 7.72kJ/kg 82.58236.97kg/s 0.0541=-=-=s L
h h m Q and
23.4kW 1.83kW 7.72COP in
R ===
W Q L
Then the percentage increase in Q
and COP becomes
4.2%
4.2%=-=∆=
=-=∆=06
.406
.423.4COP COP COP in Increase 41.741.772.7in Increase R R R L L L Q Q Q
11-23
A vapor-compression refrigeration cycle with refrigerant-134a as the working fluid is
considered. The amount of cooling, the work input, and the COP are to be
determined. Also, the same parameters are to be determined if the cycle operated on the ideal vapor-compression refrigeration cycle between the same temperature limits. Assumptions 1 Steady operating conditions exist. 2 Kinetic and potential energy changes are negligible. Analysis (a) The expansion process through the expansion
s
