Handbook of air conditioning and refrigeration / Shan K

REFRIGERANTS, REFRIGERATION CYCLES, AND REFRIGERATION SYSTEMS 9.25
The enthalpy differential along the constant-entropy line within a narrower temperature range can
be calculated as
h 2 � h 1 � F(T s2 � T s1) (9.19)
where h 1, h 2 � enthalpy of refrigerant on constant-entropy line at points 1 and 2, Btu/lb (kJ/kg)
T s1, T s2 � temperature of saturated refrigerant at points 1and 2,°F (°C)
From the refrigerant tables published by ASHRAE, the following polynomial can be used to calculate
the enthalpy of saturated liquid refrigerant h lr, Btu/lb (kJ/kg), from its temperature T s1 at a saturated
temperature from 20 to 120°F (� 7 to 50°C) with acceptable accuracy:
where a 1, a 2, a 3, a 4 � coefficients. For HCFC-22,
h lr � a 1 � a 2T s1 � a 3T s1 2 � a4T s1 3 (9.20)
a 1 � 10.409 a 2 � 0.26851 a 3 � 0.00014794 a 4 � 5.3429 � 10 �7
Similarly, the polynomial that determines the enthalpy of saturated vapor refrigerant h vr, Btu/lb
(kJ/kg), from its temperature T sv in the same temperature range is
where b 1, b 2, b 3, b 4 � coefficients. For HCFC-22,
h vr � b 1 � b 2T sv � b 3T sv 2 � b4T sv 3 (9.21)
b 1 � 104.465 b 2 � 0.098445 b 3 ��0.0001226 b 4 ��9.861 � 10 �7
The polynomial that determines the enthalpy changes of refrigerant along the constant-entropy line
for an isentropic compression process between initial state 1 and final state 2 is
h 2 � h 1 � c 1 � c 2(T s2 � T s1) � c 3(T s2 � T s1) 2 � c 4(T s2 � T s1) 3
(9.22)
where c 1, c 2, c 3, c 4 � coefficients
T s1, T s2 � saturated temperature of vapor refrigerant corresponding to its pressure at initial
state 1 and final state 2,°F (°C)
For HCFC-22 within a saturated temperature range of 20 to 100°F:
c 1 ��0.18165 c 2 ��0.21502 c 3 ��0.0012405 c 4 ��8.198 � 10 �6
Computer programs are available that calculate the coefficients based on ASHRAE’s refrigerant tables
and charts.
Refrigeration Effect, Refrigerating Load, and Refrigerating Capacity
The refrigeration effect q rf, Btu/lb (J/kg or kJ/kg), is the heat extracted by a unit mass of refrigerant
during the evaporating process in the evaporator. It can be calculated as
q rf � h lv � h en
(9.23)
where h en, h lv � enthalpy of refrigerant entering and leaving evaporator, Btu/lb (J/kg). Refrigerating
load Q rl, Btu/h (W), is the required rate of heat extraction by the refrigerant in the evaporator. It