Handbook of air conditioning and refrigeration / Shan K

9.44 CHAPTER NINE
of HFC-125 c pl in the temperature range of 0 to 50°F (�17.8 to 10°C) is 0.300 Btu/lb�°F (1256
J/kg�°C).
Calculate:
1. The refrigeration effect per pound (kilogram) of refrigerant in the lower system
2. Total work input to the compressors
3. Coefficient of performance of this cascade system
4. Mass flow rates of the refrigerants in the higher and lower systems
Solution
1. From the given values, the enthalpy of the saturated vapor refrigerant at �60°F is
h1 � 63.30 � 0.14[Tsv � (�80)]
� 63.30 � 0.14[ �60 � (�80)] � 66.10 Btu/lb
Enthalpy of saturated liquid refrigerant at a condensing temperature of 20°F is
and from Eq. (9.26)
h 3 � 11.04 � 0.30T sl
� 11.04 � 0.30(20) � 17.04 Btu/lb
h 1� � h 4 � h 3 � c pl (T 3 � T 3�)
Then the refrigeration effect of the lower system is
� 17.04 � 0.30(20 � 1) � 11.34 Btu/lb
q rf,l � h 1 � h 4 � 66.12 � 11.34 � 54.78 Btu/lb (127.4 kJ/kg)
2. Enthalpy of superheated vapor refrigerant after the heat exchanger in the lower system is
h 1� � h 1 � c pv(degree of superheat)
� 66.12 � 0.14[ �20 � (�60)] � 71.72 Btu/lb
Enthalpy difference (h 2 � h 1�) along the constant-enthalpy line of the lower system is
h 2 � h 1� � 0.175 (T con � T lv)
� 0.175[20 � (�60)] � 14.0 Btu/lb
Enthalpy of the hot gas discharged from the compressor of the lower system is
h2 � h1� � (h2 � h1�) � 71.72 � 14.0 � 85.72 Btu/lb
From Eq. (9.20), the enthalpy of saturated vapor refrigerant HCFC-22 in the higher system is
h 5 � 104.465 � 0.098445T s � 0.0001226T s 2 � 9.861 � 10 �7 Ts 3
� 104.465 � 0.098445(10) � 0.0001226(10) 2 � 9.861 � 10 �7 (10) 3
� 104.465 � 0.984 � 0.012 � 0.001 � 105.436 Btu/lb