Handbook of air conditioning and refrigeration / Shan K

REFRIGERANTS, REFRIGERATION CYCLES, AND REFRIGERATION SYSTEMS 9.43
where qrc,l � refrigeration capacity of lower system, Btu/h (W)
qrf,l � refrigeration effect of lower system, Btu/lb (J/kg)
Total work input to the compressors in both higher and lower systems Win, Btu/lb (J/kg), of refrigerant
in the lower system can be evaluated as
Win � (h2 � h1�) � (9.48)
where h1� � enthalpy of vapor refrigerant leaving heat exchanger, Btu/lb (J/kg). The coefficient of
performance of the cascade system is given as
m˙ h(h6 � h5) m˙ l
COP ref � q rf
W in
�
(9.49)
Example 9.3. A cascade system for an environmental chamber is designed to operate at the following
conditions during summer:
Lower system Higher system
Refrigerant HFC-125 HCFC-22
Evaporating temperature,°F �60 10
Evaporating pressure, psia 12.75 47.46
Condensing temperature,°F 20 100
Condensing pressure, psia 78.5 210.6
Suction temperature,°F �20
Subcooling after heat exchanger,°F 1
Refrigerating load, Btu/h 100,000
Vapor refrigerant enters the compressor of the higher system at a dry saturated state, and liquid refrigerant
leaves the condenser of the higher system without subcooling. Ignore the pressure losses
in the pipelines and valves, and assume that the compression processes for both the higher and
lower systems are isentropic.
Based on the data from the Properties of Saturated Liquid and Saturated Vapor of HFC-125
listed in ASHRAE Handbook 1997, Fundamentals, the following formulas can be used to calculate
the enthalpies of the refrigerants:
Enthalpy of refrigerant HFC-125 at saturated liquid state at temperature T sl between 0 and 50°F
(�17.8 and 10°C) h sl, Btu/lb (kJ/kg), is
h sl � 11.04 � 0.300T sl
m˙ l(h 1 � h 4)
m˙ l(h 2 � h 1�) � m˙ h(h 6 � h 5)
Enthalpy of refrigerant HFC-125 at saturated vapor state at temperature T sv between �80 and
�30°F ( �62.2 and �34.4°C) h sv, Btu/lb (kJ/kg), is
h sv � 63.30 � 0.14[T sv�(�80)]
Enthalpy difference of vapor refrigerant HFC-125 along the constant-entropy line when T con �
30°F (�1.1°C) and T ev > 80°F (26.7°C) is
�h sv � 0.175(T con � T ev)
Here T con represents the condensing temperature and T ev the evaporating temperature, both in
°F (°C). The specific heat of saturated vapor of HFC-125 c pv, in the temperature range of �80 to
�40°F (�62.2 to �40°C) is 0.14 Btu/lb�°F (586 J/kg�°C). The specific heat of saturated liquid