Handbook of air conditioning and refrigeration / Shan K

the diagram and temperature T, °R, is the ordinate. A T-s diagram is more suitable for evaluating
the effectiveness of an air expansion refrigeration cycle.
Analytical Evaluation of Cycle Performance
Swers et al. (1972) proposed a thermodynamic analysis of degradation of available energy and irreversibility
in a refrigerating system, and Tan and Yin (1986) recommended a method of exergy
analysis. The exergy of a working substance e, Btu/lb (kJ/kg), is defined as
e � h � h a � T Ra(s � s a) (9.1)
where h, h a � enthalpy of working substance and ambient state, Btu/lb (kJ/kg)
T Ra � absolute temperature of ambient state, °R (K)
s, s a � entropy of working substance and ambient state, Btu/lb�°R (kJ/kg�K)
Both analyses are effective tools in the selection of optimum design and operating parameters by
means of complicated analysis. They require extensive supporting data and information.
For most analyses of refrigeration cycle performance and design and operation of refrigeration
systems in actual applications, satisfactory results can be obtained by using the steady flow energy
equation, heat and work transfer, and energy balance principle. If a more precise and elaborate
analysis is needed in research or for detailed improvements of refrigeration systems, the references
at the end of this chapter can be consulted.
9.8 CARNOT REFRIGERATION CYCLE
The Carnot refrigeration cycle is a reverse engine cycle. All processes in a Carnot refrigeration
cycle are reversible, so it is the most efficient refrigeration cycle.
Figure 9.4a is a schematic diagram of a Carnot cycle refrigerating system, and Fig. 9.4b shows
the Carnot refrigeration cycle using gas as the working substance. This Carnot cycle is composed of
four reversible processes:
1. An isothermal process 4-1 in which heat q #1 is extracted at constant temperature T R1 per lb (kg)
of working substance
2. An isentropic compression process 1-2
3. An isothermal process 2-3 in which q #2 is rejected at constant temperature T R2 per lb (kg) of
working substance
4. An isentropic expansion process 3-4
Figure 9.4c shows the Carnot refrigeration cycle using vapor as the working substance. Wet vapor
is the only working substance where heat supply and heat rejection processes can occur easily
at constant temperature. This is because the temperatures of wet vapor remain constant when latent
heat is supplied or rejected.
As in the gas cycle, there are two isothermal processes 4-1 and 2-3 absorbing heat at temperature
T R1 and rejecting heat at T R2, respectively, and two isentropic processes, one for compression
1-2 and another for expansion 3-4.
Performance of Carnot Refrigeration Cycle
REFRIGERANTS, REFRIGERATION CYCLES, AND REFRIGERATION SYSTEMS 9.19
According to the first law of thermodynamics, often called the law of conservation of energy, when
a system undergoes a thermodynamic cycle, the net heat supplied to the system is equal to the net