Modern Engineering Thermodynamics

582 CHAPTER 14: Vapor and Gas Refrigeration Cycles
EXAMPLE 14.14 (Continued )
_I evaporator = T 0
" #
_m ref ðs 1 − s 4h Þ −
_Q evaporator
T evaporator
= ð25:0 + 273:15Þ 0:500 kg
0:9332 − 0:3449 kJ
s
kg.K
= 2:15 kJ/s = 2:15 kW
and the total irreversibility rate of the system is
−
73:4 kJ/s
−15:6 + 273:15 K
b. The system coefficient of performance is given by
_I total = _I compressor + _I condenser + _I expansion valve + _I evaporator
= 7:41 + 2:55 + 2:86 + 2:15 = 15:0kW
COP =
_Q evaporator
_W compressor
= 73:9
26:4 = 2:80
The second law efficiency for a refrigeration system is discussed in Chapter 10. The relevant equation is Eq. (10.34),
1 − T
0 _Q
T L L
ε R/AC =
=
_W 1 − T 0 Q _
L =
T L _W 1 − T 0
× COP
T actual
L
R/AC
=
1 − 25:0 + 273:15 K
× 2:85 = 0:494 = 49:4%
− 15:6 + 273:15 K
c. The component with the highest irreversibility rate in this system is the compressor. It is the weak link in this system.
Improving other system components will have only a marginal effect on system performance until a more efficient
compressor is designed or found. Note that to reduce the irreversibility rate of the compressor to a value comparable to
the irreversibility rates of the other components in the system requires improving the isentropic efficiency of the
compressor from 70% to about 88%.
SUMMARY
Refrigeration is a generic term that embodies the topics of refrigeration, air conditioning, and heat pump systems. In
this chapter, we divide refrigeration into three broad categories of technology. The first is vapor refrigeration cycles,
consisting of vapor-compression cycles and absorption cycles. Vapor-compression cycles are basically reversed Rankine
power cycles, whereas absorption refrigeration has no power cycle analog. The second category is gas refrigeration
cycles, which consist of reversed versions of external combustion power cycles. The most prominent are the
reversed Brayton and reversed Stirling refrigeration cycles. The third category covers all other miscellaneous refrigeration
technologies, such as Joule-Thomson cooling; refrigerating mixtures; and evaporation, radiation, reduced pressure,
thermoelectric, and vortex tube cooling. The chapter ends with a discussion of future needs in refrigeration
technology followed by an example of how the second law of thermodynamics can assist in the design of better
refrigeration technologies by minimizing the irreversibility rate within the various system components.
As in the other chapters in this text, a historical timeline is used to develop the material. This is done to provide
a perspective on the social and cultural impact produced by the development of refrigeration technology. The
reason this approach has been followed throughout this text is to sensitize you, the next generation of engineers,
to your responsibility for understanding the enormous potential of your profession to change society.
Some of the more important equations introduced in this chapter follow. Do not attempt to use them blindly
without understanding their limitations. Please refer to the text material where they were introduced to gain an
understanding of their use.
1. The relation between the coefficient of performance of heat pumps, refrigerators, and air conditioners:
COP HP = COP R/AC + 1