Modern Engineering Thermodynamics

536 CHAPTER 14: Vapor and Gas Refrigeration Cycles
commonly called vapor-compression refrigeration cycles and reversed gas cycles are normally referred to by the
cycle name (e.g., a reversed Brayton refrigeration cycle).
Like power cycle technology, refrigeration technology has had an enormous impact on our culture and the way
we live. It changed our diet, the architecture of our buildings, the agriculture on our farms, and many other
items that touch our everyday life. Just as we would find it very difficult to return to a time without the portable
power produced by engines, we would also find life much less comfortable in a time without refrigeration and
air conditioning.
14.2 PART I. VAPOR REFRIGERATION CYCLES
The basic concepts of refrigeration, air conditioning, and heat pumps were introduced in Chapter 7. This technology
is usually modeled as a backward-running heat engine. When a heat engine runs backward (or in reverse), it
receives a net input of work W that causes an amount of heat Q L to be removed from a low-temperature region
and an amount of heat Q H to be added to a high-temperature region. So, it actually cools the low-temperature
region and heats the high-temperature region.
A backward-running heat engine is a refrigeration machine, but its exact technical name depends on exactly
how it is being used. For example, if food occupies the low-temperature region, then the device is indeed called
a refrigerator, but if people occupy the low-temperature region, then the device is called an air conditioner. 1 On
the other hand, if people occupy the high-temperature region and utilize Q H for space-heating purposes, then
the device is called a heat pump. Though the details of their design and operation differ slightly, refrigerators, air
conditioners, and heat pumps can all be modeled as backward-running heat engines. These distinctions are
shown in Figure 14.1.
HOW CAN AN ENGINE RUN BACKWARD?
To get an engine to run backward, you need to put work into it where the work normally comes out. If the engine has an
output shaft, you simply attach a motor or something to the shaft to turn it so that you are putting work into the engine
instead of having the engine produce work. A heat engine converts some of the heat from a high-temperature source into
work and rejects the remaining heat to a low-temperature sink. A “backward-running” heat engine draws heat from a lowtemperature
source, adds work energy to it, and rejects everything to a high-temperature sink (see Figure 14.1).
Environment
(warm region)
Environment
(warm region)
People
(warm region)
Q H
Q H
Q H
Backwardrunning
heat engine
W Backwardrunninrunning
R W Backward-
AC W HP
heat engine
heat engine
Q L Q L Q L
Food stuff
(cold region)
People
(cool region)
Environment
(cold region)
(a) Refrigerator (b) Air conditioner (c) Heat pump
FIGURE 14.1
Characteristics of refrigerators, air conditioners, and heat pumps.
1 In addition to lowering room or building air temperature, air conditioners also usually filter the air and alter its humidity
(see Chapter 8).