Multistage Compression Refrigeration Systems

When the fluid used throughout the cascade refrigeration system is the same,
the heat exchanger between the stages can be replaced by a mixing chamber
(called a flash chamber) since it has better heat transfer characteristics. Such
systems are called multistage compression refrigeration systems.A two-
stage compression refrigeration system is shown in Fig. 11–12.

Chapter 11 | 623

8

5 2

1

T

s

7

(^63)
9
QH
Condenser
WARM
environment
High-pressure
compressor
COLD
refrigerated space
Expansion
valve
54
Expansion
valve
3
Evaporator
QL
4
9
2
1
6
7
8
Flash
chamber
Low-pressure
compressor
FIGURE 11–12
A two-stage compression refrigeration system with a flash chamber.
(c) The COP of a refrigeration system is the ratio of the refrigeration rate to
the net power input:
Discussion This problem was worked out in Example 11–1 for a single-stage
refrigeration system. Notice that the COP of the refrigeration system
increases from 3.97 to 4.46 as a result of cascading. The COP of the system
can be increased even more by increasing the number of cascade stages.
COPR
Q

L
W

net,in

7.18 kW
1.61 kW
4.46
1.61 kW
¬ 1 0.039 kg>s 231 255.93239.16 2 kJ>kg 4
 1 0.05 kg>s 231 270.92251.88 2 kJ>kg 4