Microsoft Word - Cengel and Boles TOC _2-03-05_.doc

In the cascade system shown in the figure, the refrigerants in both cycles
are assumed to be the same. This is not necessary, however, since there is no
mixing taking place in the heat exchanger. Therefore, refrigerants with more
desirable characteristics can be used in each cycle. In this case, there would
be a separate saturation dome for each fluid, and the T-sdiagram for one of
the cycles would be different. Also, in actual cascade refrigeration systems,
the two cycles would overlap somewhat since a temperature difference
between the two fluids is needed for any heat transfer to take place.
It is evident from the T-sdiagram in Fig. 11–10 that the compressor work
decreases and the amount of heat absorbed from the refrigerated space
increases as a result of cascading. Therefore, cascading improves the COP
of a refrigeration system. Some refrigeration systems use three or four
stages of cascading.

Chapter 11 | 621

4

5

2

1

T

s

6

7

8

3

8 5

QH

Condenser

WARM

environment

QL

Evaporator

Decrease in

compressor

work

QH

QL

Increase in

refrigeration

capacity

Compressor

COLD refrigerated

space

Expansion

valve

7 6

Compressor

Expansion

valve

(^3) Condenser^2
Evaporator
A
B
QL
4
Heat exchanger
A
B
Heat
1
FIGURE 11–10
A two-stage cascade refrigeration system with the same refrigerant in both stages.
EXAMPLE 11–3 A Two-Stage Cascade Refrigeration Cycle
Consider a two-stage cascade refrigeration system operating between the pres-
sure limits of 0.8 and 0.14 MPa. Each stage operates on an ideal vapor-
compression refrigeration cycle with refrigerant-134a as the working fluid. Heat
rejection from the lower cycle to the upper cycle takes place in an adiabatic
counterflow heat exchanger where both streams enter at about 0.32 MPa.