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

11–4 ■ ACTUAL VAPOR-COMPRESSION

REFRIGERATION CYCLE

An actual vapor-compression refrigeration cycle differs from the ideal one

in several ways, owing mostly to the irreversibilities that occur in various

components. Two common sources of irreversibilities are fluid friction

(causes pressure drops) and heat transfer to or from the surroundings. The

T-sdiagram of an actual vapor-compression refrigeration cycle is shown in

Fig. 11–7.

In the ideal cycle, the refrigerant leaves the evaporator and enters the

compressor as saturated vapor. In practice, however, it may not be possible

to control the state of the refrigerant so precisely. Instead, it is easier to

design the system so that the refrigerant is slightly superheated at the com-

pressor inlet. This slight overdesign ensures that the refrigerant is com-

pletely vaporized when it enters the compressor. Also, the line connecting

614 | Thermodynamics

and the turbine would produce 0.33 kW of power. This would decrease the

power input to the refrigerator from 1.81 to 1.48 kW and increase the rate of

heat removal from the refrigerated space from 7.18 to 7.51 kW. As a result,

the COP of the refrigerator would increase from 3.97 to 5.07, an increase of

28 percent.

4

5

2

1

T

s

(^678)
3
2'
43
78
Compressor
QH
2
Condenser
WARM
environment
QL
Evaporator
1
COLD refrigerated
space
Expansion Win
valve
6
5
FIGURE 11–7
Schematic and T-sdiagram for the actual vapor-compression refrigeration cycle.
SEE TUTORIAL CH. 11, SEC. 3 ON THE DVD.
INTERACTIVE
TUTORIAL