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

Chapter 11 | 625

and

The enthalpy at state 9 is determined from an energy balance on the mixing

chamber,

Also, s 9 0.9416 kJ/kg · K. Thus the enthalpy at state 4 (0.8 MPa, s 4 

s 9 ) is h 4 274.48 kJ/kg. Substituting,

(c) The coefficient of performance is

Discussion This problem was worked out in Example 11–1 for a single-stage

refrigeration system (COP 3.97) and in Example 11–3 for a two-stage cas-

cade refrigeration system (COP 4.46). Notice that the COP of the refriger-

ation system increased considerably relative to the single-stage compression

but did not change much relative to the two-stage cascade compression.

COPR

qL

win



146.3 kJ>kg

32.71 kJ>kg

4.47

32.71 kJ/kg

win 11 0.2049 231 255.93239.16 2 kJ>kg 4  1 274.48255.10 2 kJ>kg

h 9  1 0.2049 21 251.88 2  11 0.2049 21 255.93 2 255.10 kJ>kg

112 h 9 x 6 h 3  11 x 62 h 2

E

#

outE

#

in

winwcomp I,inwcomp II,in 11 x 621 h 2 h 12  1121 h 4 h 92

8

7

2

1

T

s

4

5

6

h 7 = 55.16 h 6 = 95.47^9

h 4 = 274.48 kJ/kg

h 2 = 255.93

h 9 = 255.10

h 1 = 239.16

h 8 = 55.16

h 3 = 251.88^3

h 5 = 95.47

FIGURE 11–13

T-sdiagram of the two-stage

compression refrigeration cycle

described in Example 11–4.

Multipurpose Refrigeration Systems

with a Single Compressor

Some applications require refrigeration at more than one temperature. This
could be accomplished by using a separate throttling valve and a separate
compressor for each evaporator operating at different temperatures. However,
such a system is bulky and probably uneconomical. A more practical and