TITLE.PM5

REFRIGERATION CYCLES 745

dharm
\M-therm\Th14-3.pm5

0.2232 + x 3 ×

138

298

= 0.1251 + 0.6 ×

154

268

= 0.4698

∴ x 3 = (0.4698 – 0.2232) ×^298

138

= 0.5325

T(K)

298

268

s

4 Condensation^3

Evaporation

Compression

Throttling

12

Fig. 14.23

Now, h 3 = hf 3 + x 3 hfg 3 = 59.7 + 0.5325 × 138 = 133.2 kJ/kg

Also, h 1 = hf 4 = 59.7 kJ/kg

Theoretical C.O.P. = R

W

n = hh

hh

21

32

−

−

= −

−

123.8 59.7

133.2 123.8

= 6.82

Actual C.O.P. = ηrelative × (C.O.P.)theoretical = 0.5 × 6.82 = 3.41

Heat extracted from 1 kg of water at 20°C for the formation of 1 kg of ice at 0°C

= 1 × 4.187 × (20 – 0) + 335 = 418.74 kJ/kg

Let mice = Mass of ice formed in kg/min.

(C.O.P.)actual = 3.41 =

R

W

n(actual) = m

mh h

ice. mice

()

.(kJ/min)

()(kJ/min)

×

−

= ×

−

418 74 418 74

32 6 133.2 123.8

∴ mice = 6 133 2 123 8 3 41

418 74

(. .).

.

−× = 0.459 kg/min

=

0 459 60 24

1000

. ××
tonnes (in 24 hours) = 0.661 tonne. (Ans.)

Example 14.13. 28 tonnes of ice from and at 0°C is produced per day in an ammonia
refrigerator. The temperature range in the compressor is from 25°C to – 15°C. The vapour is dry
and saturated at the end of compression and an expansion valve is used. Assuming a co-efficient
of performance of 62% of the theoretical, calculate the power required to drive the compressor.