Page 70 - Advanced Thermodynamics for Engineers, Second Edition

3.4 REVERSED HEAT ENGINES 55

Temperature, T

Q out
40 C 3
T H p = 15.54 bar
28 C Condensation
2 Sub-cooling
Throttling W C
- 48 C Evaporation
T C p = 0.7177 bar
1 4
Q in
Saturated Saturated
liquid line vapour line
Specific entropy, s
FIGURE 3.21
T–s diagram for reversed heat pump cycles.

The values of enthalpy and entropy on the saturated liquid line at 40 C have been arbitrarily set at

zero in the tables.
Conditions at 3:
p 3 ¼ p 2 ¼ 15:54 bar
h f ¼ 371:9kJ=kg; h g ¼ 1473:3kJ=kg:

s f ¼ 1:360 kJ=kgK; s g ¼ 4:877 kJ=kgK:
Conditions at 2:
This is a subcooled state where the liquid is compressed, and it will be assumed that the enthalpy is
equal to the enthalpy on the saturated liquid line at 28 C. Hence

h f ¼ 313:4kJ=kg

Now, since the compression from 4 to 3 is isentropic s 4 ¼ s 3 . Hence,
s 3 ¼ 4:877 ¼ xs g þð1 xÞs f ¼ 5:962x
giving x ¼ 4:877=5:962 ¼ 0:8180

and
h 4 ¼ xh g þð1 xÞh f ¼ 0:8180 1390 ¼ 1137 kJ=kg
Work done in compressor
W C ¼ W 43 ¼ h 4 h 3 ¼ 1137 1473:3 ¼ 336:3kJ=kg