VAPOUR POWER CYCLES 559

dharm

\M-therm\Th15-2.pm5

2

3 2 ′

1

T

s

(b)

3

v 2

p 1

p 2

p 3

1

2 (2 bar)

Loss of work due

to incomplete

expansion

v

p

(a)

2 (1.1 bar)′

Fig. 12.13. p-V and T-s diagrams.

During isentropic expansion 1-2, we have

s 1 = s 2

6.918 = sf 2 + x 2 sfg 2 = 1.5301 + x 2 × 5.5967

∴ x 2 = 6. 1.

5.

918 5301

5967

− = 0.96.

Then h 2 = hf 2 + x 2 hfg 2 = 504.7 + 0.96 × 2201.6 = 2618.2 kJ/kg

v 2 = x 2 vg 2 + (1 – x 2 ) vf 2

= 0.96 × 0.885 + (1 – 0.96) × 0.00106 = 0.849 m^3 /kg.

(i)Ideal work :

Ideal work or modified Rankine engine work/kg,

W = (h 1 – h 2 ) + (p 2 – p 3 ) v 2

= (3037.6 – 2618.2) + (2 – 1.1) × 10^5 × 0.849/1000

= 419.4 + 76.41 = 495.8 kJ/kg. (Ans.)

(ii)Rankine engine efficiency :

ηrankine =

Work done

Heat supplied

=

495 8

(^13)
.
(hh− f)

495 8
3037 6 428 8
.
..−
= 0.19 or 19%. (Ans.)
(iii) Indicated and brake work per kg :
Indicated work/kg, Windicated =
I. P.
m&

1 3600
12.8
×
= 281.25 kJ/kg. (Ans.)
Brake work/kg, Wbrake = B. P.&
m

ηmech.×I.P.
m&

0 8 1 3600
12.8

. ××
= 225 kJ/kg. (Ans.)