Page 156 - DJJ20063- Thermodynamics 1

DJJ20063- Thermodynamics 1

Thermal efficiency of Rankine cycle

Rankine efficiency,

Net work output
 = (4.19)
R
Heat supplied in the boiler
(h − h ) − (h − h )
 R = 1 2 4 3

1
h − h 4
(h − h ) − (h − h )
or  = 1 2 4 3 (4.20)
R
(h − h 3 ) − (h − h 3 )
4
1

If the feed pump term, (h4 – h3) is neglected, equation (4.20) becomes

(h − h )
 = 1 2 (4.21)
R
(h − h 3 )
1

When the feed pump term is included, it is necessary to evaluate the quantity, W34.

From equation (4.16)

Pump work = -W34 = f (p4 – p3) (4.22)

The work ratio for Rankine cycle

It has been stated that the efficiency of the Carnot cycle is the maximum possible, but

that the cycle has a low work ratio. Both efficiency and work ratio are criteria of

performance. The work ratio is defined by

net work
Work ratio =
gross work

(h − h ) − (h − h )
i.e. Work ratio = 1 2 4 3
(h − h 2 )
1

(h − h ) − v (p − p )
or Work ratio = 1 2 f 4 3 (4.23)
(h − h 2 )
1
147 | P a g e