Page 145 - DJJ20063- Thermodynamics 1

DJJ20063- Thermodynamics 1

The work ratio for Carnot cycle

The ratio of the net work output to the gross work output of the system is called the work

ratio. The Carnot cycle, despite its high thermal efficiency, has a low work ratio.
net work
Work ratio = (4.10)
gross work

(h − h ) − (h − h )
i.e. Work ratio = 1 2 4 3 (4.11)
(h − h 2 )
1

The work output of the Carnot cycle also can be found very simply from the T-s diagram.

From the first law,
 Q =  W

therefore, the work output of the cycle is given by

W = Q1 – Q2

Hence for the Carnot cycle, referring to Fig. 4.4,

Wcarnot = area 12341 = (T1 - T2)(sB – sa) (4.12)

This cycle is never used in practice owing to:
1. The difficulty in stopping the condensation at 3, so that subsequent compression

would bring the state point to 4.

2. A very large compressor would be required.
3. Compression of wet steam in a rotary compressor is difficult as the water tends to

separate out.

4. Friction associated with the expansion and compression processes would cause

the net work done to be very small as compared to the work done in the turbine

itself.

The Carnot cycle is modified to overcome the above difficulties and this modified cycle,

known as the Rankine cycle, is widely used in practice.

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