Page 153 - DJJ20063- Thermodynamics 1

DJJ20063- Thermodynamics 1

b) Rankine Cycle

In the Rankine cycle, the exhaust

steam is completely condensed into
water in the condenser.

Many of the impracticalities associated with the Carnot cycle can be eliminated
by condensing it completely in the condenser, as shown schematically on a T-s

diagram in Fig. 11.3. The cycle that results is the Rankine cycle, is the ideal cycle
for vapour power plants. The ideal Rankine cycle does not involve any internal

irreversibility and consists of the following processes:

4,5 to 1: Constant pressure heat addition in a boiler
1 to 2: Isentropic expansion taking place in the turbine or engine

2 to 3: Constant pressure heat rejection in the condenser

3 to 4: Isentropic compression of water in the feed pump

From a comparison made between Fig. 11.1 and Fig. 11.2, the similarities between the

Carnot and the Rankine cycles can be clearly seen. In the Rankine cycle, the exhaust

steam is completely condensed into water in the condenser. It actually follows the
isentropic expansion in the turbine. This water is then pumped into the boiler by a boiler

feed pump. After the feed pump, since the water is not at the saturation temperature

corresponding to the pressure, some of the heat energy supplied in the boiler is taken

up by the water as sensible heat before evaporation can begin. This results in the boiler
process being no longer completely isothermal; the process is, therefore, irreversible,

causing the Rankine cycle to be an irreversible cycle and to have a lower efficiency than

the Carnot cycle.

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