Power Plant cycle

CLASSIFICATION OF POWER PLANT CYCLE

Power plants cycle generally divided in to the following groups,
(1) Vapour Power Cycle
(Carnot cycle, Rankine cycle, Regenerative cycle, Reheat cycle, Binary vapour cycle)
(2) Gas Power Cycles
(Otto cycle, Diesel cycle, Dual combustion cycle, Gas turbine cycle.)


CARNOT CYCLE
This cycle is of great value to heat power theory although it has not been possible to construct a
practical plant on this cycle. It has high thermodynamics efficiency.
It is a standard of comparison for all other cycles. The thermal efficiency (η) of Carnot cycle is as
follows:
η= (T1– T
2)/T
1
where, T1
= Temperature of heat source
T2
= Temperature of receiver


RANKINE CYCLE
Steam engine and steam turbines in which steam is used as working medium follow Rankine
cycle. This cycle can be carried out in four pieces of equipment joint by pipes for conveying working
medium as shown in Fig. 1.1. The cycle is represented on Pressure Volume P-V and S-T diagram as

shown in Figs. 1.2 and 1.3 respectively.


 REHEAT CYCLE
In this cycle steam is extracted from a suitable point in the turbine and reheated generally to
the original temperature by flue gases. Reheating is generally used when the pressure is high say above
100 kg/cm
2
. The various advantages of reheating are as follows:


(i) It increases dryness fraction of steam at ex-haust so that blade erosion due to impact of
water particles is reduced.
(ii) It increases thermal efficiency.
(iii) It increases the work done per kg of steam
and this results in reduced size of boiler.
The disadvantages of reheating are as follows:
(i) Cost of plant is increased due to the reheater
and its long connections.
(ii) It increases condenser capacity due to in-creased dryness fraction.
Fig. 1.4 shows flow diagram of reheat cycle. First
turbine is high-pressure turbine and second turbine is
low pressure (L.P.) turbine. This cycle is shown on T-S
(Temperature entropy) diagram (Fig. 1.5).
If,
H1= Total heat of steam at 1
H2= Total heat of steam at 2
H3= Total heat of steam at 3
H4= Total heat of steam at 4
Hw4= Total heat of water at 4
Efficiency = {(H1– H
2) + (H
3– H
4)}/{H
1+ (H3– H
2) – H
w4}