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/cm2. 
The various advantages of reheating are as follows: 

1.  It increases dryness fraction of steam at exhaust so that blade erosion due to impact of  water particles is reduced
2. It increases thermal efficiency.
3. It increases the work done per kg of steam and this results in reduced size of boiler.

The disadvantages of reheating are as follows: 

1.  Cost of plant is increased due to the reheater and its long connections.
2.  It increases condenser capacity due to increased 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 – H2) + (H3 – H4)}/{H1 + (H3 – H2) – Hw4}