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:

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

The disadvantages of reheating are as follows:

• Cost of plant is increased due to the reheater and its long connections.
• It increases condenser capacity due to increased dryness fraction.

Figure bvelow 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 (Figure below).

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}