REHEAT FACTOR

The expansion of steam through a number of stages of turbine is shown in Fig. 7.13. A₁B₁ represents isentropic expansion in the first stage. The actual state of steam with frictional reheating is shown by point A₂. Therefore, the actual heat drop in the first stage is A₁C₁. Similarly, the isentropic and actual heat drops in the succeeding stages are shown by A₂B₂, A₃B₃, ….., A₂C₂, A₃C₃, ….., and so on. A₁D represents the isentropic heat drop between supply and condenser state of steam.

Stage efficiency, 

For first stage, 

Figure 7.13 Expansion of steam through a number of turbine stages

For second stage,  and so on.

Turbine internal efficiency 

If η_s1 = η_s2 = ….. = η_s′ then

From Eqs. (7.34) and (7.35), we get

Combining Eqs (7.33) and (7.36), we get

 

η_i = η_sR_f

Reheat factor is always greater than one. Normally R_f lies between 1.02 to 1.05. The line passing through A₁, A₂, … is called the condition curve.