Practical Rankine Cycle

In actual practice, the pump and the turbine cannot be operated under isentropic condition because of irreversibilies. Therefore process 1-2 and 4-5 are non-isentropic. Applying the second law of thermodynamics to the control volumes the turbine and the pump we find that the entropy of the exit fluid is greater then the entropy of the entering fluid. (See Figure 27.6)

Figure 27.6

Therefore

and

Also

Therefore

and

The actual work delivered by the turbine is less than the work delivered by an isentropic turbine. The work spent on the actual pump is greater the work spent on the isentropic pump. Because of irreversibilties in the turbine and the pump, the actual efficiency of a practical Rankine Cycle is less than the efficiency of an ideal Rankine cycle.

The performance of an actual turbine or pump is usually expressed in terms of isentropic efficiency.

They are defined as

For the turbine,

(27.6)

Now, For the pump

(27.7)

The efficiency of an actual Rankine cycle is

(27.8)