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Efficiency & Back Work Ratio

Process flow diagram for a heatheat engine.

Recall that the performance of a

power cycle

is measured by its

thermal efficiency

, η.

Another way to describe performance is the

back work ratio

(BWR): the ratio of pump work required and turbine work generated:

Next, we will consider some easy ways to improve the efficiency the

Rankine Cycle

without adding any new equipment.

Lesson
9B Blog

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Lesson 9B: page 7 of 12

The definition of thermal efficiency is the same for any cycle, including the Rankine Cycle.
The back work ratio is another measure of performance that is sometimes used in power plants.
The back work ratio is just the fraction or percentage of the work that the turbine produces that is consumed by the pump.
The lower the back work ratio, the better the performance of the power plant.
The good news is that the back work ratio for vapor power cycles is generally very low, 10% or less.
This brings us to the beginning of the really interesting part of this chapter.
How do we IMPROVE the efficiency of a practical power cycle ?
We already have an isentropic turbine an isentropic pump, right ?
Isentropic equipment is better than any real equipment.
So, how can we still manage to improve the efficiency ?
Flip the page and let’s see.

Efficiency & Back Work Ratio

power cycle

thermal efficiency

back work ratio

Rankine Cycle

Ch 9, Lesson B, Page 7 - Efficiency & Back Work Ratio