Chapter 6-5

Reversible Steady-flow
Some questions can be answered
• Why do they dump all that
energy out by condensing
the steam in a steam power
generating system before
boosting the pressure back
• Why don’t they like the
steam to condense in the
Equations for work are similar,
but quite different!
What does that difference
The larger the specific volume, the larger the work!
Example 6-12
It takes more than 500 times the work to compress the
vapor than it takes to pump the liquid.
Look over proof in
the text.
Similarly, a reversible compressor uses
less work.
• So, can make a compressor more
efficient by:
– removing as many irreversibilities
as possible.
• not too economic or practical
– Keeping v as small as possible
• means keeping the gas as cold as
Comparison of work done by different
reversible compressors between two pressures.
Maximum cooling
The area to the left of the curves represents
the work done.
Multistage compression with intercooling.
Ideally, the intercooling is at constant pressure and gets you back
to T1 each time.
Where do you break it to minimize work?
• Intermediate pressure is Px
• differentiate equation for work
• Px/P1 = P2/Px
• Gives the result that compression
work is equal at each stage.
Example 6-13 – Work for various compression processes
What should pressure
be here?
Isentropic = 263 kJ/kg
Isothermal = 189 kJ/kg
Polytropic = 246 kJ/kg
Two stage polytropic = 215 kJ/kg
The more stages, the closer it gets to isothermal.