A feed of 5 kg/s of saturated liquid water is entering the pump in a Rankine cycle at a temperature of 423.15 K. The pump is supplied with 412.5 W of shaft work from an electric motor. What are the approximate outlet pressures from the pump, as calculated assuming that (1) the exiting stream is saturated liquid (which is NOT physically reasonable, but the result can be calculated) and (2) isochoric pumping (which is a much more reasonable model)?

Question

A. 4.8 bar; 5.5 bar
B. 7.9 bar; 300 bar
C. 9.7 bar; 500 bar
D. 10.2 bar; 11.7 bar
E. Insufficient information to solve

Answer 1

A. Correct. Using the saturated steam table for temperature and the compressed (subcooled) liquid “steam” table with the simplified energy balance you can find the approximate pressures for the two scenarios. This is not enough work to achieve a large pressure increase, but note the actual ?P is much larger than you’d expect if you arbitrarily assumed the exiting liquid is saturated.
B. Incorrect. It appears you are not tending to your units and have made a calculation error in the energy balance.
C. Incorrect. You are making a mistake somewhere in your calculations; these are both far higher than they should be.
D. Incorrect. You are making a mistake in both of your calculations.
E. Incorrect. The problem can be solved as requested.