Common mistakes on HW9

Dear class,

If you would like your HW9 back, you can retrieve it at the review session tonight or see me in my office tomorrow. Here are a few common mistakes we noticed in grading it:

Problem 49:

Some students did not realize that entropy increases for the nonideal compressor case. The corresponding curve in the Mollier diagram should start at the same place as the ideal case, but then move both up and over to the right, in the direction of increasing entropy. The endpoint will not be the same, although it will intersect the same isobar.

Problem 50:

Some students calculated for the turbine:

W_real = W_isentropic / efficiency
This is the right expression for a pump or compressor, but for a turbine it should instead be:
W_real = W_isentropic * efficiency

Cheers,
MSS