Isentropic Effeciencies of steam turbines

[benny85]

Hi,
I'm have big problems calculating the Isentropic Effeciencies of steam turbines. There is a HP, IP and LP turbine in the system. I know the calculation is (h1 - h2)/(h1 - h2'), i have the temperatures and pressures in and out for the first two turbines so i can easily get h1 and h2 from the steam tables for each of these but how do i find h2'??

any help on this would be great, its been wrecking my head for hours.

Thanks

 

[Q_Goest]

Hi benny, welcome to the board. h2' is the enthalpy of the outlet given the process is isentropic. So if you know the inlet state, you can find the enthalpy and entropy of that state from the steam tables. If you know the outlet pressure, you can find that state because you know the pressure and entropy. The entropy will be the same as the inlet entropy. All you need to do to find h2' is to locate the outlet pressure for the turbine in the steam tables and look for the temperature at which the entropy is the same as the inlet entropy. Interpolation of the tables is generally necessary.

 

[benny85]

thanks a million, i had assumed that and had started doing it, but its nice to know I am doing the right thing. Left me with a lot of interpolation to do though.

So my problem now is first calculating isentropic effeciency of the LP turbine, the output i have been given is the pressure and dryness fraction, and i don't know what to make of the input as the feed to a feedwater heater is inbetween the IP and LP turbines. Also how might i calculate the percentage of steam sent to the feedwater heater.

Your help is very much appreciated as I am doing a distance learning Masters and this stuff is blowing my mind because iv no lecturer to ask questions.