# Enthalpy with regards to heat exchangers and air

I'm just wondering how one would actually solve for the final enthalpy of air after entering a heat exchanger when the only known values are initial pressure and temperature before heating. I'm assuming that if air is modelled as an ideal gas you can just do (Cp)T, but how would you be able to find T in the first place? If the assumption of pressure changes being negligible, using the ideal has equations just leads to the final and initial temperatures being equal, unless I'm not doing it correctly.

Regards.

## Answers and Replies

russ_watters
Mentor
Well you certainly need more than just the starting conditions. You have to know something about the exit conditions or about the heat exchanger.

In a 1st semester thermo class, you'd typically either be given one critical piece of information about the exit conditions or information about the fluid on the other side of the heat exchanger.

Well the thing is, the problem I have is that the heat exchanger is part of a bigger system that includes a compressor and a turbine, in which I need to find the turbine power output. My initial plan was to take the compressor and turbine into one system, which means I then have two flow rates in (one from surroundings and one from the heat exchanger) and two flow rates out (one to the exchanger and one back to the surroundings).

So basically you're saying that knowing the final and initial pressures which are equal at the heat exchanger, and the temperature going in it is impossible to find the enthalpy coming out?