Isentropic Specific Enthelpy HELP

[bobboviking]

Hi guys,

I'm struggling with some equations regarding the enthalpy change over a compressor in a vcc system.

I'm currently working on a model for the whole system and I think I've got all equations except the one describing the enthalpy difference betweeen the inlet and outlet of the compressor.
I can't really understand how to handle the isentropic specific enthalpy, h_out,s, in the following equation:

h_out = (h_out,s - h_in)/n_s + h_in

where n_s is the isentropic efficiency

I also know that n_s = (T_out,s-T_in)/(T_out-T_in) (assuming constant specific heat)

I would prefer the model to be as "general" as possible, meaning that I don't have to perform a bunch of experimental test to determine certain coefficients.

I would greatly appreciate some help with how to calculate/estimate h_out,s. I think that the value of h_out,s depends on the suction pressure (?) so assuming a constant value (from example a table at some operating condition) isn't good enough for an accurate model? I mean that value of h_out,s does change when the system moves from that particular condition, right?

Thanks in advance,
Andreas

 

[Chestermiller]

To get the isentropic enthalpy out, you start with the inlet conditions of temperature and pressure and determine from the tables the entropy in. You must have already don this since you seem to know hin. Then you look up in your tables the enthalpy at the outlet pressure and at the same entropy as inlet. Simple as that. This is hout,s.