Gas Turbine Compressor Efficiency

[CTengineer2012]

I know that this subject has been covered before on this site. However, I was interested in learning more about tracking compressor efficiency using enthalpy. I have access to an excel add-in that will calculate enthalpies and entropies based on pressure, temperature, and humidity. This excel add-in is based on ASHRAE tables and it's accuracy has been documented.

The basic formula that I am using is eff = (H2s-H1)/(H2-H1).

Where:

H1 = Actual Enthalpy In
H2s = Isentropic Enthalpy Out
H2= Actual Enthalpy Out

I was wondering if this is an accurate way of performing a compressor efficiency calculation, and any other advice would be appreciated as well.

Thank You

 

[sheiko]

I have noticed you have mistaken in the formula. Numerator should be denominator.

"We don't believe things because they are true, things are true because we believe them."

 

[CTengineer2012]

Actual enthalpy out is always more than isentropic enthalpy out in a compressor. Enthalpy increases due temperature increase caused by friction. Isentropic enthalpy assumes no change in entropy due friction. If the numerator and denominator were switched, this would cause efficiency to be over 100%. I think you are referring to the equation for turbine efficiency.

 

[sheiko]

You are right. I had not read your query carefully enough.

"We don't believe things because they are true, things are true because we believe them."

 

[PaoloPemi]

it depends from the process,
if it's a polytropic process you need to follow a polytropic path with specified efficiency,
this is usually a iterative procedure unless you are able to integrate a simplified model,
you may find some numerical examples in

http://www.prode.com/docs/pppman.pdf

 

[CTengineer2012]

Can you elaborate a litte on this? A polytropic process is one that is reversible, the process that goes on inside the compressor is not reversible. There is always heat loss due to friction. Therefore, I can not assume that the process is reversible.

I thought that comparing real enthalpy and isentropic enthalpy was like comparing the reversible aspect of the process to the non-reversible aspect. Is this correct?

 

[PaoloPemi]

In almost all real processes you lose something (if that is what you mean),
still we can create models and simulate processes,
for a compression cycle a polytropic path can be defined as t*dS/dt = C
and since for real fluids properties are not constant you need to integrate pin=>pout to calculate tout and work.