5 replies to this topic

[DANA-IF]

hi friends

 

 

What's the formula used for  an isentropic compression to calculate effeciency ?

 

 

 

T1= 327,59 K    ;      P1=618,4 Kpa  ,Cp/Cv = 1,149

 

 

 

T2 = 379,05 K  ;P2= 1825 Kpa ,   Cp/Cv = 1,195

 

 

 

Thanks a lot

 

 

[ankur2061]

Isentropic efficiency is the ratio of the isentropic work to the actual work done by the compressor

 

Did you try google, where there are thousands of references available:

 

http://petrowiki.org/Compressors

 

 

Regards,

Ankur.

[DANA-IF]

Yes , I have used this article but the problem the ratio Cp/Cv isn't constant

Edited by N.BH, 30 January 2014 - 07:34 AM.

[ankur2061]

You can use the arithmetic mean of the two Cp/Cv values in your starting post. 

 

Regards,

Ankur

[DANA-IF]

the hysys give me an adiabatic efficiency : 75% , then I used this value to predict how much should be the  Cp/Cv ratio , I found 1,113 which is less than the initial Cp/Cv 

[PaoloPemi]

I think it would be better to put similar questions in student section,

anyway you can solve as ideal gas or real gas,
for a ideal fluid you can calculate discharge temperature
for some predefined process (for example isentropic) as

t2 = tin*(pout/pin)(k-1)/k

k = cp/cv (average as suggested by Ankur)

then calculate Hin (Pin,Tin), Hout (Pout, Tout), H2 (pout,t2)
since you know cp it's easy,

finally, efficiency = (Hout-Hin)/(H2-Hin)

replace Hout-Hin and H2-Hin with
ideal gas formulation to obtain the direct correlation

for real fluids things are a bit more complex,

if it's a pure fluid you can replicate the procedure
getting the values from a  chart (i.e. a Mollier chart
or Pressure-Enthalpy diagram where you can find H,S values)
this allows accurate results

for mixtures you would need some thermo package to calculate
H, S with a EOS or similar procedure,
I utilize Excel with Prode Properties for these calc's
but other simulators can give equivalent results.

 

Do not forget that before to use a simulator you need

to get a good knowledge of thermodynamics.