Polytropic Head

[Abualnassr]

Hi guys.

I'm trying to calculate the polytropic head for a centrifugal compressor using this equation H = (1545/Mw)*Z*T1*(n/(n-1))*((P2/P1)^(((n-1)/n))-1)

Where,

Z = 0.996
n = 1.32
P1 = 16.87 PSIA
P2 = 23.29 PSIA
T1 = 740 R
Mw = 68.5

My calculated H = 5579.62 ft

But, according the OEM data sheet the H = 8293.96 ft

So, what do you think guys ? Is there anything missing ?

Another thing, how can i calculate T2 and the volume flow rate ?

Regards

 

[pmover]

ok, at what compressor speed does the mfg state the H=8293.96 and at what flow rate?

what about other inlet conditions stated on mfg data sheet?

for T2, the mfg likely has published formulas for determining T2.

as for flow rates, a flow measurement device is the preferred method. if no flow measurement devices are available, there are alternatives techniques to estimate flow. knowing power consumed, the head developed, and efficiencies of the compressor, the flow rate can be estimated.

you really ought to gather all essential information so the other forum participants can assist you further.

good luck!
-pmover

 

[Abualnassr]

Thanks pmover.

As per the OEM manual speed = 5813 RPM.
Mass flow rate = 21655 Kg/hr
No inlet conditions mentioned in the manual.

Regards,

 

[25362]

The compressor's operating curve provided by the vendor would help to answer your questions.

 

[milltir]

For T2 and volumetric flow you need to get your thermodynamics books out.

However, in short to calculate the discharge temperature
T2=T1*r^((n-1)/n) where r is the pressure ratio

As you have the mass flow calculate the density at suction conditions from P1/(Z1*R*T1) where R is the Gas Constant. Volumetric flow at suction conditions is then mass flow divided by suction density.

Be careful with your units as you are quoting imperial (English/US) units in your first post, then SI in your second.

 

[TPL]

There are many reasons why your calculated value of head will differ from that shown by the OEM spec sheet. The main one results from the more complex method (CFD) that the OEM will likely use for calculating these values, based on a knowledge of their own machine - the formula shown is relatively simplistic and deals with bulk values; for example - the values of n and z will vary with pressure and temperature throughout the gas path

 

[wimple]

TPL,

You are right, however here we are talking about OEM value that is 1.5 times greater than calculated Head...

Regards
Wimple

 

[DLite30]

Abu:

Your equation and caluclations are correct per the values you gave.

My guess the difference lies in the values used by the OEM...the values on the data sheet may not be the same as you used.

Their value and your value should be pretty close.

I would contact them and verify the values they used...they may have screwed up one your operating conditions (which NEVER happens--yea, right)