Polytropic Head
Polytropic Head
(OP)
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
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





RE: Polytropic Head
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
RE: Polytropic Head
As per the OEM manual speed = 5813 RPM.
Mass flow rate = 21655 Kg/hr
No inlet conditions mentioned in the manual.
Regards,
RE: Polytropic Head
The compressor's operating curve provided by the vendor would help to answer your questions.
RE: Polytropic Head
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.
RE: Polytropic Head
RE: Polytropic Head
You are right, however here we are talking about OEM value that is 1.5 times greater than calculated Head...
Regards
Wimple
RE: Polytropic Head
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)