How to calculate compressor discharge temperature by hand?
How to calculate compressor discharge temperature by hand?
(OP)
Hi,
I came across a problem when calculating the compressor discharge temperature. When using the GPSA databook to calculate a single stage centrifugal compressor discharge temperature, the result is much higher than that we gained using the simulation software ProII or a compressor sizing program.
The formula in the GASP is:
T2=T1(P2/P1)^[(n-1)/n]
(n-1)/n=(k-1)/(k*Eta)
Eta - Polytropic efficiency
Who knows how to estimate the centrifugal compressor's discharge temperature? What is the actual formula most vendors use?
What is limitation of the formula in GASP databook?
Thanks.
I came across a problem when calculating the compressor discharge temperature. When using the GPSA databook to calculate a single stage centrifugal compressor discharge temperature, the result is much higher than that we gained using the simulation software ProII or a compressor sizing program.
The formula in the GASP is:
T2=T1(P2/P1)^[(n-1)/n]
(n-1)/n=(k-1)/(k*Eta)
Eta - Polytropic efficiency
Who knows how to estimate the centrifugal compressor's discharge temperature? What is the actual formula most vendors use?
What is limitation of the formula in GASP databook?
Thanks.





RE: How to calculate compressor discharge temperature by hand?
David
RE: How to calculate compressor discharge temperature by hand?
TD = (TS + 460!) * ((Ratio ^ CON - 1!) * C1P10 * (1.0 + .078 / (R - .793)) + 1.0) - 460
C1P10 = 1.10675
CON = (VN - 1!) / VN
VN = e^(.35 - .188 * SPGR)
SPGR = specific gravity of the mostly hydrocarbon gas.
Ratio = Pd/Ps pressure discharge/pressure suction in absolute
TD & TS are temperatures in degrees F
RE: How to calculate compressor discharge temperature by hand?
Where is your formula coming from? Can you please let me know the book or references?
RE: How to calculate compressor discharge temperature by hand?
RE: How to calculate compressor discharge temperature by hand?
If you post the info, I can double check the numbers.
Mark
RE: How to calculate compressor discharge temperature by hand?
In my application, the pressure ratio is 4.6. This means that the GPSA equation will not work precisely. The simulation software like ProII or Hysis and my compressor software give more accurate result.
I don't know other limitation of the GPSA equation. Please comments if you have.
Hippo, is your pressure ratio near 1.15?
Frank
RE: How to calculate compressor discharge temperature by hand?
Mark
RE: How to calculate compressor discharge temperature by hand?
What we tend to forget about is valve loss ends up in the equation for discharge temp. We try to simulate this loss as polytropic efficiency. Then when we do an anlysis of calculated versus observed we get excited when the calcs say 275 Degrees and the Observed is 280 degrees, this is less than 1% error (remember, use absolute temperatures).
I've attached some sample Td calcs from all sources posted here.
RE: How to calculate compressor discharge temperature by hand?
RE: How to calculate compressor discharge temperature by hand?
However, for centrifs it's a different case entirely. For lower flow higher ratio machines I'd use a polytropic efficiency on the order of 68-73%. The narrower the impellers, the lower the efficiency. The older the design (ie. 2D wheels) the lower the efficiency...that is, closer to the 68% number.
For higher flow, lower ratio machines, then I'd be looking at 76-82% polytropic ifficiency, the same qualifies applying as before re: impeller vintage and passage width.
Your best bet is to obtain the PTC-10 based performnace curves for the compressor, which should include a plot of efficiency vs speed and volumetric flow. And then subtract 3-4 percentage points for laby wear.
One well known manufacturer boasts polytropic efficiencies in the low 80's. However, these efficiencies fall off 5-7% in service to normal laby wear...they fall off further than you would expect an older design machine with lower initial efficiency to fall off. I'd recommend caution when using their as-new efficiencies for economic analysis.