Polytropic efficiency for two compressors in parallel
Polytropic efficiency for two compressors in parallel
(OP)
I am looking for calculating the overall polytropic efficiency for of two compressors in parallel.
I have the performance map from the supplier for both compressors with the polytropic head depending on the flow rate; the speed and the efficiency are also represented.
But for two different speeds with the compressor running in parallel, I wonder how calculated the overall efficiency if I have for example 78% for one compressor and 82% for second one. What is the overall efficiency?
regards,
Rafcu
I have the performance map from the supplier for both compressors with the polytropic head depending on the flow rate; the speed and the efficiency are also represented.
But for two different speeds with the compressor running in parallel, I wonder how calculated the overall efficiency if I have for example 78% for one compressor and 82% for second one. What is the overall efficiency?
regards,
Rafcu





RE: Polytropic efficiency for two compressors in parallel
RE: Polytropic efficiency for two compressors in parallel
Nowhere does this thread mention what type of compressor(s) are being discussed. This is not the first engineer who assumes that every compressor in the world is a centrifugal. It just so happens that Polytropic efficiency is normally only attributed to centrifugals - but is everyone supposed to know that?
This is analogous to giving temperature readings as "degrees" without mentioning anything about which scale. Or how about discussing automobile engines without revealing that the engine in question is not a "clunker", but really a Mazda rotary?
RE: Polytropic efficiency for two compressors in parallel
How obtain the new efficiency to the group, knowing that I have just the curve and the table associated no polynomial models are available from the constructor.
Thanks,
RE: Polytropic efficiency for two compressors in parallel
if A compressor operates at X% and B compressor operates at X +/- 5%, i know of no known mathematical formula to determine "overall efficiency". the results for efficiency simply state how well the compressor is performing.
i would further ask you for the purpose/justification in wanting "overall efficiency". perhaps an explanation may be helpful . . .
now, if you want both centrifugal compressors operating at same efficiency, then that is a control system matter and can be address/handled as such.
goodl uck!
-pmover
RE: Polytropic efficiency for two compressors in parallel
RE: Polytropic efficiency for two compressors in parallel
efficiency for centrifugal compressor is defined as a
e=Wp/(Hs-Hd), where Wp is "net reversible process mechanical energy input"; Hs and Hp - enthalpy on the suction and discharge correspondingly. That's why I agree with DickRussell that you can talk about "overall efficiency" only if there are a junctions between the suction (s) and discharge (d) lines, may be drams or something like that. Then you can determine the polytropic
efficiency between those points or, more formal, for the "path of constant efficiency" between s and d points. You need to calculate temperatures in these points, Ts and Td. Discharge temperatures for each compressor you can calculate using given compressor maps and then calculate the temperature of mix flow. Suction and discharge pressures for both compressors are probably equal. So, you know Ts, Ps, Td, and Pd. Now you can calculate the polytropic efficiency. Easy way for ideal gas is e=(k-1)/k * ln(Pd/Ps) / ln (Td/Ts). Hard way for real gas - use the eq. e= v* dP/dH, where v is specific volume; solve it iteratively to find the efficiency. Or you can use method from PTC-10.
I used the article:
Journal of Engineering for Power; Vol 84; January 1962; J.M. Schultz; "The Polytropic Analysis of Centrifugal Compressors"