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besides being a user of Prode Properties I work in a factory where there are several centrifugal compressors and I am interested to the different topics concerning design and rating of these complex machines,
I post a question about Polytropic Compression Of Wet Gas (Efficiency Evaluation) with Prode Properties and the answer given by PaoloPemi in Cheresources, it includes several examples which I think could be of interest for the readers

question
recently we changed the inlet spec's of our (two stages) centrifugal compressor (natural gas), now we have about 1% fraction of condensate (value estimated with both Peng Robinson or Soave thermo packages) at inlet,
the manufacturer said it's acceptable and we are reevaluating efficiency (and power consumption),
My questions concern the evaluation of polytropic efficiency in case of wet gas,
we have measured pressures, temperatures etc. and calculated the polytropic efficiency with Prode Properties (selecting the procedure for polytropic solution with phase equilibria), at the new operating condition the value is close to the original number (0.76) for dry gas (that could be Ok since we have only 1% condensate on inlet line and dry gas at outlet),

1) to improve the accuracy, should I attempt (and why) to evaluate directly (i.e. measure) the amount of condensate on inlet line?
Actually the liquid fraction is calculated by Prode with Peng Robinson model.

2) the outlet temperature estimated with the correlation for ideal gas tout = tin*(pout/pin)^(k*eff) is very different from the value calculated with Prode Properties (Prode calculates lower temperatures), I suppose it is Ok considering we have a pressure ratio of about 4 and some condensate to evaporate.

answer

I am not aware of standards discussing the calculation procedure to estimate the polytropic efficiency of a compressor under wet gas (gas + liquid) conditions, ...

Tags: centrifugal, compressor, polytropic, prode, properties

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Oil And Gas Process

you can easily solve the polytropic compressor with Prode Properties,
there is a specific unit in extended version (please contact Prode at 'www.prode.com'),
that unit solves the model with the desired level of accuracy (it's a iterative procedure),
however you can obtain equivalent (or very close) results by modeling the polytropic compressor as a series of isentropic units with predetermined polytropic efficiency,
this is very simple with Prode Properties, for additional information see

'http://www.prode.com/docs/pppman.pdf'

-decide the number of steps (I suggest 5 steps) and calculate the dp as (Pout-Pin) / nr. of steps
-start from compressor inlet temperature and pressure
-at these conditions calculate cp with the method =StrGCp and cv with the method =StrGCv , calculate k = cp/cv
-calculate n (polytropic exponent) as n = polytropic efficiency * (k-1)/k
-calculate P1=Pin+dp and T1 = Tin*(P1/pin)^(n-1)/n
-at these conditions (P1,t1) calculate cp with the method =StrGCp and cv with the method =StrGCv , calculate k = cp/cv
-proceed until to complete the steps
-....
-once you have calculated the final (outlet) temperature you can calculate the enthalpy at inlet conditions (with Prode Properties) as H1 = StrH() and outlet conditions H2 = StrH() the difference being the enthalpy rise in the gas, you can then calculate the polytropic head from polytropic efficiency[/QUOTE]

Tags: centrifugal, compressor, polytropic, prode, properties, simulation

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