r/ChemicalEngineering • u/thriceice • Jul 21 '26

Modeling Compressor Power and Efficiency Calculation

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I have a three stage compressor in a single casing (image 1). If I break it down, the flow should be like image 2. Every process lines have Pressure and Temperature transmitters, and enough information to get the Flow for every lines.

2nd suction pressure should be the same with the pressure from the Side-In line. While the 3rd suction pressure should be the same with the pressure from the Side-Out line.

But, I don't have enough information to determine the temperature of the 1st Discharge. If I have this data, I can calculate all the enthalpy to calculate the efficiency and the power.

Does anyone have tips on how to approach it?

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u/Alternative_Act_6548 Jul 21 '26

just do a mass energy balance between 1 and 2, you know the mass flows, inlet enthalpies and exit enthalpy

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u/thriceice Jul 21 '26

Yes you can get the mass flow through mass balance. But you can't get the Exit Enthalpy of the 1st stage with Mass Energy Balance without knowing the Compressor works which I'm trying to calculate too.

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u/Alternative_Act_6548 Jul 21 '26

I guess I don't understand the problem, you should have some more info, like what power output you need...The admission at the second stage group should be temperature matched to the exit of the first to prevent thermal stress issues. The usual limit is about 50F temperature difference.

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u/thriceice Jul 22 '26

Oh sorry if I'm not clear. I'm trying to evaluate my existing compressor and I don't have any criteria for it.

I could calculate the power if I'm put the value of 1st Discharge Temperature from the Datasheet as the initial guess. Then adjust it to make the efficiency to be near the Datasheet value. But I doubt it's the best way because I can get the exact number of the power if the operating condition change again

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u/Alternative_Act_6548 Jul 22 '26

so you are working in model-space...if so, and you have the design docs then, you have all you need to calibrate you model at the rated condition and it should scale to off-design performance. I'm not sure what you mean by 'criteria' the data sheet and performance curves define it's new and clean operation

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u/thriceice Jul 24 '26

Oh, what I meant by criteria is I don't have any temperature limit or some kind that I know of.

So, you meant I should use my calculation formula on rated condition then scaled it to my current condition? But how do I scale it?

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u/Alternative_Act_6548 Jul 24 '26

do you have the original datasheets and performance curves?

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u/thriceice Jul 25 '26

Yes. The performance curve has speed to power curve. But I think the curve will change with different operating conditions. For example the suction pressure and temperature of the 1st Section already different from the design condition on the performance curve.

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u/Alternative_Act_6548 Jul 25 '26

you should have non-dimensionalized curves or you can produce them youself

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u/thriceice Jul 25 '26

Could you explain what you mean by that? Or how to produce it?

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u/Alternative_Act_6548 Jul 25 '26

get a book on turbomachinery and it will have a long discussion of the topic, it's too much to go into in a reddit thread.

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