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Effect Of Molecular Weight On Compressor Shaft Speed, Polytropic Head

centrifugal compressor natural gas process simulation molecular weight multistage compressor gas compressor polytropic head gas power

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#1 rha257

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Posted 15 November 2019 - 01:37 PM

Hello All,

 

I am currently working on a fixed speed multistage gas centrifugal compressor package (including motor, transformer, lube oil system, DGS, etc.) I have a couple of understandings that I would like to get feedback on from the chemical engineers with experience in such compressors:

 

For a fixed speed multistage compressor, with fixed inlet temperature, pressure and compression ratio:

 

1. If the Molecular weight of the gas increased, the gas becomes easier to compressor since it has a higher density, which means that the polytropic head required to achieve the compression ratio becomes lower. Since the mass flow rate is higher (because the of the higher density), the BHP required by the compressor increases. On a separate note, the compressor required speed (i.e. RPM) is lower since the gas requires a lower polytropic head to achieve the required compression ratio. 

 

2. If the Molecular weight of the gas decreased, the gas becomes harder to compressor since it has a lower density, which means that the polytropic head required to achieve the compression ratio becomes higher. Since the mass flow rate is lower (because the of the lower density), the BHP required by the compressor decreases. On a separate note, the compressor required speed (i.e. RPM) is higher since the gas requires a higher polytropic head to achieve the required compression ratio.

 

Is my basic understanding correct? (I know that there are other factors like impeller tip speed and gas relative velocity that can play a part but I just want feedback on my basic understanding)

 

Also, does anyone have a spreadsheet that simulates the outlet parameters of a centrifugal based on the polytropic process?

 

Many thanks.

 



#2 Bobby Strain

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Posted 15 November 2019 - 02:00 PM

You only need to look at the equations that describe behaviour of gases in a centrifugal compressor. Then the answer to your query is obvious. And, you don't appear to be a student. You can go to some of the vendors for online calculators. If you get a spreadsheet, you won't know if it is correct. So, you should start with the basics. It's pretty simple.

 

Bobby



#3 rha257

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Posted 15 November 2019 - 02:59 PM

You only need to look at the equations that describe behaviour of gases in a centrifugal compressor. Then the answer to your query is obvious. And, you don't appear to be a student. You can go to some of the vendors for online calculators. If you get a spreadsheet, you won't know if it is correct. So, you should start with the basics. It's pretty simple.

Bobby

Dear Mr. Bobby,

Thank you for your response. However, I have tried doing what you suggested and I am not getting a definite answer. If you have an answer that could help me out then I would appreciate if you can share it with me otherwise I will wait for someone who would be willing to help out.

Thank you

Edited by rha257, 15 November 2019 - 05:27 PM.


#4 breizh

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Posted 15 November 2019 - 11:58 PM

Hi ,

Consider the resources attached to support your work .

 

http://yrsp.elliott-..._perf/index.jsp

 

good luck

Breizh

Attached Files


Edited by breizh, 16 November 2019 - 12:07 AM.





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