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Oil-cooled Rotary screw compressor - volumetric efficiency by compression ratio?

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YungPlantEng

Chemical
Jan 19, 2022
82
Vendor documentation for an oil-cooled rotary screw compressor in compressed air service indicates multiple flowrate-pressure data points at "full loading".

Assuming a temperature change of 10 F between the two values we would achieve 11% more volumetric flowrate going from 490 cfm @ 100 psig to 444 cfm @ 125 psig. Is there any way to substantiate this flow improvement? Presumably since we're increasing our compression ratio we'd end up with higher heat losses but this can't really be accounted for with an oil-cooled system using our current instrumentation. I'm not even sure if using the discharge temperature to calculate SCFM since the system is continuously cooled throughout the compression.

Anyone have some pointers on this? We're planning to run at a higher pressure and regulating pressure down after our dry receivers for distribution to hopefully incur less low pressure events.
 
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The heat transfer of the compression and cooling process determines the final temperature of the gas, but once at that final temperature PV still must equal nZRT. The gas does not care what thermo process was used to get to its discharge conditions. Cooling load and Power will increase with the higher discharge conditions. Calculate the new initial discharge temperature at the higher pressure ratio, then calculate the heat removed by the cooling system and you get the resulting final gas discharge temperature. From discharge P and T conditions, you can determine volume and flow rate

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."
 
Wouldnt know why flow increases with increasing Pd.
Can you tell us why you are getting these low pressure dips? Is it from intermittent plant air users or from one or more instrument air users?
 
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