Vi Ratio screw compressors

Hi,

Many fundamentals books about refrigeration systems say about screw compressors that the pressure ratio is = Vi^k...so far so good....but they say that k is the isentropic exponent.

The above statement is wrong. Even for piston compressor I don't think correct...and for screw compressors is totally wrong because the oil will cool a good part of the compressed gas...so my opinion is that the politropic exponent will be between an isothermal process and the isentropic.

Because the gas is been cooling during the compressing process the final pressure at the discharge port will be lower than if it will been cooled. So the pressure ratio for the same Vi ratio is less than the obtained for the isentropic compressing process.


Awaiting your comments

Correct me if I'm wrong, but in absence of an oil cooler, the heat remains within the system. The oil cools the discharge, but superheats the suction. The net result is zero.

Hi Sandro

NoNIckName is 100% right ..... without an oil cooler the compressor/oil heat is transfered to the refrigerant increasing suction/discharge superheat

R's chillerman

NoNickName. I don't agree at all. I think the effect of the oil cooler will have much more impact on lower the discharged superheated gas » less specific volume at the discharged gas.

Chillerman. I can't see why the suction superheat will increase without oil cooler.

Hi Sandro

the compressors heat is transferred to the oil

the oil cooler rejects the oils heat

if no oil cooler, then the heat generated by the compressor stays in the compressor/oil

and this heat is transfered to the suction vapour as it enters the compressor

the suction superheat is added to the heat of compression to give your discharge superheat

so yes you are correct an oil cooler lowers discharge superheat by lowering compressors internal suction superheat

R's chillerman

Quote:

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Originally Posted by Sandro Baptista

NoNickName. I don't agree at all. I think the effect of the oil cooler will have much more impact

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Chillerman got it right.