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  1. #1
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    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


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  2. #2
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    Re: Vi Ratio screw compressors

    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.

  3. #3
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    Re: Vi Ratio screw compressors

    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
    If the World did not Suck, We would all fall off !

  4. #4
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    Re: Vi Ratio screw compressors

    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.
    To make progress is never good enough, I want to do better and better and better

  5. #5
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    Re: Vi Ratio screw compressors

    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
    If the World did not Suck, We would all fall off !

  6. #6
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    Re: Vi Ratio screw compressors

    Quote Originally Posted by Sandro Baptista View Post
    NoNickName. I don't agree at all. I think the effect of the oil cooler will have much more impact
    Chillerman got it right.

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