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Head still intact and believe I understand most of it, but I REALLY need to take a A/C and refrigeration class at Vo-Tech!

I've been trying to understand that <A href="http://www.rawal.com/">Rawal Valve</A> for a <I>long time.</I>

Do I have this anywhere near correct?

It tries to vary the effective size of the expansion valve to maintain a constant evaporator temperature regardless of total (latent and sensible) heat of the air passing across the evaporator. In other words, it strives to maintain a constant rate of evaporator superheat regardless of load on the evaporator.

It does this by refusing to allow the evaporator to superheat beyond some point by forcing the condensing unit to de-superheat. Since this means that some of the low temp, low pressure vapor in the suction line to bypasses the compressor, the load on the the compressor is reduced.


  • Mike T., Swampeast MO
    Mike T., Swampeast MO Member Posts: 6,928

    Thanks for the great explanation of superheat in another thread Prof.

    Now can you explain de-superheating?

    Thanks and I hope my head doesn't explode from the explanation...
  • Eugene Silberstein
    Eugene Silberstein Member Posts: 1,380
    No Head Explosion...

    When refrigerant leaves the compressor, it is a high pressure, high temperature superheated vapor. Inorder for the refrigerant to condense, the refrigerant must be cooled from the compressor discharge temperature down to the condenser saturation temperature.

    For example, if the refrigerant leaves the compressor at a temperature of 180 degrees and the condenser saturation temperature is 110 degrees, the superheated refrigerant must cool from 180 degrees to 110 degrees before the condensing process can begin.

    The process of giving up superheat from the point where the refrigerant leaves the compressor to the point where the refrigerant begins to condense is referred to as desuperheated.


    If you are dealing with a hermetically sealed compressor, the outlet of the compressor is inside the shell of what we often refer to as the "compressor". The compressor, the motor, lubricating oil, etc are all enclosed in the shell. The "dome" or inside of the shell is filled with low pressure, low temperature vapor from the suction line. Now, as the refrigerant leaves the compressor, which is in the shell, the hot gas gives up heat to the suction gas in the shell before leaving the shell. So, the temperature of the gas that leaves the hermetic compressor is, in reality, already somewhat desuperheated.


    The note does not apply to semi hermetic compressors, as the discharge gas leaves the compressor almost immediately without coming on contact with the suction gas. This is one of the reasons why the discharge lines on semi-hermetic compressors is so much higher than those on hermetic compressors.

    Hope this helps.

  • carol_3
    carol_3 Member Posts: 397

    delighted to see you on this site, Eugene! And thanks for the explanation. I had promised to give you a call, lost your number, and just recently got it again from Dan's office. I'm still looking forward to your making all that refrig stuff easy for me!
  • Eugene Silberstein
    Eugene Silberstein Member Posts: 1,380
    Thanks Carol

    Give me a shout whenever you are free.

    My 5 minute guarantee: You will learn the operation of the basic vapor-compression refrigeration cycle in less than 5 minutes and be able to teach it to someone like you've being doing ot all of your life. How's that?
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