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  1. #1
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    Post Air-conditioning superheat query



    I have two questions, firstly when measuring superheat on a refrigeration plant it is a lot simpler as you can normally get the saturation temp of the evaporator outlet, where as on an air conditioning system you can only take the suction temp at the condenser to determine the evaporator superheat.

    My question is, will the method of taking the evaporator outlet pipe temperature but using the suction line port just before the compressor to get the pressure/temp be accurate for determining the evaporator superheat.

    Following on from that question, is the total/compressor superheat just as good to use to determine whether a system is short or had too much refrigerant in it?

    Secondly, for r410a what would be the target superheat? I realise that is a vague questions and there are other factors to consider, but what would should it be roughly or how do I work this out?

    Id appreciate your help

    Thanks!



  2. #2
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    Re: Air-conditioning superheat query

    The whole reason that we want superheat is to ensure that no liquid is entering the compressor.
    3K of superheat is useful and any additional superheat is unuseful, i.e. it just adds to the compressor load.

    It doesn't matter what the refrigerant type is, superheat is technically the amount of additional heat added to the vapour after the saturation point.

    If you measure the suction line close to the compressor inlet you stand the chance of getting a false reading due to the compressor heat being transmitted up the pipework. Superheat should be measured close to the evaporator outlet so that your measurement does not include pipework heat gains.
    Last edited by frank; 03-09-2015 at 06:31 PM.

  3. #3
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    Re: Air-conditioning superheat query

    Cheers frank,

    On a standard air conditioning unit rather than refrigeration unit, they don't have a shrader port on or just after the unit so the only place to get a pressure reading is the condenser suction port?
    I'm sure I remember someone saying add about 2-5 psi and that would be about the right difference, although I'm not sure how that would work on vrvs vrfs etc.

    Thanks

  4. #4
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    Re: Air-conditioning superheat query

    You are quite right, so if the only place you can take your pressure is at the compressure, yoy have to deduct an amount of superheat temperature for the pipework gain to estimate what you are getting at the evaporator. Not ideal but I suppose the only alternative is to fit a schraeder on the suction near the indoor fan coil.

    You also have to take into account the length of the pipework when estimating the heat gain(superheat) in the pipework

  5. #5
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    Re: Air-conditioning superheat query

    [I am not an installing engineer, but I do oversee installs and have a question about checking the superheat.]

    Could one just use a surface contact thermometer (one of those that has a probe to press onto the target surface) to measure the temperature of suction pipe leaving the evaporator? Is that not accurate enough?

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