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Does it matter when measuring superheat , if you are lower or higher, as long as you within.

For example: Measured low side temp of pipe is 58*F, should manifold gauge read, 79psig or 117psig on a R-22 system with a target of 10*F superheat.?


  • Wayne_16
    Wayne_16 Member Posts: 130
    use the R-22 charging chart to answer the question

    R-22 At 76.9 psi(g) gauge is equal to 75 degrees F., at 84.1 psig is equal to 80 F.  Your 79 psig is close to the middle of the 2 numbers so lets say 77-78 degrees.

    If you measured 58 degrees on the low side or your suction line, then the superheat is 78 - 58 for a difference of 20 degrees. 

    What pressure is the 117 for?  High side maybe.

    If it is then there may be a problem with the compressors capacity to pump the refrigerant. 

    There is an acceptable range, and "it depends" upon the type of system, capillary tube, fixed orifice, or expansion valve.  If you have a true expansion valve ac system then 12-15 degrees of superheat would be in the range, depending upon manufacturer of equipment. A fixed orifice or capillary  tube will have a different value.

    Too fully identify the problem, we would need the system pressures, indoor and outdoor temperature, liquid line and suction line temperatures, and compressor amp draw.  State the problem or concern.


    Minnesota Wayne
  • Devan
    Devan Member Posts: 138

    no problem with any system wayne, but not sure how you come up with 78 *f at 79psig. check your gauge again.

    let me put it another way. if measured suction temp is 58*f , pt chart range 48*f or 68*f , both within targeted superheat of 10*f.

    target superheat = measured outside dry bulb and interior wet bulb, not manufacturers rating.

  • Techman
    Techman Member Posts: 2,144
    edited May 2010

    Hi Devan,on average there should be about 8-10* of superheat at outlet of the evap coil,that temp is above the equivelant temp off of the P/T chart for the given pressure. And then depending on a few factors  about another 10* rise  at the inlet to the compressor, for a split system. That 117 psi you mentioned ,is that for r410a? 
  • Wayne_16
    Wayne_16 Member Posts: 130
    Sorry readers

    I used a on line chart for my information.  The numbers more represent R-12 rather than R-22.  I should have known better too, in a hurry.

    Devan, to answer your question; superheat is additional heat added to the refrigerant vapor once all of the liquid refrigerant has boiled off.  So the measured temperature reading of the suction line will be higher than the measured suction gas pressure converted to temperature using a pressure temperature (pt) chart. subtract the converted temperature from the measured suction line temperature to arrive at the superheat value.

    Sub cooling is measured on the liquid line.  Liquid line pressure converted to temperature,  Measured liquid line temperature minus the liquid line converted temperature to arrive at sub-cooling.  Sub-cooling will be the amount of additional heat removed from the liquid refrigerant. 1 degree sub-cooling equal about 2% efficiency.

    Sorry for the inaccurate information posted previously;

    Minnesota Wayne
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