Monday, April 11, 2011

Superheat - Target and Actual

Superheat is the temperature rise in the refrigerant after it boils in the evaporator.  The refrigerant leaves the throttle valve (fixed orifice, capillary tube, TXV) and enters the evaporator as a low pressure, low temperature liquid.  As air from the building is blown across the evaporator, the heat is taken out of the air and added to the refrigerant.  As heat is added to the liquid refrigerant the refrigerant boils and changes to a gas.  The temperature at which a substance (in this case a refrigerant) changes from a liquid to a gas is known as the boiling point or saturation temperature.  The change of state happens in the evaporator.  As the refrigerant continues out of the evaporator and onto the compressor heat is added to the refrigerant.  The added heat to the refrigerant between the boiling point and the compressor is the superheat.
Superheat is used for properly charging fixed orifice systems.  Superheat is important for two reasons:
  1. The first reason is to know if the evaporator is doing its job and the A/C system is running efficiently.  If there is no superheat, very little heat is being taken out of the return air and hence the air being supplied to the space is the same temperature as the air being returned to the evaporator.
  2. The second reason is to save the life of the compressor.  The compressor is designed to compress (add pressure) to gas refrigerant.  It is not powerful enough to compress liquid refrigerant and it will burn up the compressor.  It’s like pushing on a brick wall.  The compressor is also designed to be cooled by the refrigerant it compresses.  When the superheat is too high the compressor is working at higher than normal temperatures and can cause it to overheat.

To measure the superheat, first you place a thermocouple (there are pipe clamps specifically designed for this) on the suction line near the compressor and record the measured temperature.  Next attach a pressure gauge to the suction line service valve, measure and record the reading.  Using a pressure/temperature chart or pressure temperature calculator find the boiling point (saturation temperature) of the refrigerant at that specific pressure.  Subtract the saturation temperature from the temperature measure from the thermocouple and that is the total system superheat.
We now have the actual superheat but how do we know that is an acceptable superheat?  Using the target superheat table from California’s Title 24, which can be found at, we can look up the target superheat based on the indoor and outdoor conditions.
Important: Rules of thumb are out.  Between 8 and 12 degrees of superheat is out.  Sophisticated target superheats derived by the experts are in!
Measure and record the ambient air temperature going into the condenser.  Make sure to measure in the shade and get the average temperature going into the condenser.  Next measure the wet bulb temperature going into the evaporator.  Place the thermocouple in the center of the airflow in the duct near the evaporator.  If using a wet sock thermocouple, watch the temperature stabilize before recording it.  Use the outdoor ambient air temperature and the return air wet bulb to look up the target superheat on the chart.
Compare the actual superheat to the target superheat.  If the actual is greater than the target, add refrigerant.  If the actual is less than the target, remove refrigerant.  Adjust the charge until the actual superheat is within ±3 degrees Fahrenheit of the target superheat.  How much refrigerant to add or remove will be based on the size of the system and how far off the superheat is.  A good way to dial in the correct charge is by using a digital superheat tool such as Fieldpiece’s ASX14 accessory head, or SSX34 superheat subcooling standalone meter.  This way you can watch the superheat live rather than continually recalculating the superheat.

(From A/C Sytem Testing Basic By Russell Harju – Product Manager, Fieldpiece Instruments)

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