Superheat Overview

When technicians take an air conditioning course, one of the first things they learn is to use superheat to charge a fixed orifice air conditioning system.

Superheat is not hard to deal with, but the technician needs to take four good measurements.

To get the actual superheat, the technician measures suction line pressure and suction line temperature. When he reads suction line pressure, he reads the oF scale on his gage, not the PSIG scale. That’s the boiling point of that refrigerant at that pressure. To get actual superheat, subtract the suction line boiling point temperature from the measured temperature of the suction line. The Fieldpiece ASH3 makes it easier. It measures both suction line pressure and suction line temperature, and calculates and displays actual superheat directly. You don’t need to do any math or use any lookup tables.

To read get the required superheat from the most common A/C manufacturer’s superheat charts, the technician measures indoor “wet bulb” temperature and outdoor air temperature (“dry bulb”). Using these two temperatures the technician can look up the required superheat on most A/C manufacturers’ superheat charts. Required superheats can vary from 5 oF to over 45 oF depending on the conditions (indoor wet bulb and out door dry bulb). The higher the load, the higher the required superheat. The Fieldpiece ARH4 measures both indoor wet bulb and out door dry bulb, the two measurements needed to get the manufacturer’s required superheat.

The technician adds or subtracts refrigerant to decrease or increase the actual superheat to match the required superheat.

Superheat is the temperature difference between the boiling point of the refrigerant in the evaporator and the actual temperature of the refrigerant gas after the evaporator. It is the “extra” temperature (or temperature rise) the refrigerant picks up in the evaporator after it boils.

When charging the system, the technician adds as much refrigerant as he can. But if he adds too much (overcharge), he risks flooding the compressor with liquid refrigerant.

The biggest risk of flooding is under low load conditions: low outside temperatures and low indoor wet bulb temperatures. The refrigerant boils off late in the evaporator. To make sure the refrigerant is all boiled off before the end of the evaporator, the the A/C manufacturer’s required superheat chart directs the technician to stop adding refrigerant when the suction line temperature gets down to within a few degrees of the boiling point inside the evaporator. The “few degrees” is the superheat. At low load conditions, the superheat is often specified as five or six degrees. It’s a safety factor to make sure no liquid gets to the compressor.

At other load conditions, as determined by outdoor air and indoor wet bulb temperatures, the required superheat is given by a the superheat chart supplied by the A/C manufacturer. The higher the temperatures, the higher the load and the higher the required superheat.

Doing a superheat analysis is the best way to insure that an air conditioner has the proper charge.

Fieldpiece Instruments, Inc.
580 W. Central, Suite A
Brea, CA 92821
(714) 257-9060 FAX (714) 257-9069
fporders@fieldpiece.com