Finding the correct superheat is not enough. Hvacr mechanics and technicians must be aware of the proper superheat on any given type of equipment. Generally, tables that give approximate superheat temperatures are referring to superheat readings that are taken when the equipment is at the design conditioned load or space temperature.
Under normal operating conditions, superheat measurements that are taken when the box is hot are high and don't give an accurate picture of what's happening inside the coil. Although manufacturers differ slightly on what they consider "correct" superheat, there are some general rules a technician can use.
For air-conditioning applications, the superheat at the coil should be between 10° F and 15° F depending on the ambient temperature and load conditions. For medium-temperature applications, 6° F to 10° F superheat is normal. On low-temperature boxes, 4° F to 8° F is generally recommended. Remember, these figures are meant to be used only as a guide and the manufacturers' recommended superheat settings should be used at all times when possible.
You may wonder why this is important? Having the correct superheat at the evaporator is important. Superheat is a technician's window to how the evaporator is performing. Too much superheat (see Figure 4) means that the evaporator is not receiving enough refrigerant and, therefore, its capacity is diminished.
Too little superheat (see Figure 5) means that the evaporator is being over-fed with refrigerant. When the superheat is correct we know that the proper amount of refrigerant is inside the evaporator and the coil is operating at its maximum capacity.
Superheat readings taken at the compressor allow the technician to determine if liquid refrigerant is flooding back to the compressor. The temperature you found in the evaporator is the saturated temperature of the refrigerant being used at a certain pressure.
In our example, the pressure is 21 psig and the saturation temperature is 20° F. This is the temperature at which the refrigerant is "boiling" or evaporating, but you have to remember that some liquid is still present. To ensure that the remaining liquid will boil off, additional heat above the saturation temperature must be added to the refrigerant, thus the term superheat.
So, if there is superheat at the compressor, we know that vapor is entering the compressor. However, it's not enough to know that vapor is entering the compressor; equally important is the temperature of the vapor.
Remember, reciprocating compressors use suction gas as a method of cooling the motor windings. If the suction gas is too hot, the temperature of the windings also will be too hot. Proper insulating of the suction line and correct sizing of heat exchangers, along with correct evaporator superheat, will assist in achieving the correct return gas temperature. Most compressor manufacturers recommend return gas temperatures to the compressor no higher than 65° F or 70° F to guarantee proper cooling of the motor windings.