Manufacturers will stipulate the setting of new expansion valves. They are generally set to maintain a superheat of 3.5 to 5o C or 7 to 9o F
Different evaporators require different settings of the valve to keep the coils correctly flooded. It is accepted that optimum operating conditions are achieved when thermostatic expansion valves are set to within the following:
Dry expansion, gravity coil: 5 to 7o C or 9 to 12o F
Forced air evaporator coil: 1.5 to 3o C or 3 to 5o F
It is evident that if a factory set valve with a 7o C setting is fitted to a forced air evaporator, too much superheating will take place. By adjusting the valve to within the accepted range, say 4o C less superheating will occur and the evaporator will be more efficient. Figure 19 shows two conditions of the refrigerant in a gravity coil at
different settings of the expansion valve.