The term " overheating", when used with a thermostatic expansion valve, refers to the temperature difference between the steam in the low side and the sensing bulb. The system is adapted for operation in the normal 10F (5.6C) overheating shown in Fig. 5-20. Increase of superheat, as a rule, to starve the evaporator. Starving evaporator means that only part of the evaporator is filled with liquid refrigerant. Fig. 5-21 illustrates the overheating of-15SF (8.3C). On this setting, the evaporator is said to be starved.
As evaporator temperature drops, the amount of overheating increases. The volume of overheating in the suction line is largely determined by the refrigerant control. Best overheating to the evaporator when the thermal bulb temperature change least in a running system. This parameter is called the Minimum Stable Signal (MSS), clause or a parameter.
For example, if the valve and the evaporator combination behaves as follows:
- 12F overheating, bulb temperature changes from 14F 10F.
- 10F overh
eating, bulb temperature changes from 11T 9F.
- 8F overheating, bulb temperature changes from 8.5F 7,5F,
- 6F overheating, bulb temperature varies from 8F 4F.
slightest change (the most stable condition) is 8SF overheating, because it gives the smallest variation bulb temperature.