From the positioning of its parts, it may seem like the principle of TEV operation is bulb pressure opposing spring pressure. But this is not the case. Evaporator pressure is a key factor in the operation of a thermostatic expansion valve. It is always equal to the pressure corresponding to refrigerant saturation temperature.
A thorough knowledge of the relationship among these three pressures is needed for complete understanding of valve operation.
An excerpt from the Technical Institute Manual #1 from the Refrigeration Service Engineers Society.
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