Final answer:
The primary function of the thermostatic expansion valve is to regulate the flow of refrigerant in a heat pump or air conditioning system.
Step-by-step explanation:
The primary function of the thermostatic expansion valve is to regulate the flow of refrigerant in a heat pump or air conditioning system. It controls the amount of liquid refrigerant that enters the evaporator coils, ensuring that the correct amount of refrigerant is supplied to the coils for efficient cooling or heating. The valve works by sensing the temperature of the refrigerant and adjusting the size of the valve opening accordingly, allowing for precise control of the system's operation.