Expansion Valve: Description and Operation

MECHANISM




The expansion valve is attached to the evaporator inlet. It converts high pressure liquid refrigerant which comes from the liquid tank to misty, low pressure refrigerant, and delivers to the evaporator. Being at low pressure and low temperature, the liquid refrigerant evaporates in the evaporator removing heat from the compartment. It automatically controls the flow rate of refrigerant to obtain the necessary cooling ability required by the fluctuating heat load.

The expansion valve is composed of a sensing bulb with a capillary tube that senses the temperature at the evaporator outlet, an equalizer that transmits the pressure of the refrigerant in the evaporator to the diaphragm and other parts such as the spring, ball and diaphragm.

FUNCTION




The operation of the internal equalizer type is as shown.

The expansion valve opening is automatically controlled by the functions of three force: the pressure of the temperature-sensing bulb (P(1)) containing a liquid or vapor, the pressure at the expansion valve outlet (P(2)), and the force of the spring (F).

Temperature-sensing bulb pressure (P(1)) ...... Forces the diaphragm downward (opens the valve).

Pressure at the expansion valve outlet (P(2)) ...... Forces the diaphragm upward (closes the valve).

Force of the spring (F) ...... Forces the ball upward (closes the valve).

A: Condition of the valve with the compressor in off.

Since the temperature around the evaporator is constant, the pressure in the evaporator is equal to that of the temperature-sensing bulb. Therefore, the force of the spring is greater than these pressures and the pressures are in the following condition:
P(1) = P(2)
P(1) < P(2) + F

That means, the valve is closed with the ball pushed upward.

B: Operation of the valve with the sensing bulb at low temperatures (When the temperature is low at evaporator outlet)

As the temperature of the compartment decreases; the heat load applied to the evaporator becomes less, and the temperatures at the evaporator outlet, and the sensing bulb drop. Accordingly, the valve tends to close, reducing the amount of refrigerant.

P(1) < P(2) + F

The amount of refrigerant is reduced.

C: Operation of valve with the sensing bulb at high temperatures
(When the temperature is high at evaporator outlet)

As the temperature of the compartment rises, the heat load applied to the evaporator becomes greater, and the temperatures at the evaporator outlet, and the sensing bulb increase. Therefore, the valve tends to open, allowing more refrigerant to flow.

P(1) > P(2) + F

The amount of refrigerant is increased.