control valve

What is an expansion valve?

These valves are used to control the refrigerant flow in a refrigeration system. Expansion valves are used to decrease or remove the pressure from the refrigerant fluid so that it can change its state from fluid to liquid in an evaporator. The expansion valves are widely used for many industrial applications. So basically expansion valves help to change the higher pressure of the liquid refrigerant in the condensing unit to a low-pressure gas refrigerant in the evaporator. The term low side is used to indicate the part of the system that operates under low pressure and the term high side is to indicate the part of the system that operates under high pressure, and in this case, it will be the condenser.

How does an expansion valve work?

The high-pressure liquid refrigerant that enters the valve will be very warm. The expansion valve would detect this by feeling the liquid line at its connection to the expansion valve. The liquid refrigerant that leaves the expansion valve will be cold. The orifice in the valve does not remove the heat but it will reduce the pressure.

The heat molecules which is present in the liquid refrigerant are thus allowed to spread as the refrigerant moves out of the orifice. So under greatly reduced pressure, the liquid refrigerant will be really cold when it leaves the expansion valve and enters the evaporator. The pressure at the inlet-outlet of the expansion valves will closely approximate the gauge pressures at the inlet and the outlet of the compressor in most systems. The similarity of the pressures is created by the closeness of the components to each other.

What are the types of expansion valves?

Hand operated expansion valves

This valve is also known as throttle valves, this type of valve consists of a body, valve seat, and handwheel. The handwheel is used to change the opening area around the valve seat in order to adjust the frictional resistance to the refrigerant flow. The refrigerant flow through the valve is dependent upon the pressure differential across the valve and also the opening of the valve. This is the simplest type of expansion valve but it needs an operator to regulate the flow of the refrigerant to the evaporator manually. The major advantage of a hand-operated valve is that it is not responsive to the changes in the system load. The disadvantage of the hand-operated expansion valve is that it must be manually readjusted if the load on the system changes. So the use of the hand-operated valve is limited for systems that operate under constant loads.

Automatic expansion valves

This type of valve is also called a constant pressure expansion valve due to its ability to maintain constant evaporator pressure. Mostly this type of expansion valves is used with dry expansion evaporators where the load is constant. This valve consists of a needle valve and a seat metallic diaphragm or a bellow, spring, and an adjusting screw. The opening and closing of the valve are dependent upon the opposing forces which act on a diaphragm. The spring pressure and the atmospheric pressure acts on the top of the diaphragm and the evaporator pressure would act below the diaphragm. So when the compressor is running the valve would maintain an evaporator pressure in equilibrium with the spring pressure and the atmospheric pressure.

The spring pressure can be varied by adjusting the tension of the spring with the help of the spring adjusting screw. So if the spring is adjusted for the desired evaporator pressure then the valve would operate automatically to maintain a constant evaporator pressure by controlling the flow of refrigerant to the evaporator. So when the evaporator pressure falls the diaphragm would open the valve because of this more liquid will flow to the evaporator till the required pressure is reached. If the evaporator pressure rises the diaphragm would move upwards in order to reduce the valve opening.  

Thermostatic expansion valves

A thermostatic expansion valve is the most commonly used expansion device in commercial and industrial refrigeration systems. These valves are also known as a constant superheat valve because they can maintain a constant superheat of vapor refrigerant at the end of the evaporator coil by providing the proper flow of the refrigerant to the evaporator. The major components of this valve is a diaphragm, seat, needle valve, and also an adjusting screw

This device also has a feeler or a thermal bulb which is mounted on the suction line near the outlet line on the evaporator coil. The feeler bulb will also be filled with the refrigerant fluid partly. The opening and closing of the valve are depended upon the forces that act on the diaphragm. These forces are the spring pressure which acts on the diaphragm’s bottom, then the evaporator pressure which also acts on the diaphragm’s bottom, and the feeler bulb pressure which acts on the top of the diaphragm. The feeler bulb is installed in the suction line and because of this, it has the same temperature as the refrigerant at that point. If there is any change in the temperature of the refrigerant then it would change the pressure in the feeler bulb and this would be transmitted to the top of the diaphragm.

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