Expansion Valves

Expansion valves are devices used to control the refrigerant flow in a refrigeration system. They help to facilitate the change of higher pressure of liquid refrigerant in the condensing unit to lower pressure gas refrigerant in the evaporator.

The term "low side" is used to indicate the part of the system that operates under low pressure, in this case the evaporator. The "high side" is used to indicate the part of the system that operates under high pressure, in this case the condenser.

Types of Expansion Valves

There are basically four types of valves that are in used. These valves are also refer to as metering devices.

  • Automatic Exp. Valves
  • Thermostatic Exp. Valves
  • Capillary Tubes
  • Float Valves

Automatic Expansion Valve regulates the flow of refrigerant from the liquid line to the evaporator by using a pressure-actuated diaphragm. It maintains a constant pressure in the evaporator.

The setback is that it is not efficient if the load fluctuates hence this type is not suitable for use in air conditioning as the load fluctuates a lot during its operation.

Thermostatic Expansion Valve uses a valve mechanism to control the flow of liquid refrigerant into the evaporator coil. The flow is controlled by the pressure in the evaporator.

This type of metering device is able to operate well when the load fluctuates and hence is suitable for use in air conditioning system. When the evaporator warms, the valve provides a higher flow rate amd when it cools, it reduces the flow rate.

It is also commonly refer to as TXV, TEV or TX valve. There is a sensing bulb which detects the temperature of the coil and is usually located at a higher temperature within the evaporator.

The bulb must be clamped firmly to the coil to ensure proper sensing. When the temperature of the evaporator increases due to the demand for cooling, the pressure in the bulb will also increase hence pushing the spring to open the valve.

Similarly, when the temperature of the evaporator reduces due to a lack of demand for cooling, the pressure in the bulb will drop hence causing the spring to close the valve.

Capillary Tube is a tube with small internal diameter and could be coiled for part of its length. It is installed to the suction line. A filter-drier is sometimes fitted before the tube to remove dirt or moisture from the refrigerant.

This device is simple, does not have any moving part and lasts longer. In order to use this device, the amount of refrigerant in the system must be properly calibrated at factory level.

Due to its lower cost compared to TXV, this metering device is used in units that are produced in large quantity such as room or window air conditioners.

Depending on the capacity design of the system, the capillary tube internal diameter that is commonly used range from 0.031" to 0.065" and the outer diameter from 0.083" to 0.130".

Float Valve is actuated by a float that is immersed in the liquid refrigerant. Both low-side float and high side-float are used to control the flow of liquid refrigerant.

The low-side float helps to maintain a constant level of liquid refrigerant in the evaporator. It opens when there is no liquid in the evap. and closes when there is liquid in the evap.

The high-side float is located at the high pressure side of the system and maintain a constant level of refrigerant in the condenser. When the compressor operates, the condensed refrigerant flows to the float chamber and opens the valve.

This causes the refrigerant to flow into the evaporator where it is stored. As the liquid level falls in the float chamber, the valve opening will close hence preventing the liquid from flowing to the evap.

Related Link 

Filter Drier
Find out the functions of the suction line and liquid line filter driers that are used in bigger HVAC units. 

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