What is an expansion valve and what are its types

What is expansion valve and what does it do

The expansion valves are used to decrease or change the pressure in a liquid, expansion valves are used in the refrigeration system. The function of an expansion valve in a refrigeration system is to reduce or remove the pressure in liquid refrigerant. The expansion will take place in liquid because the pressure is removed in certain conditions the liquid would change its state into a vapor. In the expansion valve, the high-pressure liquid that enters the valve would be warm because of its high pressure but when the liquid leaves the valve it won’t be warm because the pressure is reduced. The pressure is reduced by the valve orifice and it won’t reduce the heat. So expansion valves can be used to reduce the pressure and because of this throttling would occur.

What is a hand expansion valve

These valves are also known as throttling valves, this type of expansion valve consists of handwheel, valve seat and valve body. The handwheel in this valve is actuated so that it can vary the opening area of the valve seat to adjust the frictional resistance to the refrigerant flow. The hand expansion valve won’t be affected by the changes in the system.

What are the functions of expansion valves

Expansion valves are the major component of the refrigeration system, these valves would reduce the pressure and temperature of the refrigerant that comes from the condenser according to the system requirements. The flow of the refrigerant would be regulated according to the load on the evaporator.

How to select expansion valve

  • Refrigerant must be known
  • Evaporator capacity
  • Evaporating pressure
  • Condensing pressure
  • Sub-cooling
  • Pressure drop across the valve
  • Internal or external pressure equalization

What are the types of expansion valves

Expansion valves can be divided into two categories

  • Variable restriction type
  • Constant restriction type

Variable restriction type

In this type of expansion valve, the flow opening area would keep on varying according to the type of control. The automatic expansion valve, thermostatic expansion valve, and float valves are examples for variable restriction type.

Constant restriction type

Capillary tube suits in this category which is merely a long tube with a narrow diameter bore.

Automatic expansion valves

These valves use a sensor to meter the refrigerant to the evaporator. This device would maintain constant pressure in the evaporator. These valves are capable to maintain constant pressure and constant temperature in the evaporator. In this valve, the adjustment spring is usually set in a way that during the off-cycle the valve will be fully closed. This valve would operate according to the pressure changes in the evaporator due to an increase or decrease in load. Two opposing force controls the opening of the valve in the seat, they are the tension in the spring and the pressure in evaporator acting on the diaphragm. If we adjust the spring for the desired evaporator pressure and given load, then the valve would operate automatically with changing load conditions in the evaporator. These valves are mostly used when the constant temperature is needed

What are the applications and advantages of automatic expansion valve

  • These valves are cheaper
  • It can be used where the constant and continuous load is maintained in evaporator
  • Used in milk chilling units
  • Water coolers
  • Air conditioning system

Thermostatic expansion valve

This valve can be considered as a throttling device that works automatically. It would maintain proper liquid flow according to the load on the evaporator. It can be used for any kind of dry expansion. These valves are operated automatically and have high efficiency. Due to these features, these valves are widely used. These valves are capable to keep the evaporator active. It can reduce the pressure of the liquid from condenser pressure to evaporator pressure. These valves can operate at varying pressure resulting from varying temperatures. The thermostatic expansion valve is build up around a thermostatic element separated from the valve body by a diaphragm.

The working of the thermostatic expansion valve is mostly dependent on pressure acting in the valve. The operation takes place with three pressure that acts on the valve. The first pressure can be considered as A that acts inside the power element above the diaphragm, because of this pressure the diaphragm will move down and causes the valve to open. When the evaporator temperature has increased the gas in the bulb would expand and because of this, the gas pressure inside the power element increases this would cause the needle to move downward and the valve would open. The pressure that acts on the lower side of the diaphragm can be considered as B. This pressure is because of the refrigerant pressure inside the evaporator because of this the diaphragm would move upwards and close the valve. The spring pressure can be considered as C it would close the opening of the valve. The pressure B and C would act against A. So the pressure A must be greater than the combined pressure of B and C, in order to open the valve.

Float valve

This type of expansion valves are used in the large capacity refrigeration system and they are also used in flooded evaporators. This valve would open or close the valve according to the liquid level sensed by the float. The major function of a float valve is to stop the flow of water from an inlet supply line. If the water level rises then the float would rise too and if the water level reaches a specific level, the one-way valve would be activated and shut off the valve entry. Float valves are of two types they are low side float valve and high side float valve.

** Low-pressure float valve**

These valves can maintain a constant level of liquid in the evaporator, and it is done by regulating the flow of liquid refrigerant into that unit according to the rate at which the supply of liquid is being depleted by the vaporization. This valve would maintain the liquid refrigerant level in the evaporator according to the desired level, in all conditions without regarding the evaporator temperature and pressure.

** High-pressure float valves**

In this type of float valve, the flow of the liquid is regulated in the evaporator according to the rate at which the liquid is being vaporized. This valve would be installed on the high-pressure side of the system and controls the amount of liquid in the evaporator.

Capillary tubes

This device is mostly used as an expansion device in small vapor compression refrigeration systems. Such as household refrigerators and room air conditioners. In this system, the capillary tube is wound into coils for direct expansion. This device is very simple and it doesn’t have any moving parts and it is of low cost.

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What is air conditioner expansion valve

In an air conditioner, an expansion valve is an important component. The expansion valve in the air conditioner is the divided point between the low side and high side of the air conditioner unit. These valves are also called meter devices and they are located in indoor units with the evaporator coil. The AC expansion valve would control the amount of refrigerant flow to the evaporator coil. The AC expansion valves are of three types they are automatic expansion valve, capillary tubes, and thermostatic expansion valve.

What is an electronic expansion valve and how does it work

The electronic expansion valve can be considered as the improved version of the thermostatic expansion valve. These valves would allow the refrigeration system to operate very effectively. This device would bring down the condensing pressure to evaporating pressure and it also controls the refrigerant flow. An electronic expansion valve consists of a pulse motor that is controlled by a microprocessor. A needle for the opening and closing of the orifice. Spring is used for the backing force for the needle. The temperature sensor in this valve would detect the temperature and sends the data to the microprocessor and then it would calculate the flow of refrigerant and according to that, the valve opening is regulated.

How to install expansion valves

The installation of the expansion valve must be done in the liquid line, ahead of the evaporator, with its bulb fastened to the suction line as close to the evaporator as possible. The bulb must not be located at the bottom of the suction line because there is a possibility that the oil laying in the bottom of the pipe can cause false signals.