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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. This article will take a closer look at a common type of valve called the automatic expansion valves. This guide will help you understand what is automatic expansion valve and its function. Read on this new blog in Linquip to find out more.
What is an Automatic Expansion Valve?
Automatic expansion valves or constant pressure expansion valves are a type of valve used to regulate a refrigerant’s pressure in a cooling or refrigeration unit. It helps meter liquid refrigerant to the evaporator, ensuring that the pressure is where it needs to be to achieve the state change from liquid to gas. The automatic expansion valve maintains constant pressure inside the evaporator no matter what the load inside the evaporator is.
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Automatic Expansion Valve Construction
The automatic expansion valve consists of the metallic body inside which is the metallic diaphragm or bellow. On the upper side of the diaphragm is the spring which is under pressure and its pressure is controlled by the adjusting screw.
Below the diaphragm, there is a thin plate or seat that has a small opening. The opening in the seat is controlled by the needle or stem connected to the diaphragm. As the diaphragm moves down the needle also moves down thus opening the valve. The seat and the needle form the orifice for the constant pressure valve.
There are also two openings in the valve. From one side, the refrigerant from condenser enters the constant pressure expansion valve and from the other side, the refrigerant leaves the valve to enter the evaporator.
The constructional details of the automatic expansion valve are shown in the figure below.
Automatic Expansion Valve Working Principle
- In the automatic expansion valve, the spring pressure along with the atmospheric pressure acts on the diaphragm and pushes it down. As a result, the needle also tends to move down away from the seat leading to the opening of the valve.
- The evaporator pressure pushes the diaphragm in an upward direction. Due to this, the needle tends to move in an upward direction towards the seat to close the valve.
- In the normal running condition of the plant, the valve maintains equilibrium between the evaporator pressure and the spring pressure and maintains a certain opening of the valve to allow the flow of refrigerant through it.
- The automatic expansion valve maintains the pressure inside the evaporator constant and automatically as per the setting of the spring pressure.
- When the refrigeration plant with the constant pressure valve is stopped, a certain amount of the refrigerant remains inside the evaporator and creates pressure inside the evaporator. This pressure acts against the diaphragm and keeps the valve closed. When the refrigeration plant is started again, the evaporator pressure reduces and the constant pressure expansion valve opens again.
Automatic Expansion Valve Features
- The automatic expansion valve Operates with prefixed constant pressure in the evaporator.
- It only has the bellows which is pressure operated.
- This is the only type that can be used in any machine with any refrigerant where there is a constant cooling requirement.
- When cooling decreases, pressure in the evaporator decreases. But as it has to maintain constant pressure in the evaporator, it will open more and supply more liquid refrigerant to the evaporator. Finally, it will result in sending liquid refrigerant to the compressor. The compressor will be broken as all liquids are incompressible.
- When cooling increases, pressure in the evaporator increases. But as it has to maintain constant pressure in the evaporator, the valve will open less and supply less liquid refrigerant to the evaporator. More heat is not picked by the less flow of refrigerant and hence there will be an unbalance.
- Its construction is simple.
- It is used in small cooling plants.
- It is cheap.
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Automatic Expansion Valve Advantages
The automatic expansion valve provides constant pressure. This makes it easier to adjust once it has been installed when compared to other valve options. The consistent pressure also makes the compressor more efficient because it reduces the surface area and use of lower-capacity compressors.
Automatic expansion valves are also better at preventing coil freeze because it maintains a higher suction pressure at low load conditions. They also make superheat adjustments easier because the pressure the valve operates at can be adjusted after installation.
Automatic Expansion Valve Disadvantages
The main aim of the automatic expansion valve is to maintain constant pressure inside the evaporator no matter what the refrigeration load inside the evaporator is. When the load on the evaporator is high it won’t allow more flow of the refrigerant as per the requirement. Similarly, when the load is lesser, it won’t allow a reduction of the flow of the refrigerant as per the requirement. Thus the automatic expansion valve does not allow the control of the flow of the refrigerant through it which is very important for the efficient operation of the refrigeration plant.
Automatic Expansion Valve Application
The automatic expansion valves are used wherever a constant temperature is required, for instance, milk chilling units and water coolers.
They are also used in home freezers and small commercial refrigeration systems where hermetic compressors are used.
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