Constant Pressure Expansion Valve or Automatic Expansion Valve
The constant pressure expansion valve keeps the pressure inside the evaporator constant, hence the name. The constant pressure valve maintains constant pressure inside the evaporator no matter what the load inside the evaporator is. It does not allow the control of the flow of the refrigerant through it even when lesser or more refrigerant is required in the evaporator since the primary function of the constant pressure valve is to maintain constant pressure inside the evaporator.
The constant pressure valve is also called as the automatic expansion valve but this is the misnomer since it is not automatic for it does not allow automatic control of the flow of refrigerant through it. In the very earlier days only hand operated expansion valve or throttling valve was available, however since this valve allowed automatic constant pressure in the evaporator it was given the name automatic expansion valve.
Constant Pressure Expansion Valve
Construction of the Constant Pressure Valve
The constructional details of the constant pressure expansion valve are shown in the figure above. It comprises 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 thin plate or seat that has the 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 the seat and the needle form the orifice for the constant pressure valve.
There are also two opening in the valve. From one side the refrigerant from the condenser enters the constant pressure expansion valve and from the other side the refrigerant leaves the valve to enter the evaporator.
How the Constant Pressure Expansion Valve Works?
The spring above the diaphragm is under compression thus the spring pressure along with the atmospheric pressure acts on the diaphragm. Due to the pressure the diaphragm tends to move down due to which the needle also tends to move down away from the seat leading to the opening of the valve.
Below the diaphragm there is refrigerant at the evaporator pressure thus the evaporator pressure tends to move the diaphragm in the upward direction. Due to this the needle tends to move in the upward direction towards the seat to close the valve.
Thus the spring pressure and the evaporator pressure act against each other and whichever is greater would determine the position of the needle and the opening of the orifice of the valve. In the normal running condition of the plant the valve maintains equilibrium between the evaporator pressure and the spring pressure and maintains certain opening of the valve to allow the flow of refrigerant through it.
The tension of the spring can be adjusted as per the requirements by the adjusting screw. The constant pressure expansion valve maintains the pressure inside the evaporator constant and automatically as per the setting of the spring pressure. This means that the evaporator pressure can be varied by changing the position of the spring.
When the refrigeration plant with the constant pressure valve is stopped, certain amount of the refrigerant remains inside the evaporator and create 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.
Limitations of the Constant Pressure Expansion Valve
The main aim of the constant pressure expansion valve is to maintain constant pressure inside the evaporator no matter what the refrigeration load inside the evaporator is. When the load on 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 reduction of the flow of the refrigerant as per the requirement. Thus the constant pressure 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.
Sometimes the load on the compressor is very less in such cases the liquid refrigerant floods to the compressor. This can be very dangerous as the piston and cylinder of the compressor may break due to compression of the liquid.
By changing the position of the spring one can change the pressure inside the evaporator. One can change the position of the spring to allow for higher flow of the refrigerant though the valve, but in that case the compressor will work at high suction pressure or higher capacity even if the load is lower. This will lead to lots of wastage of the power. On the other hand if the spring pressure is adjusted to allow lesser flow of the refrigerant due to lesser evaporator load the compressor would not work at full capacity when required.
Thus the constant expansion valve actually puts the limitation on the capacity of the compressor and the whole refrigeration plant. It is not really automatic in the real sense since it cannot automatically adjust the flow of the refrigerant as per the requirement. Due to these limitations the constant pressure expansion valve is seldom used these days. In fact even the name automatic expansion valve is not correct for this valve; it was probably given to it since it maintains the pressure inside the evaporator constant automatically.
Reference and Images Courtesy
1) 1) Book: Basic Refrigeration and Air Conditioning by P. N. Ananthanarayanan, Second Edition, Tata Mc-Graw-Hill Publishing Company Limited
This post is part of the series: Throttling Devices, Expansion Valves in Refrigeration & Air Conditioning Systems
- Throttling Devices or the Expansion Valves Used in the Refrigeration and Air Conditioning Systems
- Capillary Tube for Refrigeration and Air Conditioning Systems
- Constant Pressure Expansion Valve or Automatic Expansion Valve
- Thermostatic Expansion Valve or TEV
- Float Valve used as the Throttling Device in Refrigeration Systems