Valves are parts that are used in systems to regulate the flow of fluids. They can be used to initiate, terminate, control, and reroute fluid flow within the system. The availability of various valve types with various operations aids in controlling and adjusting the flow to meet our needs. They are an essential part of the mechanical systems and are designed to perform crucial tasks.
In air conditioning and refrigeration systems, an automatic expansion valve (AEV) is a component that controls the flow of refrigerant into the evaporator. Throughout the refrigeration cycle, the automatic expansion valve ensures that the flow of refrigerant remains optimal and constant. AEVs are able to modulate or regulate the flow in response to changes in pressure and temperature without help from an outside source. This helps the refrigeration unit to operate effectively and efficiently, ensuring that the proper evaporator temperature is also maintained. They are made from materials that can withstand the system’s operating conditions and are compatible with the refrigerant. It includes materials like brass, stainless steel, and different polymers like Polytetrafluoroethylene and various elastomers. Some of the additional purposes of Automatic Expansion Valves include ensuring ideal heat flow, preventing the growth of ice and frost on the evaporator coils, and safeguarding the compressor from harm that might result from the unit’s operation.
Types of Automatic Expansion Valves
i) Self-acting expansion valve: This is the most basic system that consists of a diaphragm-spring arrangement for the control of fluid flow. The spring detects the fluid pressure and causes a corresponding deflection in the diaphragm.
ii) Electric Expansion Valve: It is the most commonly used and consists of an electric controlling unit that senses the external condition and provides feedback based on a pre-defined data set.
iii) Bi-Flow Expansion Valve: In this type of valve, the direction of flow could be reversed. They are commonly used in heat pumps.
Components of an Automatic Expansion Valve
1) Valve Body: The valve body acts as a housing for the automatic Expansion Valve, providing room for the internal components of the valve as well as maintaining the structure and integrity of the valve. They are usually made of materials that possess good corrosion resistance and high strength. The exact nature of the material is selected after considering what type of refrigerant is used and under what conditions the valves are made to operate.
2) Adjusting Screw: The adjusting screw is a mechanism within the valve body that controls the flow of refrigerant into the evaporator. The system’s operating conditions can be identified and the size of the hole or orifice can be changed as needed. The screw can be turned and the size of the orifice can be adjusted with high precision.
3) Controlling spring: The controlling spring is responsible for actions of force on the diaphragm which determines the degree of valve opening. The spring is subjected to tension and compression in order to achieve this. It also has the functions of maintaining optimal superheat and enabling adaptability to varying loads.
4) Diaphragm: The major function of the diaphragm is to modulate the flow of refrigerant by identifying and responding to changes in temperature or pressure. The diaphragm makes sure that the pressure is equalised at both sides of the valve to prevent abrupt behaviour. They are made using flexible and durable materials such as neoprene or nitrile rubber.
5) Strainer: The strainer carries out the function of filtering out solid particles, contaminants, and debris from the refrigerant before entering the valve. It protects the evaporator, compressor, and other parts of the valve ensuring high efficiency and reliability. The strainer also assists in the cleaning process of the valve and helps in the maintenance process. The strainer can be removed, cleaned, and put back in if it is clogged.
6) Sensing element: It detects the pressure of the refrigerant in the evaporator or the outlet of the Automatic Expansion Valve and converts the pressure measurement into an electrical or mechanical signal that can be interpreted by the control system. The pressure variation information is transmitted in real-time which helps in maintaining the stability of the valve.
Working of an Automatic Expansion Valve
The sensing element in the AEV detects the pressure at the outlet of the evaporator and provides feedback on the operating conditions, mainly the evaporator pressure. There is a desired setpoint of pressure for superheat that is fed into the control system beforehand. The feedback of evaporator pressure received from the sensing element is compared with the desired setpoint. The control system makes necessary adjustments in the form of valve openings to reach the desired level of superheat. The opening of the valve is performed as a combined effort of the diaphragm, controlling spring, and adjustable screw. They are together called the valve mechanism. And based on the input provided by the control system, the valve mechanism performs the task of valve opening. The controlling spring can expand as well as retract. When the controlling spring expands, it pushes the diaphragm down in turn turning the adjustable screw for the opening of the valve and entry of refrigerant. The adjustable screw regulates the amount of flow of refrigerant within the valve in such a way that the superheat is maintained.
Superheat is the temperature difference between the refrigerant vapour and its saturation temperature. An AEV also performs a function known as system load adaptability where system conditions including system load and temperature besides pressure are constantly monitored with the help of the sensing element ensuring optimal flow control and superheat regulation. The stable operation of an AEV is made possible by creating an effective feedback loop involving the sensing element, control system, and valve mechanism. Overall, the working of an AEV involves the functions of adapting to varying system conditions, maintaining desired superheat, and ensuring precise flow control. These functions help in preventing undercooling, and overcooling and improve the efficiency and reliability of the cooling or refrigeration system.
Advantages of Automatic Expansion Valve:
i) The number of moving components in an Automatic Expansion Valve is less as compared to normal valves. This reduces the need for maintenance as well as prolongs the life of the device.
ii) Automatic Expansion Valves can give a quick and precise response to varying environmental conditions. Since the valve adjusts itself based on inputs from the feedback mechanism, great working efficiency is achieved.
iii) Automatic Expansion Valves work well even with a minimum amount of refrigerant. This makes the overall process optimal and cost-effective.
iv) The Automatic Expansion Valves have a compact size and design. This makes the installation easier.
Disadvantages of Automatic Expansion Valve:
I) Automatic Expansion Valves are more complex as compared to other types of expansion valves as they require additional components like sensors, controllers, etc.
ii) They are more expensive.
iii) The evaporator pressure decreases when the refrigeration load decrease due to which the valve opens fully which attempts to bring the evaporator pressure up to the pressure setting of the valve. As a result, the liquid refrigerant goes back to the compressor which causes serious damage to the compressor.
iv) The evaporator pressure increases above the pressure setting of the valve when the refrigeration load increases due to which the valve closes until the evaporator pressure is reduced up to pressure setting of valve. As a result of which the evaporator starves.
Applications of Automatic Expansion Valves:
I) Air Conditioning System
ii) Refrigeration System
iii) Heat Pumps
iv) Cold Room and Freezers
v) Milk chilling Units.