In HVAC systems, the expansion valve plays a vital role as a throttling device. Its main function is to throttle the saturated liquid (or subcooled liquid) in the condenser or receiver to the evaporation pressure and evaporation temperature, and adjust the refrigerant flow entering the evaporator according to load changes. Common throttling devices include thermal expansion valves, capillary tubes, and electronic expansion valves. Next, we will introduce these three types of expansion valves one by one.
The thermal expansion valve is widely used in HVAC systems using Freon refrigeration. It automatically adjusts based on temperature changes at the evaporator outlet. This regulation ensures proper refrigerant flow into the system.
Most thermal expansion valves are preset with a superheat range of 5-8℃ before leaving the factory. The valve design ensures it opens fully when superheat increases by 2℃. When the superheat is about 2℃, the valve closes. The spring controlling superheat can be adjusted within a range of 3-6℃.
A higher superheat reduces the evaporator’s heat absorption capacity. This happens because the increase in superheat uses up the heat transfer surface at the evaporator’s tail. The saturated vapor gets superheated, reducing the available area for refrigerant vaporization. In this case, the evaporator surface is not fully utilized.
If the superheat is too low, liquid refrigerant may enter the compressor. This could lead to liquid hammer, which is harmful to the system. Therefore, the superheat must be properly adjusted. It should allow enough refrigerant into the evaporator, while preventing liquid refrigerant from entering the compressor.
The thermal expansion valve consists of the valve body, temperature sensing package, and capillary tube. There are two types of thermostatic expansion valves: internal balance type and external balance type. The difference lies in how the diaphragm balance is achieved.
The capillary tube is a simple, low-cost throttling device consisting of a copper tube with a very small diameter, and the inner diameter is generally 0.5~2mm. Its working principle is to limit the refrigerant flow through the small inner diameter of the pipe, thereby achieving throttling.
The advantages of the capillary tube are simple structure, low cost and no moving parts, so it is not easy to fail and leak. In addition, the capillary tube can quickly balance the high and low pressures of the system after the refrigeration compressor stops running, so that the compressor will not be subjected to excessive shock when restarting.
However, the capillary tube has poor regulation accuracy and cannot be dynamically adjusted according to load changes. It is suitable for small equipment or small refrigeration systems with small load changes. Compared with thermal expansion valves and electronic expansion valves, capillaries are not suitable for systems with large load fluctuations because they cannot adjust the flow in real time, resulting in reduced refrigeration efficiency.
Compared with thermal expansion valves and electronic expansion valves, capillaries are suitable for small equipment with small load changes, but the regulation is not accurate enough in large-scale systems.
Electronic expansion valve is mainly used in variable frequency air conditioning system with intelligent control. It uses electronic control method, which is more accurate than traditional thermal expansion valve, and can dynamically adjust the refrigerant flow according to the real-time load changes of the system, thereby improving energy efficiency.
The structure of electronic expansion valve consists of three main parts: detection, control and execution. The detection part monitors the data in the system in real time through sensors, such as the temperature and pressure of the evaporator; the control part transmits the data to the controller, and the controller adjusts the valve opening according to the data; the execution part adjusts the valve core position according to the control signal to accurately control the flow of refrigerant. Through this structure, the electronic expansion valve can respond quickly to load changes and ensure that the refrigerant flow of the system matches the load demand.
The advantages of electronic expansion valve are large flow adjustment range and high precision, which is especially suitable for systems with large load changes or high energy efficiency requirements. Electronic expansion valve can not only improve the energy efficiency of variable frequency compressor, but also quickly adjust the temperature and improve the seasonal energy efficiency ratio of the system. For high-power variable frequency air conditioning system, electronic expansion valve is almost a must.
Electronic expansion valves are divided into electromagnetic and electric types according to the driving mode. Among them, electric electronic expansion valves are further divided into direct-acting type and reduction type. They can flexibly adjust the flow rate according to different application requirements to ensure optimal system control.
In HVAC systems, each type of expansion valve has its own advantages and applicable scenarios:
