Analysis: Electronic Expansion Valve
In the refrigeration and air conditioning industry, expansion valves, four-way valves, and stop valves all play a certain role. These components not only affect the efficiency and reliability of the system, but are also elements in realizing advanced refrigeration technology. This article will conduct an in-depth technical analysis of these components and discuss their structure, working principles, common faults, and detection and maintenance methods.
Electronic expansion valve
The electronic expansion valve adjusts the liquid supply volume of the evaporator according to a preset program. It is an electronic adjustment mode, so it is called an electronic expansion valve.
The driving method of the electronic expansion valve is that the controller calculates the parameters collected by the sensor and issues adjustment instructions to the drive board. The drive board outputs an electrical signal to the electronic expansion valve to drive the electronic expansion valve, as shown in the figure below. It only takes a few seconds for the electronic expansion valve to go from fully closed to fully open. It has fast reaction and action speeds. There is no static superheat phenomenon, and the opening and closing characteristics and speed can be set artificially. It is especially suitable for severe fluctuations in working conditions. The use of heat pump units.
From the perspective of control implementation, the electronic expansion valve consists of three parts: controller, actuator and sensor. The core hardware of the electronic expansion valve controller is a single-chip microcomputer.
From the classification point of view, according to the driving method, there are two types: electromagnetic type and electric type. The most commonly used one at present is the electric electronic expansion valve driven by a four-phase stepper motor.
Thermal expansion valve
The top of the expansion valve is composed of a sealed box cover, a corrugated film temperature sensing bag and a capillary tube to form a closed container, which is filled with Freon to form a sensing mechanism. The refrigerant filled in the sensing mechanism can be the same as that of the refrigeration system, or it can be different.
The thermal expansion valve senses the change in superheat at the outlet of the evaporator through the temperature sensing bulb, causing the temperature sensing system to A closed system) filled with substances to produce pressure changes and act on the transmission diaphragm. It causes the diaphragm to move up and down, and then transmits this force to the transmission rod through the transmission plate to push the valve needle up and down, making the valve close or open, which plays the role of reducing pressure and throttling and automatically adjusting the refrigerant supply of the evaporator. And maintain a certain degree of superheat at the outlet end of the evaporator to ensure full utilization of the heat transfer area of the evaporator and reduce the occurrence of liquid impact on the cylinder.
At present, the adjustment range of thermal expansion valves is generally narrow. Some heat pump units require both cooling and heating, and the ambient temperature range of applicable occasions is from - 15 ℃ to + 43 ℃, and the corresponding refrigerant evaporation temperature will work in the range of - 25 ℃ to 5 ℃. Moreover, if there are multiple compressors in the refrigeration circuit, the number of operating compressors in the unit will change accordingly as the user load changes, causing drastic changes in refrigerant flow.
Therefore, a single thermal expansion valve is far from capable of operating in large heat pump units. At present, many large-scale heat pump products adopt a single-circuit design system equipped with a single compressor, and adopt independent expansion valve systems for cooling and heating modes, which will inevitably increase the complexity and manufacturing cost of the system.
According to the different structures of thermal expansion valves, they are divided into two types: internal balanced type and external balanced type.
Considering that the refrigerant flows through the evaporator and produces a certain pressure loss, in order to reduce the opening superheat and improve the utilization of the heat transfer area of the evaporator, the evaporation temperature corresponding to the pressure drop of the refrigerant from the expansion valve outlet to the evaporator outlet is generally If the temperature drop exceeds 2~3°C, an externally balanced thermal expansion valve should be used.
