Comparison Between Electronic Expansion Valve and Thermal Expansion Valve
Now more and more civil and commercial refrigeration equipment use electronic expansion valve to replace the original thermal expansion valve. The electronic expansion valve and the thermal expansion valve have the same basic use, but different in performance.
1. Adjustment range
At present, the regulating range of thermal expansion valve is generally narrow. The heat pump unit should not only refrigeration, but also heating, and the ambient temperature range of the suitable occasions is from – 15 ℃ to + 43 ℃, and the corresponding refrigerant evaporation temperature will work in the range of – 25 ℃ – 5 ℃. In addition, if there are multiple compressors in the refrigeration circuit, the number of compressors in operation will change accordingly with the change of user load, resulting in dramatic change of refrigerant flow.
Therefore, a single thermal expansion valve is far from competent for the operation of large heat pump units. At present, many large-scale heat pump products are designed with single loop and single compressor, and the expansion valve system with independent refrigeration mode and heating mode is adopted, which will increase the complexity and manufacturing cost of the system. The electronic expansion valve can be adjusted accurately in the range of 15% ～ 100%.
According to the current use effect, a single electronic expansion valve can meet the regulation of heat pump unit under the above conditions. The adjustable range can be set according to the characteristics of different products, which increases the flexibility.
2. Control of superheat
(1) Superheat control point:
For the thermal expansion valve, generally only the superheat at the evaporator outlet can be controlled. In the semi closed and fully closed compressor system, the control point can be set not only at the evaporator outlet, but also at the compressor suction port, which can control the suction superheat of the compressor to ensure the efficiency of the compressor.
(2) Superheat setting value:
For thermal expansion valve, its superheat setting value is generally set by the manufacturer in the manufacturing process, usually 5 ℃, 6 ℃ or 8 ℃. The superheat degree of the electronic expansion valve can be set manually according to the different characteristics of the product. For example, the superheat of the evaporator outlet is set to 6 ℃, and the superheat of the compressor suction can be set to 15 ℃, which is very flexible.
(3) Stability of superheat control under non-standard operating conditions:
The superheat setting values of thermal expansion valve are all set under standard conditions. However, due to the characteristics of charging working fluid, when the system deviates from the standard working condition, the superheat will deviate from the set value with the change of condensation pressure, which will not only cause the decrease of system efficiency, but also cause the fluctuation of the system. The superheat degree of the electronic expansion valve is set by the controller artificially, and the actual superheat degree of the system is calculated by the parameters of the control point collected by the sensor, so there is no such problem.
(4) Intelligence of system regulation:
The superheat control of thermal expansion valve is based on the state of the current control point, which is determined by the characteristics of working fluid filled. It can not judge the change trend of the system. The control logic of the electronic expansion valve can adopt various intelligent control systems according to the design and manufacturing characteristics of different products. It can not only adjust the current state of the system, but also distinguish the characteristics of the system according to the change rate of superheat degree and other parameters. The corresponding control methods are adopted for different system change trends. Therefore, its response speed and pertinence to system changes are superior to thermal expansion valve.
3. Reaction rate
The thermal expansion valve is driven by taking advantage of the thermal characteristics of the filling working medium, so its opening and closing characteristics are as follows:
(1) The sensitivity of reaction and the speed of opening and closing are slow.
(2) Generally speaking, the opening and closing speed of thermal expansion valve is relatively consistent.
(3) In the process of unit start-up, there is static superheat. The superheat (SH) of thermal expansion valve is composed of static superheat (SS) and opening superheat (OS). Due to the existence of static superheat, there will be a tendency to delay the opening of thermal expansion valve during startup.
The driving mode of the electronic expansion valve is that the controller calculates the parameters collected by the sensor, sends the regulation command to the driving board, and the driving board outputs the electric signal to the electronic expansion valve to drive the action of the electronic expansion valve. It takes only a few seconds for the electronic expansion valve to change from fully closed to fully open. It has fast reaction and action speed, and there is no static superheat phenomenon. Moreover, the opening and closing characteristics and speed can be set manually, which is especially suitable for the use of heat pump units with severe fluctuation of working conditions.
4. Diversity of control functions
In order to prevent compressor overload caused by excessive refrigerant pressure and flow at evaporation side during the initial start-up of the unit, the thermal expansion valve is generally equipped with mop function, that is, the expansion valve can only be opened when the evaporation pressure is lower than the set value. However, compared with the electronic expansion valve, its function is still monotonous.
The structure of the electronic expansion valve can be regarded as the organic combination of throttle mechanism and solenoid valve, and it can be adjusted by the controller. Therefore, according to different product characteristics, it shows the diversity and superiority of its control function under the conditions of unit start-up, load change, defrosting, shutdown and fault protection. For example: the electronic expansion valve to regulate the refrigerant flow can not only control the evaporator, but also can be used to adjust the condenser.
When the evaporation condition allows, if the condensation pressure is too high, the expansion valve can be properly closed to reduce the refrigerant flow in the system and the condenser load, so as to reduce the condensation pressure and realize the efficient and reliable operation of the unit.