The throttling device commonly used in Freon refrigeration warehouses is a thermal expansion valve. The working principle of the thermal expansion valve is to adjust the refrigerant flow rate by sensing the superheat degree of the refrigerant vapor at the evaporator outlet to maintain a constant superheat degree. In terms of control principle, it belongs to a proportional regulator. Although the thermal expansion valve can automatically regulate the flow of refrigerant, its drawbacks are also significant.
1. The delay time for superheat response is long, especially for volume delay.
The superheated steam at the outlet of the evaporator first transfers heat to the shell of the temperature sensing bag. The shell of the temperature sensing bag itself has a large thermal inertia, causing a certain volume delay. The shell of the temperature sensing bag transfers heat to the temperature sensing medium, which further delays. The delayed result will cause the thermal expansion valve to alternately open or close, resulting in oscillation.
When the expansion valve is opened too large, the superheat at the outlet of the evaporator is low, and the suction pressure increases; When the valve is too small, the evaporator lacks liquid supply and the suction pressure decreases. This will have adverse effects on the economy and security of the entire system. The experiment shows that the regulating effect of the thermal expansion valve takes more than ten minutes for small devices, and 30 to 40 minutes for large devices to stabilize.
2. The adjustment range is limited.
Due to the limited deformation of the diaphragm connected to the valve needle, the movement displacement of the valve needle is small, resulting in a small flow adjustment range. For refrigerated warehouses with significant load changes or systems using variable frequency compressors, the thermal expansion valve cannot meet the requirements.
3. Low adjustment accuracy.
Due to factors such as machining accuracy and installation, the actuator diaphragm of a thermal expansion valve may experience deformation and affect deformation sensitivity, making it difficult to achieve high adjustment accuracy.
The electronic expansion valve adjusts the liquid supply to the evaporator according to a preset program, and is called an electronic expansion valve because it belongs to the electronic adjustment mode. It meets the development requirements of refrigeration mechatronics integration and has excellent characteristics that cannot be compared to thermal expansion valves, providing conditions for intelligent control of refrigeration systems. It is a promising self-control and energy-saving component. The basic purpose of electronic expansion valves and thermal expansion valves is the same, with various structures, but there are significant differences in performance between the two.
From the perspective of control implementation, electronic expansion valves are composed of three parts: a controller, an actuator, and a sensor. Generally speaking, electronic expansion valves only refer to actuators, namely controllable driving devices and valve bodies. In fact, only this part cannot complete the control function. The core hardware of the electronic expansion valve controller is a microcontroller. If the controller simultaneously completes control functions such as frequency conversion of the compressor and fan, it generally adopts the form of multi machine cascade connection. Sensors for electronic expansion valves typically use thermocouples or thermal resistors.
Electronic expansion valves have significant advantages in the following areas:
(1) The driving method of the electronic expansion valve is that the controller calculates the parameters collected by the sensor, sends adjustment instructions to the driving board, and the driving board outputs electrical signals to the electronic expansion valve, driving the action of the electronic expansion valve. The electronic expansion valve takes only a few seconds to move from fully closed to fully open state, with fast reaction and action speed, no static overheating phenomenon, and the opening and closing characteristics and speed can be manually set, especially suitable for the use of heat pump units with severe working conditions fluctuations.
(2) The applicable temperature of the electronic expansion valve is low. For thermal expansion valves, when the ambient temperature is low, the pressure change of the temperature sensing medium inside the temperature sensing bag is greatly reduced, seriously affecting the regulating performance. For electronic expansion valves, their temperature sensing components are thermocouples or thermal resistors, which can accurately reflect changes in superheat at low temperatures. Therefore, in low-temperature environments such as freezing rooms in refrigerated warehouses, electronic expansion valves can also provide good flow regulation.
(3) The superheat setting value of the electronic expansion valve is adjustable. Simply change the source code in the control program to change the set value of superheat. Unlike the thermal expansion valve, which needs to enter the cold storage, the pre tightening force of the spring is adjusted on site to change the set value of the superheat. The regulating effect of the electronic expansion valve can be completely controlled remotely, and the electronic expansion valve can flexibly adjust the superheat according to different needs to reduce the temperature difference between the evaporator surface and the environment inside the cold storage, thereby reducing frost formation on the evaporator surface, thereby improving the freezing capacity, It can also reduce the dry consumption of food.
(4) Electronic expansion valves can play an energy-saving role. During the shutdown of the refrigeration system in the cold storage, if the high and low pressure sides are connected, a phenomenon called refrigerant migration will occur, which means that the normal temperature and high pressure liquid in the condenser will gradually flow into the evaporator, causing the temperature and pressure of the evaporator to increase. When starting up again, rebuilding the pressure difference also requires an additional amount of energy consumption from the compressor. On the contrary, if the high and low pressure sides are cut off during the shutdown period, although maintaining the low temperature and low pressure of the evaporator, when the compressor is restarted, it belongs to a loaded start, and the current impact is large, which will also increase energy loss. But if electronic expansion valves are used, the above problems will be solved.
The specific method is to fully close the expansion valve during shutdown to prevent high-temperature liquid from the condenser from flowing into the evaporator, causing energy loss during restart. Before starting the machine, fully open the expansion valve to balance the high and low pressure sides of the system, and then start the machine. This not only achieves a light load start, but also reduces the loss of hot sticks during shutdown. In addition, the use of electronic expansion valves can shorten the freezing time. The electronic expansion valve can achieve a balance between load and cooling capacity throughout the freezing process, improve the freezing efficiency, and shorten the freezing time by 10% compared to the thermal expansion valve. At the same time, it also reduces the energy consumption of the compressor. The use of electronic expansion valves to control the compressor exhaust temperature can prevent adverse effects on system performance caused by the increase in exhaust temperature, while also eliminating the need for specialized safety protectors, saving costs and electricity consumption by about 6%.
(5)Electronic expansion valves meet the development requirements of mechatronics integration. With the rise of microcomputer control technology, mechatronics integration has become a new trend in the development of refrigeration systems. Compared to thermal expansion valves, electronic expansion valves have evolved from mechanical control to computer control, fully reflecting the development trend of mechatronics integration. At present, in the field of household air conditioning, the system composed of electronic expansion valves and variable frequency compressors has achieved good results. Its principle is to combine the wide range flow regulation characteristics of electronic expansion valves with the variable frequency characteristics of variable frequency compressors.