Performance comparison between electronic expansion valve and thermal expansion valve

More and more refrigeration equipment is using electronic expansion valves to replace the original thermal expansion valves. The basic purpose of electronic expansion valves and thermal expansion valves is the same, with various structures, but there are significant differences in performance. Today, let me share with you the performance comparison between electronic expansion valves and thermal expansion valves

1. Adjustment range

At present, the regulating range of thermal expansion valves is generally narrow. The heat pump unit needs both cooling and heating, and the applicable ambient temperature range is from -15 ℃ to+43 ℃. The corresponding refrigerant evaporation temperature will work within the range of -25 ℃ to 5 ℃. Moreover, if there are multiple compressors in the refrigeration circuit, the number of compressors in operation of the unit will change accordingly with the change of user load, resulting in a drastic change in refrigerant flow rate.

Therefore, a single thermal expansion valve is far from capable of handling the operating conditions of large heat pump units. At present, many large heat pump products adopt a design system with a single circuit equipped with a single compressor, and adopt an expansion valve system that is independent of the refrigeration mode and heating mode, which will inevitably increase the complexity and manufacturing cost of the system. The electronic expansion valve can be precisely adjusted within a range of 15% to 100%.

From the current usage effect, a single electronic expansion valve can meet the regulation of heat pump units under the above working conditions. And the adjustment range can be set according to the characteristics of different products, increasing flexibility.

2. Control of superheat

(1) Control point for superheat: For thermal expansion valves, they can generally only control the superheat at the outlet of the evaporator. The electronic expansion valve demonstrates its superiority. In semi enclosed and fully enclosed compressor systems, its control point can be set not only at the evaporator outlet, but also at the compressor suction port to control the suction superheat of the compressor and ensure its efficiency.

(2) The set value of superheat: For thermal expansion valves, the set value of superheat is generally set by the manufacturer during the manufacturing process, usually 5 ℃, 6 ℃, or 8 ℃. The overheating degree of the electronic expansion valve can be manually set according to the different characteristics of the product. For example, the superheat degree at the evaporator outlet can be set to 6 ℃, and the superheat degree at 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 the thermal expansion valve are all set under standard operating conditions. However, due to the characteristics of the filled working fluid, when the system deviates from the standard operating conditions, its superheat often deviates from the set value with changes in condensation pressure, etc. This not only causes a decrease in system efficiency, but also causes system fluctuations. The overheating degree of the electronic expansion valve is manually set by the controller, and the actual overheating degree of the system is calculated by the parameters of the control points collected by sensors, so this type of problem does not occur.

(4) The intelligence of system regulation: The control of superheat by the thermal expansion valve is based on the current state of the control point, determined by the characteristics of the filled working fluid, and it cannot make judgments on the trend of system changes. The control logic of electronic expansion valves can adopt various intelligent control systems based on 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 based on parameters such as the rate of change of superheat, and adopt corresponding control measures for different system change trends. Therefore, its response speed and specificity to system changes are superior to thermal expansion valves.

3. Reaction speed

The drive of the thermal expansion valve utilizes the thermal characteristics of the filled working fluid, therefore, its opening and closing characteristics have the following characteristics:

(1) The sensitivity of the reaction and the speed of the opening and closing actions are relatively slow.

(2) Generally speaking, the opening and closing speeds of thermal expansion valves are relatively consistent.

(3) During the start-up process of the unit, there is static overheating. The superheat (SH) of a thermal expansion valve is composed of static superheat (SS) and open superheat (OS). Due to the presence of static superheat, there is a tendency for the expansion valve to open delayed during the startup process.

The driving method of the electronic expansion valve is that the controller calculates the parameters collected by the sensor, sends adjustment instructions to the drive board, and the drive board outputs electrical signals to the electronic expansion valve, driving its action. The electronic expansion valve only takes 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 use in heat pump units with severe working condition fluctuations.

4. Diversity of control functions

To prevent compressor overload caused by excessive refrigerant pressure and flow rate on the evaporator side during initial start-up of the unit, the thermal expansion valve is generally equipped with MOP function, which means that the expansion valve only opens when the evaporator pressure is below the set value. However, its function still appears relatively monotonous compared to electronic expansion valves.

The electronic expansion valve can be seen as an organic combination of a throttling mechanism and an electromagnetic valve in structure, and can be adjusted through a controller. Therefore, according to different product characteristics, its control functions are diverse and superior in situations such as unit startup, load changes, defrosting, shutdown, and fault protection. For example, the electronic expansion valve can not only control the evaporator but also be used to adjust the refrigerant flow rate in the condenser.

When the evaporation condition allows, if the condensation pressure is too high, the expansion valve can be closed appropriately to reduce the flow of refrigerant in the system, lower the load on the condenser, and thus lower the condensation pressure, achieving efficient and reliable operation of the unit.