Electronic Expansion Valve vs. Thermal Expansion Valve: Which Has More Obvious Advantages?

The Position of the Throttle Structure in the Refrigeration System:
In the refrigeration system, the common throttle structures/expansion devices are as follows:
01) Capillary tube
02) Thermal expansion valve
03) Electronic expansion valve
Many colleagues who are new to the industry have certain doubts about how to choose between the thermal expansion valve and the electronic expansion valve. Today, let's take a look at the advantages and disadvantages of these two throttle mechanisms:
I. Thermal Expansion Valve:
Currently, the thermal expansion valve is mostly used as the throttle device in the Freon cold storage. The working principle of the thermal expansion valve is as follows:
It regulates the flow rate of the refrigerant by sensing the degree of superheat of the refrigerant vapor at the outlet of the evaporator to maintain a constant superheat, and it belongs to a proportional regulator in terms of control principle. Although the thermal expansion valve can automatically regulate the flow rate of the refrigerant, its disadvantages are also very obvious.
(1) There is a long delay time in responding to the superheat, especially the volume delay.
The superheated vapor at the outlet of the evaporator first transfers heat to the outer shell of the thermostatic bulb.

The outer shell of the thermostatic bulb itself has a large thermal inertia, causing a certain volume delay. The outer shell of the thermostatic bulb transfers heat to the temperature-sensing medium, which further causes a delay.

The result of the delay will lead to the thermal expansion valve alternately opening wider or closing narrower, that is, the oscillation phenomenon occurs.
When the expansion valve is opened too wide, the superheat at the outlet of the evaporator is low, and the suction pressure rises;
When the valve is opened too narrow, the liquid supply to the evaporator is insufficient, and the suction pressure decreases.
This will have an adverse impact on the economy and safety of the entire system. Experiments show that it takes more than ten minutes for the thermal expansion valve to achieve a stable adjustment effect on a small device, and 30min to 40min for a large device.
(2) The adjustment range is limited.
Because the deformation of the diaphragm connected to the valve needle is limited, the movement displacement of the valve needle is small, so the flow adjustment range is small. For cold storage with large load changes or systems using variable-frequency compressors, the thermal expansion valve cannot meet the requirements.
(3) The adjustment accuracy is low.
Due to factors such as processing accuracy and installation, the diaphragm of the actuator of the thermal expansion valve will deform and affect the deformation sensitivity, so it is difficult to achieve a high adjustment accuracy.
II. Electronic Expansion Valve:
The electronic expansion valve regulates the liquid supply to the evaporator according to a preset program. Since it belongs to the electronic regulation mode, it is called an electronic expansion valve.
It meets the development requirements of the integration of refrigeration machinery and electronics, has excellent characteristics that the thermal expansion valve cannot match, provides conditions for the intelligent control of the refrigeration system, and is a self-controlled energy-saving component with great development prospects.
The basic purpose of the electronic expansion valve is the same as that of the thermal expansion valve, and there are various structures. However, in terms of performance, there are significant differences between the two.
From the perspective of control implementation, the electronic expansion valve is composed of three parts: a controller, an actuator, and a sensor. The electronic expansion valve usually referred to mostly only refers to the actuator, that is, the controllable driving device and the valve body. In fact, only this part cannot complete the control function.
The core hardware of the electronic expansion valve controller is a single-chip microcomputer. If the controller also needs to complete control functions such as the variable frequency of the compressor and the fan, it generally adopts the form of multi-machine cascade connection. The sensors of the electronic expansion valve usually use thermocouples or thermal resistors.
Compared with the thermal expansion valve, the electronic expansion valve has significant advantages in the following aspects:
(1) The driving mode of the electronic expansion valve is that the controller calculates the parameters collected by the sensor and sends an adjustment instruction to the drive board. The drive board outputs an electrical signal to the electronic expansion valve to drive the action of the electronic expansion valve.
It only takes a few seconds for the electronic expansion valve to move from the fully closed state to the fully open state. It has a fast response and action speed, there is no static superheat phenomenon, and both the opening and closing characteristics and speed can be artificially set, especially suitable for the use of heat pump units with drastic working condition fluctuations.
(2) The electronic expansion valve is suitable for low temperatures.
For the thermal expansion valve, when the ambient temperature is low, the pressure change of the temperature-sensing medium inside the thermostatic bulb is greatly reduced, which seriously affects the adjustment performance. For the electronic expansion valve, its temperature-sensing components are thermocouples or thermal resistors, which can still accurately reflect the change of superheat at low temperatures. Therefore, in low-temperature environments such as the freezing rooms of cold storage, the electronic expansion valve can also provide good flow regulation.
(3) The superheat setting value of the electronic expansion valve is adjustable.
Just by changing the source code in the control program, the superheat setting value can be changed. It is completely different from the thermal expansion valve, which requires entering the cold storage and adjusting the pre-tightening force of the spring on-site to change the superheat setting value.

The adjustment of the electronic expansion valve can be completely remotely controlled, and the electronic expansion valve can flexibly adjust the superheat according to different needs to reduce the temperature difference between the surface of the evaporator and the environment inside the cold storage, thereby reducing the frost formation on the surface of the evaporator. In this way, it not only improves the freezing capacity but also reduces the dry shrinkage of food.
(4) The electronic expansion valve can play a role in energy conservation.
During the shutdown period of the refrigeration system of the cold storage, if the high-pressure and low-pressure sides are connected, the so-called working medium migration phenomenon will occur, that is, the normal-temperature high-pressure liquid in the condenser will gradually flow into the evaporator, causing the temperature and pressure of the evaporator to rise. When starting the machine again, it also takes additional energy of the compressor to re-establish the pressure difference. On the contrary, if the high-pressure and low-pressure sides are cut off during the shutdown period, although it maintains the low temperature and low pressure of the evaporator, when starting again, the compressor starts with a load, and there is a large current impact, which also increases the energy loss. However, if an electronic expansion valve is used, the above problems will be solved.
The specific approach is: when shutting down, fully close the expansion valve to prevent the high-temperature liquid in the condenser from flowing into the evaporator, resulting in energy loss during the next start. Before starting, fully open the expansion valve to balance the high-pressure and low-pressure sides of the system, and then start the machine. In this way, it not only achieves a light-load start but also reduces the heat loss during the shutdown. In addition, using an electronic expansion valve can shorten the freezing time. The electronic expansion valve can achieve a balance between the load and the cooling capacity throughout the freezing process, the freezing efficiency can be improved, the freezing time can be shortened by 10% compared with that of the thermal expansion valve, and at the same time, the energy consumption of the compressor is reduced. Using the electronic expansion valve to control the discharge temperature of the compressor can prevent the adverse effects on the system performance caused by the increase in the discharge temperature. At the same time, a specially set safety protector can be omitted, saving costs and about 6% of electricity consumption.
(5) The electronic expansion valve meets the development requirements of the integration of machinery and electronics. With the rise of microcomputer control technology, the integration of machinery and electronics has become a new development trend of the refrigeration system. Compared with the thermal expansion valve, the electronic expansion valve has developed from the original mechanical control to computer control, fully reflecting the development trend of the integration of machinery and electronics. Currently, in the field of household air conditioners, the system composed of an electronic expansion valve and a variable-frequency compressor has achieved very good results. Its principle is to combine the large-range flow regulation characteristics of the electronic expansion valve with the variable-frequency characteristics of the variable-frequency compressor.
III. Forms of Electronic Expansion Valves
Currently, the research and development of electronic expansion valves mainly focus on electromagnetic expansion valves and electric expansion valves.
Before the electromagnetic coil of the electromagnetic expansion valve is energized, the needle valve is in the open position;
The opening degree of the needle valve is controlled by the voltage applied to the coil, thereby regulating the flow rate of the expansion valve. This valve has a fast action response, but it always requires power supply when the refrigeration system is working.
The electric expansion valve, that is, the electronic expansion valve driven by a stepper motor, makes the stepper motor drive the valve needle to move forward and backward through the thread by applying a digital signal with a certain logical relationship to the motor drive, thereby changing the flow area of the valve port to achieve the purpose of controlling the flow rate.
The electric expansion valve has two types: direct-acting type and reduction type. In the direct-acting type, the stepper motor directly drives the valve needle; in the reduction type, the stepper motor drives the valve needle to move through a reduction gear set. A large thrust can be generated through the reduction gear set, which is a commonly used driving method.
The electronic expansion valve driven by a stepper motor is adopted by more refrigeration systems because it is more suitable for microcomputer control and has better stability.
IV. Conclusion
Currently, more and more refrigeration devices use electronic expansion valves for throttling. Their comfort, energy conservation, and flexibility to meet special work requirements fully demonstrate the characteristics of electronic control.
To achieve intelligent and optimized operation at the refrigeration cycle level for large refrigeration devices, the application of electronic expansion valves will also be essential. In the refrigeration systems of the marine air conditioning system and the marine refrigerated containers on ocean-going ships, due to their relatively harsh and variable working conditions and high working requirements, it is of great practical significance to replace the thermal expansion valve with an electronic expansion valve. It can be predicted that as one of the breakthroughs for electronic control to intervene in the refrigeration cycle, the technology of electronic expansion valves has broad prospects.