In the refrigeration system, the throttling device is one of the four indispensable basic components of the compression refrigeration cycle and plays a vital task. Its main function is to throttle and depressurize the high-pressure liquid refrigerant coming out of the condenser so that it becomes a low-pressure liquid under the evaporation pressure. At the same time, it adjusts the amount of refrigerant liquid entering the evaporator according to changes in system load. This process is critical to ensuring the efficient operation of your refrigeration system. Next, we will introduce several common throttling devices and their working principles and characteristics in detail.
1. Throttling principle
The working principle of the throttling device is based on the throttling effect in fluid mechanics. When the refrigerant fluid passes through a small hole, its static pressure is converted into dynamic pressure, and the flow rate increases sharply, forming a turbulent flow. With the violent disturbance of the fluid, the frictional resistance increases, causing the static pressure to drop significantly, thereby achieving the purpose of pressure reduction. In addition, the pressure loss during the throttling process is closely related to the fluid flow rate. By adjusting the size or opening of the small holes, the refrigerant flow rate can be effectively controlled.
2. Common throttling devices
1. Capillary tube
Capillary tube is an extremely simple throttling device, usually composed of a slender copper tube with an inner diameter of 0.4~2.0mm. It has a simple structure, low cost and easy maintenance, and is widely used in small refrigeration devices such as household air conditioners and refrigerators. Capillary tubes determine the throttling effect through their fixed inner diameter and length. Once sized, they cannot be adjusted for load changes, so capillary tubes may not be the best choice for systems with large load changes.
2. Throttle short pipe
The throttling short pipe consists of a section of pipe with a smaller diameter and two connectors. Its working principle is similar to that of a capillary tube, but the pipe diameter is relatively larger and the length is shorter. This design allows the throttling spool to have better flow regulation capabilities while providing sufficient pressure drop. The throttling short tube has a simple structure, is easy to manufacture, and is cheap. It is suitable for small refrigeration systems, such as automobile air conditioners.
3. Thermal expansion valve
The thermal expansion valve is an automatic adjustment device that senses the change in superheat at the evaporator outlet through a temperature sensing package and automatically adjusts the opening of the valve to achieve precise control of the refrigerant flow. When the superheat at the evaporator outlet increases, the temperature of the working fluid in the temperature sensing bag increases, and the pressure above the diaphragm increases, pushing the valve stem to move downward, causing the valve opening to increase and the flow rate to increase; conversely, the valve opens The degree decreases and the flow rate decreases.
Characteristics of thermal expansion valve:
Automatic adjustment: The thermal expansion valve can automatically adjust the refrigerant flow according to changes in system load to keep the superheat at the evaporator outlet stable.
Strong adaptability: suitable for a variety of refrigerants and different system configurations, especially in situations where load changes are large.
Classification:
Internal balance type: The temperature sensing bulb collects the evaporation pressure at the evaporator inlet, which is suitable for systems with small flow pressure loss in the evaporator.
External balance type: The temperature sensing bulb collects the evaporation pressure at the evaporator outlet, and is suitable for systems with large flow pressure losses in the evaporator to improve adjustment accuracy.
4. Electronic expansion valve
The electronic expansion valve uses a stepper motor to drive the valve needle to rotate to control the opening of the valve, thereby regulating the refrigerant flow. Compared with thermal expansion valves, electronic expansion valves have higher response speed and wider adjustment range, and can be dynamically adjusted according to various parameters of the system. They are especially suitable for intelligent control systems such as inverter air conditioners. The high-precision control of the electronic expansion valve helps to improve the energy efficiency ratio and refrigeration effect of the system. It is suitable for a variety of refrigerants and different types of refrigeration systems, especially for occasions with high energy efficiency and stability requirements.
Features of electronic expansion valve:
High-precision control Through electronic controllers and sensors, the electronic expansion valve can achieve precise control of refrigerant flow, ensuring that the system can maintain optimal performance under various working conditions.
Fast response: The response speed of the electronic expansion valve is much higher than that of the thermal expansion valve. It can complete the flow adjustment in a short time and is suitable for rapidly changing load conditions.
Combining frequency conversion technology and intelligent control systems, electronic expansion valves can implement more complex control strategies, such as superheat control, temperature control, and pressure control.
5. Throttle orifice plate
The throttling orifice plate is a throttling device with a simple structure, mainly composed of a circular metal plate and a plurality of evenly distributed small holes. Its working principle is to rapidly increase the flow rate of high-pressure liquid refrigerant through small holes and rapidly reduce the pressure, thereby achieving throttling and pressure reduction. The throttling orifice plate has the advantages of easy manufacturing, low cost, and strong flow control capability. It is suitable for large-scale refrigeration systems, such as centrifugal chillers, and can handle large amounts of refrigerant flow.
However, the throttle orifice plate is a fixed-section throttle orifice and cannot automatically adjust the flow rate according to load changes. Therefore, in systems with large load changes, it usually needs to be used in conjunction with a thermal expansion valve or an electronic expansion valve. In addition, the small holes of the orifice plate are easily blocked by impurities and require regular cleaning and maintenance. By carefully designing the size and layout of the small holes, effective control of the refrigerant flow can be achieved and the stability and reliability of the system can be improved.
6. Float valve
The float valve is mainly used in evaporators with a free liquid surface. The float moves up and down with changes in the liquid level, driving the valve to open or close to regulate the refrigerant flow. At the same time, the float valve also has the function of controlling the liquid level to ensure that a certain amount of liquid is always maintained in the evaporator. Although the float valve has a simple structure, when the liquid level fluctuates greatly, the impact of the float on the valve core may cause damage, so it has been gradually replaced by other types of throttling devices in modern systems.
7. Manual regulating valve
The manual regulating valve is one of the most traditional throttling devices and relies entirely on manual operation to adjust the valve opening. Although its control accuracy is low, in some industrial applications, such as chillers with Freon refrigerants, manual regulating valves are often installed on the bypass pipeline as a backup valve for use during maintenance of the automatic throttling device.
PS: When selecting a suitable throttling device, its compatibility with the entire refrigeration system must be considered. For example, for small and medium-sized refrigeration units, for systems with large load changes, it is recommended to use thermal expansion valves or electronic expansion valves. They can automatically adjust the refrigerant flow rate according to changes in system load to maintain stable operation of the system. For small fixed-load systems, such as household air conditioners and refrigerators, capillary tubes are more suitable because of their simple structure and low cost.
In addition, there are differences in maintenance and reliability between different types of throttling devices. Capillary tubes and throttling short tubes have a low failure rate due to their simple structure, but once they are clogged or damaged, replacement is more troublesome. Although thermal expansion valves and electronic expansion valves have complex structures, their service life can be extended through regular maintenance and inspection. Float valves and manual control valves require more daily maintenance, while the throttle orifice plate is a fixed-section throttle orifice and cannot automatically adjust the flow rate according to load changes. Therefore, in systems with large load changes, it usually needs to be combined with thermal power. expansion valve or electronic expansion valve to ensure proper operation.
