Common throttling devices for refrigeration equipment
The throttling device is one of the important components in the refrigeration system. Its function is to depressurize the saturated liquid (or subcooled liquid) under the condensing pressure in the condenser or liquid receiver to the evaporation pressure and evaporation temperature after throttling. To achieve the purpose of cooling and cooling, adjust the flow of refrigerant entering the evaporator to adapt to the load change of the evaporator. Common throttling mechanisms are as follows.
1. Capillary
The capillary tube is the simplest throttling device. Due to the small aperture, when the fluid flows through the copper tube, it needs to overcome the resistance in the pipeline, resulting in a certain pressure drop. The diameter of the tube decreases, and the longer the tube length, the greater the pressure drop. big. The advantage is that it has a simple structure and no moving parts, while the disadvantage is that it has no adjustment ability and poor adaptability to working conditions. Mainly used in some cost-effective small equipment, such as air conditioners, refrigerators and so on.
2. Orifice throttling
For large-scale equipment with large cooling capacity, such as centrifugal chillers, the amount of refrigerant circulation is large, so the capillary tube is obviously not enough. When the pressure difference between the front and rear of the pipeline is large, the method of increasing the throttling orifice is often used. The principle is: fluid When flowing in the pipeline, due to the local resistance of the orifice plate, the pressure of the fluid is reduced and the energy is lost. This phenomenon is thermodynamically called the throttling phenomenon. This method is simpler than using a regulating valve, but it must be selected properly, otherwise, the liquid is prone to cavitation, which affects the safe operation of the pipeline.
Common throttling mechanism of refrigeration equipment
The function of the throttling orifice is to reduce the aperture at the appropriate place in the pipeline. When the liquid passes through the constriction, the flow beam will become thinner or contracted. The smallest cross-section of the stream appears downstream of the actual constriction and is called the systolic cross-section. At the systolic flow section, the flow velocity is the largest, and the increase of the flow velocity is accompanied by a great decrease in the pressure at the systolic flow section.
3. Thermal expansion valve
The thermal expansion valve uses the temperature sensor to sense the superheat degree of the refrigerant. When the superheat degree is high, it means that the evaporation is sufficient, the refrigerant has become a gaseous state, and there is still overheating. At this time, the pressure in the diaphragm cavity increases. Then push the valve stem downward, and finally increase the valve opening. If the superheat is low, it means that the evaporation is not enough. At this time, the pressure in the diaphragm cavity is reduced, and the diaphragm pushes the valve body to move upward, thereby reducing the valve opening. Through the above process, the control of flow and pressure drop is finally realized.
4. Electronic expansion valve
Compared with the thermal expansion valve, the electronic expansion valve uses a stepper motor for active adjustment, and its control target can be the superheat degree or the liquid level of the evaporator or condenser. For the thermal expansion valve, due to the thermal inertia of the temperature sensor itself, that is, the high degree of superheat at the outlet cannot immediately cause the operation of the expansion valve, so the operation is prolonged. The electronic expansion valve can act according to the real-time measurement of the liquid level or exhaust superheat, and immediately after the controller has calculated, there is basically no delay, and the adjustment performance is good.
Common throttling mechanism of refrigeration equipment
5. Floating ball throttle valve
For evaporators with free liquid surface, such as horizontal shell and tube evaporator, vertical tube or spiral tube evaporator, automatic adjustment of liquid supply. By means of the regulating action of the float regulating valve, a substantially constant liquid level can be maintained in these devices. At the same time, the float regulating valve has the function of throttling and reducing pressure. It can be divided into two types: straight-through and non-straight-through. The straight-through floating ball control valve has a relatively simple structure, but due to the impact of the liquid, the liquid level in the shell fluctuates greatly, which makes the operation of the control valve unstable, and the liquid flows from the shell into the evaporator, depending on the height of the hydrostatic column. poor, so the liquid can only be supplied below the liquid level of the container.
The non-straight-through float regulating valve works relatively stably, and can supply liquid to any part of the evaporator.
