The throttling mechanism is one of the important parts in the refrigeration device, its function is to reduce the pressure of the saturated liquid (or supercooled liquid) under the condensing pressure in the condenser or the liquid receptacle to the evaporating pressure and the evaporating temperature after throttling, to achieve cooling purposes, adjust the flow of refrigerant into the evaporator to adapt to the evaporator load changes, the common throttling mechanism has the following several.
1. Capillaries
Capillary is the most simple structure of the throttle device, because of small aperture, fluid flow through the copper pipe, need to overcome the resistance in the pipe, resulting in a certain pressure drop, pipe diameter is reduced, the longer the pipe, the greater the pressure drop. The utility model has the advantages of simple structure, no moving parts, and the disadvantage of the utility model is that the utility model does not have the adjustment ability, and the adaptability to the working condition is poor. Mainly used in some cost-effective small equipment, such as air conditioning, refrigerators and so on.
2，Orifice plate throttling
For large-scale equipment with large refrigeration capacity, such as centrifugal water chillers, the refrigerant circulation is large, so the capillaries are obviously not enough. When the pressure difference between the front and the back of the pipeline is large, the method of increasing the orifice plate is often adopted, the principle is: fluid flow in the pipe, due to the orifice of local resistance, so that the fluid pressure reduction, energy loss, the phenomenon in thermodynamics called throttling phenomenon. This method is simpler than using the control valve, but must be selected properly, otherwise, the liquid is easy to produce cavitation phenomenon, affect the safe operation of the pipeline.
The function of orifice plate is to reduce the diameter of orifice in proper place of the pipe. When the liquid passes through the orifice, the stream will become thin or shrink. The minimum cross section of the stream appears downstream of the actual necking, which is called the necking section. The velocity is the maximum at the section of contraction, and the increase of velocity is accompanied by the decrease of pressure at the section of contraction.
The thermal expansion valve uses the temperature-sensing package to sense the superheat of the refrigerant. When the superheat is high, it means that the evaporation is sufficient, the refrigerant has become gaseous, and there is also superheat. At this time, the pressure in the diaphragm cavity increases, then push the stem down to increase the valve opening. If the superheat is low, the evaporation is not sufficient, at this time the pressure in the diaphragm chamber is reduced, the diaphragm pushing the valve body upward movement, thereby reducing the valve opening. Through the above process, the flow and pressure drop control is finally realized.
4. Electronic expansion valve
Compared with the thermal expansion valve, the electronic expansion valve uses a stepping motor for active regulation, its control target can be overheating, but also can be evaporator or condenser level. For the thermal expansion valve, because the temperature package itself has thermal inertia, that is, the export of high superheat can not immediately cause the action of the expansion valve, so there is action extension. Electronic expansion valve can be based on real-time measurement of the liquid level or exhaust overheating, after the controller operation immediately after action, basic no delay, regulation performance is good.
5，Float ball throttle
For evaporators with a free surface, such as horizontal shell-tube evaporator, vertical tube or spiral tube evaporator for the automatic adjustment of liquid supply. The level of liquid in these devices can be kept approximately constant by means of a float regulating valve. At the same time, the floating ball control valve has the function of throttling pressure reduction. Can be divided into straight-through and non-straight-through two types. The structure of the straight-through floating ball control valve is simple, but the fluctuation of the liquid level in the shell caused by the impact of the liquid is large, which makes the operation of the control valve unstable, and the liquid flows into the evaporator from the shell, it depends on the height difference of the hydrostatic column, so the liquid can only be supplied below the level of the container.
Non-straight-through floating ball control valve work more stable, and can supply liquid to any part of the evaporator.
