In the air conditioning refrigeration system, the throttle device is one of the four basic components of the compression refrigeration system, which can reduce the high-pressure liquid from the condenser to the low-pressure liquid, and adjust the refrigerant flow into the evaporator.
1、The principle of throttling
The basic principle of the throttling device makes the refrigerant liquid flow through a small hole, the fluid channel first shrinks suddenly and then expands suddenly, the refrigerant flow rate first increases suddenly and then decreases suddenly, the refrigerant undergoes a process of violent turbulence and then slow expansion.
Because of the drastic change of the flow state, the pressure loss of the fluid flowing through the pores is very large, which makes the refrigerant liquid drop from high pressure to low pressure. The size and opening of the orifice determine the final level of depressurization and limit the amount of refrigerant flow through.
In the air conditioning refrigeration system, the commonly used throttle devices are capillary, throttle tube, thermal expansion valve, electronic expansion valve, floating ball valve, orifice plate, manual control valve. These throttling devices are different in structure and regulation, but in general can not be separated from the above throttling principle.
2、Throttling device
Capillaries：
A capillary is a long thin copper tube through which a small tube is throttled to reduce pressure and limit the flow of fluid. The utility model has the advantages of simple structure, high reliability and low price, so it is widely used in household air-conditioning, refrigerator and other small refrigeration devices.
The capillary belongs to the throttle orifice with fixed section, and its throttling effect is determined by its length and inner diameter. The disadvantage is that once the size is determined, it can not be adjusted according to the change of load conditions.
Short throttling tube：
The principle of the short throttle tube is to use the sudden change of the cross section of the tube to reduce the pressure of the refrigerant liquid by throttling, and also to limit the refrigerant flow.
The utility model has the advantages of simple structure, convenient manufacture, low price and convenient installation, and is suitable for small refrigeration systems such as automobile air conditioning.
Compared with the capillary, the diameter of the throttle tube is relatively large and the length is small, and the regulation accuracy is not as good as the capillary under the same pressure and flow regulation requirements.
The thermal expansion valve controls the opening degree of the valve through the change of the superheat degree of the evaporator outlet, thus throttling the pressure reduction and regulating the refrigerant flow, in turn also controls the superheat degree.
When the superheat of the refrigerant at the outlet of the evaporator increases, the working fluid temperature in the temperature sensing package increases, and the pressure above the diaphragm increases, pushing the valve stem down, making the valve opening increase and the flow rate increase. Conversely, the valve opening reduced, flow reduction.
Thermal expansion valve is divided into internal balance type and external balance type, the evaporating pressure at the inlet and outlet of the evaporator were collected. When the flow pressure loss in the evaporator is large, the external balance should be chosen, so that the measurement is more accurate and the adjustment error can be avoided.
Electronic expansion valve
Electronic expansion valve, the use of stepper motor drive valve needle rotation to control the opening of the valve, thereby regulating the refrigerant flow.
The electronic expansion valve, like the thermal expansion valve, is mainly regulated according to the superheat, but the reaction and action speed are faster in the regulation, and can work normally at lower temperature, and can be regulated according to other parameters of the system, so the control function is more extensive.
Float valve
The floating ball valve is suitable for evaporator with free liquid surface. When working, the floating ball moves up and down with the change of liquid level. At the same time, the floating ball valve also plays the role of controlling the liquid level, so that a certain amount of liquid is always maintained in the evaporator.
The floating ball valve has simple structure and is mostly used for the full liquid evaporator of the water chiller. But because of the fluctuation of the liquid level in the valve chamber, the impact of the floating ball on the valve core is big and easy to be damaged.
Throttle orifice plate
For centrifugal chillers and other large refrigeration capacity of equipment, such as capillary throttling device of the refrigerant flux is obviously not enough. At this point can choose throttle orifice plate, in a circular plate on a number of throttle holes, so that more refrigerant can be through.
The structure of orifice plate is simple and the price is cheap. Through the careful design of orifice size and layout, the effective control of refrigerant flow can be realized.
But the throttle orifice plate is also a constant section of the throttle orifice, can not be automatically adjusted according to the load changes. When precise control is required, it is usually used in conjunction with a thermal expansion valve or an electronic expansion valve.
There is also an adjustable section of the orifice plate, to a certain extent, according to the load changes can be adjusted refrigerant flow.
Manual control valve
Manual throttle valve, also known as the control valve or expansion valve, completely rely on manual regulation, is the most old-fashioned throttle valve. In industrial chillers using Freon refrigerants, manual throttling valves are usually installed on bypass lines as backup valves for use in automatic throttling unit maintenance.
The above throttling devices are widely used in refrigeration, chemical industry, petroleum and other fields. In practical application, they should be selected according to their different regulation modes and characteristics, combined with specific requirements and conditions.
