Expansion valve, also known as throttle valve, is a pressure and temperature sensing valve. It is a main component of automobile air conditioning and refrigeration system.
At present, the expansion valve mainly has four structural forms: internal balance thermal expansion valve, external balance thermal expansion valve, H-type expansion valve and expansion throttle pipe (orifice pipe).
(1) Internal balance thermal expansion valve
The internal balance thermal expansion valve throttles and depressurizes the high-pressure liquid refrigerant from the receiver dryer, that is, the liquid high-pressure refrigerant is ejected from its orifice 6 and expands rapidly into a low-pressure mist for endothermic gasification. In addition, it also regulates and controls the liquid refrigeration dose entering the evaporator to adapt to the change of refrigeration load, and prevents the liquid hammer of the compressor and abnormal overheating of the steam at the outlet of the evaporator. Use the temperature sensing package installed at the outlet of the evaporator to sense the superheat of refrigerant vapor,
Thus, the opening of the liquid refrigerant expansion valve is controlled. The temperature sensing package is in contact with the evaporator outlet pipe. When the evaporator outlet temperature decreases, the liquid volume in the temperature sensing package 1, capillary 2 and film 3 shrinks, the pressure decreases, and the valve port will be closed to restrict the refrigerant from entering the evaporator. Instead, the orifice opens and the refrigerant flows into the evaporator.
As the needle valve opens, more refrigerant enters the evaporator, the pressure in the evaporator rises, the return air temperature decreases, the pressure under the diaphragm increases, and the valve closes. Because the pressure on the upper and lower sides of the diaphragm is in an unbalanced state, the orifice is constantly opened and closed to match the refrigeration device with the load.
The temperature sensing package and evaporator must be in close contact and cannot be connected with the atmosphere. If the contact is poor, the temperature sensing package cannot correctly sense the temperature at the outlet of the evaporator. If the seal is not tight, the induced temperature is the atmospheric temperature. Therefore, a special kind of air conditioning tape should be used to bind and seal the temperature sensing package.
2) External balance thermal expansion valve
The structure of external balance and internal balance thermal expansion valve is similar, and the pressure under the internal balance diaphragm
The force is the inlet pressure of the evaporator, while the pressure under the outer balanced diaphragm is the outlet pressure of the evaporator. Due to pressure loss inside the evaporator, the outlet pressure of the evaporator shall be less than the inlet pressure. To achieve the same valve opening, the external balance type needs less superheat, and the volumetric efficiency of the evaporator can be improved. External balance thermal expansion valve shall be selected for bus air conditioning system.
(3) H-type expansion valve H-type expansion valve is named because its internal channel is H-shaped. It cancels the external balance pipe and temperature sensing bag of the external balance expansion valve and is directly connected with the inlet and outlet of the evaporator. It has four interfaces to the air conditioning system, two of which are the same as the ordinary expansion valve, one is connected to the outlet of the drying filter and the other is connected to the inlet of the evaporator. The other two interfaces are connected to the evaporator outlet and the compressor inlet. The temperature sensing element is in the refrigerant flow entering the compressor. H-type expansion valve has the advantages of compact structure, reliable use and simple maintenance, which meets the requirements of automobile air conditioning.
The expansion valve is installed between the inlet and outlet pipes of the evaporator. The induction temperature is not affected by the environment, nor does it need to pass through the capillary tube to cause time delay. The adjustment sensitivity is high. Since there is no temperature sensing package, capillary tube and external balance pipe, the normal operation of the expansion valve will not be affected by the fracture and leakage of the filling system and the loosening of the temperature sensing package due to the bumping of the vehicle.
(4) Expansion throttle pipe (orifice)
Expansion throttling tube is a throttling device with fixed orifice used in many car refrigeration systems. Someone said
It is a hole tube and a fixed hole tube. The expansion throttle pipe is directly installed between the outlet of the condenser and the inlet of the evaporator to throttle and depressurize the liquid refrigerant. Since the flow cannot be adjusted, the liquid refrigerant is likely to flow out of the evaporator and enter the compressor, resulting in liquid hammer of the compressor. Therefore, for the system equipped with expansion throttle pipe, a liquid collector must be installed between the evaporator outlet and the compressor inlet at the same time to implement gas-liquid separation and avoid liquid hammer of the compressor.
At present, the temperature control methods used in the expansion throttle pipe system include: circulating clutch expansion throttle pipe system (CCOT), variable volume expansion throttle pipe system (VDOT), fixed expansion throttle pipe clutch system, etc. The structure of expansion throttle pipe is shown in the figure. It is a thin copper tube, which is installed in a plastic sleeve. A sealing ring is installed in the outer annular groove of the plastic sleeve. Some also have two outer annular grooves, each with a sealing ring. Insert the plastic sleeve together with the expansion throttle pipe into the evaporator inlet pipe, and the sealing ring is used to seal the matching gap between the outer diameter of the plastic sleeve and the inner diameter of the evaporator inlet pipe. Filter screens are installed at both ends of the expansion throttle pipe to prevent system blockage. Pollution in the system after installation and use
The debris gathered behind the sealing ring, making the blockage worse. It is the pollutants in this system that block the orifice pipe and its filter screen. The expansion throttling pipe cannot be repaired. If it is broken, it can only be replaced.
Because the expansion throttle tube has no moving parts, it has the advantages of simple structure, high reliability and energy saving, which is adopted by many high-end cars. The disadvantage is that the refrigerant flow cannot be adjusted according to the change of working conditions.