1. The role of thermostatic expansion valve:
The thermostatic expansion valve is installed at the evaporator inlet, often called the expansion valve, and has two main functions:
1) Throttling effect: after the high temperature and high pressure liquid refrigerant is throttled through the throttle hole of the expansion valve, it becomes a low-temperature and low-pressure mist hydraulic refrigerant, creating conditions for the evaporation of the refrigerant;
2) Control the flow of refrigerant: the liquid refrigerant entering the evaporator, after passing through the evaporator, the refrigerant evaporates from the liquid to the gaseous state, absorbs heat and reduces the temperature in the car. The expansion valve controls the flow of refrigerant to ensure that the outlet of the evaporator is completely gaseous refrigerant, if the flow is too large, the outlet contains liquid refrigerant, which may enter the compressor to produce liquid shock; If the refrigerant flow is too small, the evaporation is completed in advance, resulting in insufficient refrigeration;
2. Types of thermostatic expansion valves:
Thermostatic expansion valve according to different balancing methods, divided into internal balance type and external balance type; The externally balanced thermostatic expansion valve is divided into two structural types: F type and H type.
1) Structure and working principle of internal balance expansion valve:
Structural diagram of internally balanced F-type thermostatic expansion valve. The temperature sensing package is filled with refrigerant and placed on the evaporator outlet pipeline, and the upper part of the temperature sensing package and the diaphragm are connected by a capillary tube to feel the temperature of the refrigerant at the outlet of the evaporator, and the inlet pressure of the evaporator is felt under the diaphragm. If the load of the air conditioner increases and the hydraulic refrigerant is evaporated in advance in the evaporator, the temperature of the refrigerant outlet of the evaporator will increase, the pressure on the diaphragm will increase, the opening of the expansion valve will increase by pushing the valve stem, the flow of refrigerant entering the evaporator will increase, and the refrigeration capacity will increase; If the load of the air conditioner is reduced, the temperature of the refrigerant outlet of the evaporator is reduced, and the valve opening is reduced by the same principle of action, thereby controlling the flow of the refrigerant.
2) Structure and working principle of external balance expansion valve:
The principle of the external balanced expansion valve and the internal balanced expansion valve is basically the same, the difference is: the inlet pressure of the evaporator is felt under the diaphragm of the internal balanced expansion valve; Underneath the diaphragm of the externally balanced expansion valve is the outlet pressure of the evaporator.
3) H-type expansion valve
H-type thermostatic expansion valve has four interfaces connected to the refrigeration system, two of which are the same as ordinary thermostatic expansion valves, one connects to the reservoir dryer and one connects to the evaporator inlet; Two other connections, one for the evaporator outlet and one for the compressor inlet. The bulb is directly in the refrigerant gas stream at the evaporator outlet. The expansion valve improves the adjustment sensitivity, compact structure and reliable vibration resistance due to the elimination of the temperature sensing package, capillary tube and external balance pipe in the F-type thermostatic expansion valve.
