Electrothermal (Solid State) Expansion Valve
Thermal electric controlled expansion valve depends on the use of thermistors, directly exposed to refrigerant in the suction line to control the expansion valve needle opening. He does not use elements of pressure, as in the thermostatic expansion valve.
Resistance electrical flow in thermistor changes with temperature. The increase in temperature reduces resistance. Therefore, given the voltage rise of temperature also increases the speed of the current. This increased the current heats and bends bimetallic valve body, opening the valve. Fig. 5-26 illustrates a typical thermo-electrical expansion valve installation. Thermistor, C, is in direct contact with the refrigerant vapor inside the suction line from the evaporator.
Low voltage transformer in the power supply. It is connected with the expansion valve control mechanism " b". transformer in series with the thermistor and electrical devices on B. Increase of the current through the thermistor increases the outlet of expansion valve. This discovery allows to increase the flow of the refrigerant in the evaporator.
Refrigerant flow is controlled by the temperature in the suction line.
The control mechanism does not depend on pressure in the evaporator. On thermal expansion valve controls the suction line of overheating in order to avoid flooding of the compressor.
Fig. 5-27 shows a cross-section of electro-thermal expansion valve. It shows in some detail, the electrical connections and the mechanisms that control the operation of this expansion valve. The bleed valves have a small crack in the valve seat. This causes the system pressure on the balance during the off cycle. When on the next business cycle starts, the engine is started with almost no load. This allows the use of low starting torque of the electric motor of the compressor. Another goal is to prevent complete shut off at the end of the cycle of the machine On. The refrigerant is allowed to flow at discounted prices.
Full thermo-electrical expansion valve thermistor and ready for installation is shown in Fig. 5-28 view, A. Tine thermistor, electrically connected to the expansion valve, is displayed in a view B. out-of-cycle operation is possible in two ways. On thermal expansion valve can be electrically connected in parallel to the operating system. During the off cycle, thermistor becomes warm and thermal power expansion valve remains open. This discharge is similar to equalize the pressure in force when the capillary tube refrigerant control is used.
In the second case, the valve can be electrically connected (lock) in the motor circuit. It only under strain when the compressor is running. This type of connection, the valve will be closed outside the loop.