A TXV or thermostatic expansion valve is a component in a refrigeration system (an air conditioner is a type of refrigeration system) that regulates the flow of refrigerant to the evaporator coil. The refrigerant on the leaving side of TXV enters the evaporator coil as a low-pressure low temperature liquid and boils into a gas as it absorbs heat that from warm air passing through the evaporator. The TXV controls the amount of refrigerant passing through the coil to control the temperature of the coil. Too much refrigerant and the coil will be too cold and potentially ice up as well as some refrigerant may not boil completely prior to returning to the compressor. Too little refrigerant and the evaporator may not get cold enough to remove the necessary heat from the air.
A TXV uses a bulb and small tube called a capillary tube to sense the temperature of the suction line leaving the evaporator. The bulb is filled with refrigerant which expands and contracts based off the line temperature that it is connected to. as it expands and contracts it applies pressure to a diaphragm which has a spring inside which applies pressure the opposite side of the diaphragm. when the pressure produced by the bulb exceeds the spring pressure it allows more refrigerant to pass through the metering device into the evaporator coil.