The thermal expansion valve is an integral component in many parts that control the cooling of air. It can typically be found in a refrigerator or air conditioning unit, even if that unit happens to be in a vehicle. It can be abbreviated as TEV or TXV.
Refrigeration Cycle
The refrigeration cycle begins in the compressor when gas is transformed into a high pressure, high temperature state. In the condenser, this gas is condensed into a high temperature liquid, which enters the thermal expansion unit and is expanded into a low pressure, low temperature liquid. Finally, the evaporator takes in heat and changes the refrigerant from a liquid form into a vapor by boiling it. This cycle repeats within the refrigeration unit.
Function
The primary function of the thermal expansion valve is to meter out the total amount of refrigerant that flows into the evaporator. The amount of refrigerant allowed is dependent upon a sensing bulb that is typically found along the suction line at the place where the refrigerant leaves the evaporation coil. The sensing bulb is filled with liquid refrigerant, which is maintained continuously, and it is connected to the diaphragm of the TEV through a capillary tube. The reason why many vehicles use the TEV is because systems with a thermal expansion valve are often quite efficient at regulating the flow of refrigerant. This is due to the pressures that regulate the amount of refrigerant, and thus temperature, that the evaporator receives.
Three Forces
There are three forces within the thermal expansion valve: the bulb pressure, which senses the temperature leaving the evaporator and acts to open the valve, the evaporator pressure, which works to close off the flow of refrigeration just below the diaphragm, and the spring pressure, which represents the heat that the valve is controlling and also acts to close off the flow. In a state of equilibrium, the pressure of the first is usually equivalent to the pressure of the last two. If there is any change, the flow will either open wider or constrict.
Superheating
Any change in temperature along the suction line causes the refrigerant in the evaporator to boil at a quicker pace. This superheating increases the bulb pressure, which expands the diaphragm and allows more refrigerant to enter the valve in order to compensate for the high temperature. The increased flow rate is sensed by the evaporator bulb and restricts the refrigeration once more, returning the valve to a state of equilibrium. The opposite sequence can also occur, so that the flow is initially restricted and then returned to a state of equilibrium.
Maintenance
When the valve is opened to its maximum, then 10 to 12 degrees of superheat may be added. If there is too little superheat, then refrigerant may get into the compressor and wash out the oil or cause damage to the vehicle. Too much superheat could result in overheating issues. A working thermal expansion valve is important to the proper functioning of the vehicle.
