Piston vs. TXV
The piston (fixed orifice) and TXV (Thermostatic Expansion Valve) are the two most common metering devices in use today. It should at least be noted that a third type, the capillary tubes, were also used on straight cool devices, but their use is not common on most modern A/C systems.
The metering device is that component of the refrigeration circuit that creates pressure differential on the two sides of the system and allows the process of boiling to begin. It can be thought of as “the gatekeeper” of the refrigerant circuit. Without the metering device, the compressor would essentially act as a pump, very little pressure differential would be noticed in the system and the heat removal process would not function because changes from liquid to vapor and vapor to liquid would not occur.
A piston is a fixed size metering device with a fixed “bore”. The piston also allows the reverse flow of refrigerant as shown in the diagram to the left. In a heat pump system when the reversing valve is energized (cool mode), the unit will run in cool mode and the refrigerant will follow the path indicated on the bottom. This seats the piston so refrigerant must pass through the orifice. With the reversing valve de-energized the flow reverses as in the upper portion of the diagram. This unseats the piston and allows the free flow of refrigerant. In this case, there is a metering device in the condenser that meters the flow of refrigerant in heat mode.
The TXV is a device by which varying amounts of refrigerant are allowed to pass based on internal parts of the TXV and external environmental factors in the refrigerant circuit. Because the TXV can vary the amount of refrigerant, it operates more efficiently in varying environmental conditions (theoretically at least).
To operate, the TXV has a needle and seat that restricts the flow of refrigerant. This needle, when opened, allows more refrigerant to flow and, when closed, restricts refrigerant flow. There are three factors that affect the flow of refrigerant flow through a TXV. A sensing bulb filled with refrigerant exerts force to open the TXV. Since refrigerant pressure increases with heat, the bulb, which is attached to the suction line after the evaporator coil, senses how much heat the refrigerant has absorbed. If the suction line becomes too warm, the additional pressure created by the heated refrigerant opens the TXV more to allow additional refrigerant flow. A spring inside the bottom of the TXV exerts pressure to close the valve. An external equalizer, senses pressure in the suction line after the evaporator, and also works to close the valve. In essence the TXV is a constant superheat device, it sets a constant superheat at the evaporator outlet.
These two types of devices are charged in a system differently. The piston uses the superheat method of charging and the TXV uses the subcool method of charging. These are two very different methods so it is very important to know the type of metering device you are dealing with when charging a system. We will cover that next time.