The piston (fixed orifice) and TXV (Thermostatic Expansion Valve) are the two most common metering devices in use today, with some modern systems utilizing an electronically controlled metering device called an EEV (Electronic Expansion Valve). It should at least be noted that there are other types of fixed orifice metering devices like capillary tubes, but their use is not common on most modern A/C systems though you will see them in refrigeration.
While the compressor creates the pressure differential to get the refrigerant moving, by decreasing the pressure on the suction and increasing the pressure on the discharge side, the purpose of the metering device is to create a pressure drop between the liquid line and the evaporator coil or expansion line (the line between the metering device and the evaporator when there is one). When the high-pressure liquid refrigerant is fed into the metering device on the inlet the refrigerant flows out the other side and the immediate pressure drop results in an expansion of a percentage of the liquid directly to vapor known as “flashing”. The amount of refrigerant that “flashes” depends on the difference in temperature between the liquid entering the metering device and the boiling temperature of the refrigerant in the evaporator. If the difference is greater, more refrigerant will be “flashed” immediately and if the difference is less than less refrigerant will be flashed.
A piston is a replaceable metering device with a fixed “bore”. It is essentially a piece of brass with a hole in the center, the smaller the bore the less refrigerant flows through the piston and vice versa. The advantage of a piston is that it is simple and it can still be removed, the bore size changed and cleaned if required.
Some piston systems also allow the reverse flow of refrigerant as shown in the diagram to the above. 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. This unseats the piston and allows the free flow of refrigerant. In this case, there is a metering device in the condensing unit (outside unit) that meters the flow of refrigerant in heat mode and one inside that meters in cooling mode.
The TXV can vary the amount of refrigerant flow through the evaporator by opening and closing in response to evaporator heat load. compared to a fixed orifice a TXV 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 and acts as the orifice. 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 gas pressure increases with a rise in temperature, the bulb, which is attached to the suction line after the evaporator coil, “senses” the temperature of the suction line. 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 (relatively) constant superheat at the evaporator outlet by balancing bulb, spring and equalizer pressures.
The primary method of charging a system changes based on the type of metering device. A piston system uses the superheat method of charging and the TXV uses the subcooling method of charging.
No matter what primary method of charging you use it is still important to monitor suction pressure (Evap temperature) head (condensing temperature), Superheat, subcool and delta t (or some other method of air flow verification).
While a TXV and a piston function differently the end result is a pressure drop and boiling refrigerant in the evaporator.