A TXV or Thermostatic Expansion Valve is a refrigerant metering device design to maintain a constant level of superheat. It regulates the superheat over a wide range of loads and outdoor temperatures. Most manufacturers of HVAC equipment designed for the TXV to maintain about 10-15 of superheat. A TXV maintains superheat by regulating the amount of refrigerant metered to the evaporator coil. This regulation occurs in response to changes in heat load and liquid pressure.
There are forces that control the TXV regulation, an opening force and a closing force. The opening force comes from the pressure from the sensing bulb which pushes down on the diaphragm. The closing force comes from internal spring tension pushing up from the bottom. The evaporator outlet pressure is directed by a small tube to the bottom of the diaphragm opposing the sensing bulb pressure. The amount of spring pressure (which can be adjusted) determines how much superheat is maintained by the TXV.
The sensing bulb is filled with an internal charge of refrigerant. When sensing a warm suction line, the pressure in the sensing bulb rises. So, if the evaporator needs more refrigerant, the suction line becomes warmer, the pressure in the sensing bulb increases. This increased pressure is sent to the top of the diaphragm, pushing down on the metering rod, and opening the orifice. When the evaporator coil floods, the suction line becomes cold, the pressure in the sensing bulb decreases. The pressure to the top of the diaphragm also decreases, thus restricting the flow of refrigerant.
How the two opposing forces regulate the refrigerant is simple:
There are forces that control the TXV regulation, an opening force and a closing force. The opening force comes from the pressure from the sensing bulb which pushes down on the diaphragm. The closing force comes from internal spring tension pushing up from the bottom. The evaporator outlet pressure is directed by a small tube to the bottom of the diaphragm opposing the sensing bulb pressure. The amount of spring pressure (which can be adjusted) determines how much superheat is maintained by the TXV.
The sensing bulb is filled with an internal charge of refrigerant. When sensing a warm suction line, the pressure in the sensing bulb rises. So, if the evaporator needs more refrigerant, the suction line becomes warmer, the pressure in the sensing bulb increases. This increased pressure is sent to the top of the diaphragm, pushing down on the metering rod, and opening the orifice. When the evaporator coil floods, the suction line becomes cold, the pressure in the sensing bulb decreases. The pressure to the top of the diaphragm also decreases, thus restricting the flow of refrigerant.
How the two opposing forces regulate the refrigerant is simple:
- If the spring tension plus the evaporator outlet pressure are greater than the sensing bulb pressure, the orifices is moved to a more closed position, throttling back the flow of refrigerant.
- If the spring tension plus the evaporator outlet pressure are less than the sensing bulb pressure, the orifices is moved to a more open position, increasing the flow of refrigerant.
- How the system reacts to changes in evaporator heat load
- System charge
- Changes in indoor temperatures