Metering Devices & Expansion
TXV operation, capillary tubes, electronic expansion valves, and proper superheat settings for each device type.
- Explain TXV operation and its three controlling forces
- Describe capillary tube function and applications
- Compare electronic expansion valves to thermostatic types
- Set and verify proper superheat for each metering device type
Lesson 1
Thermostatic Expansion Valve (TXV) Operation
TXV Function
The function of a thermostatic expansion valve (TXV) is to meter refrigerant flow based on superheat. The TXV is the most common metering device in commercial and residential systems because it automatically adjusts refrigerant flow to match the evaporator load.
Three Controlling Forces
The TXV operates based on the balance of three forces acting on a diaphragm:
When the suction line temperature rises (increasing superheat), bulb pressure increases and opens the valve wider, allowing more refrigerant to flow. When superheat decreases, the valve closes to reduce flow.
Normal TXV Superheat
The normal superheat range for a TXV system is 8 to 12 degrees F. The superheat spring is factory set or field-adjusted to maintain this range. If superheat is too low, the evaporator floods and liquid may reach the compressor. If superheat is too high, the evaporator is underutilized.
A TXV meters refrigerant flow based on superheat using three forces: bulb pressure (opens), evaporator pressure (closes), and spring pressure (closes). Normal TXV superheat is 8 to 12 degrees F.