3.3.1 — Metering Devices
Metering devices create the pressure drop between the high side and the low side and regulate refrigerant feed to match load. They are central to system stability: too much feed can cause floodback to the compressor; too little causes evaporator starvation and low capacity.
Thermostatic Expansion Valves (TXVs / TEVs)
A TXV meters refrigerant to maintain a target superheat at the evaporator outlet, using a sensing bulb and three internal forces that act on the valve pin: evaporator pressure (closing), bulb pressure (opening), and spring force (closing). When the forces balance, the valve holds its position and steady superheat results.
TXV Superheat Targets
Typical superheat targets in comfort and many medium-temperature applications are in the range of 5–8 K (approximately 9–14 °F), but the correct value depends on evaporator design, distributor/nozzle selection, and compressor protection strategy. Always use manufacturer guidance.
When adjusting: change the setting slowly and verify stability over time — TXV response is not instantaneous, and chasing short-term fluctuations leads to over-correction and hunting.
Fixed Metering Devices (Capillary Tubes, Fixed Orifices)
Fixed metering devices provide a near-constant restriction, so mass flow changes mainly with pressure differential and refrigerant properties. They are simple and reliable but less forgiving of charge errors and contamination.
Common indicators of a restriction in a fixed metering device: a pronounced temperature drop across the device (frost or icing at the inlet), low evaporator pressure, and low suction superheat. Because there is no active control, the system cannot compensate — these symptoms persist until the restriction is cleared or the device is replaced.
Electronic Expansion Valves (EEVs)
EEVs use a controller and sensors to regulate refrigerant feed, often improving part-load efficiency and control stability compared to TXVs. They require correct sensor placement and reliable electrical/control integrity — a sensor fault can drive the valve to an unsafe position (fully open or fully closed).
EEV Troubleshooting Sequence
- Check controller status and active fault codes.
- Verify sensor readings (°C / °F) are plausible for current operating conditions.
- Check valve steps or position command output from the controller.
- Confirm results with system pressures and superheat measurement at the evaporator outlet.