Expansion Devices and Refrigerant Flow Control
Metering Refrigerant
Expansion devices control refrigerant flow into the evaporator: thermostatic expansion valves (TXV), orifice tubes, and block-type valves. The TXV modulates flow based on evaporator outlet temperature/pressure to maintain a slight superheat, preventing liquid slugging the compressor. Orifice-tube systems rely on an accumulator to trap liquid and cycle the compressor via a pressure switch. Understanding which system you are looking at determines how you diagnose it.
Superheat and Subcooling
Superheat is the difference between actual suction line temperature and the saturation temperature at suction pressure; it confirms the evaporator is fully boiling refrigerant and no liquid returns. Subcooling is the difference between liquid line temperature and saturation temperature at high-side pressure; it confirms the condenser is fully condensing. TXV systems are tuned by superheat; orifice systems are not adjustable.
Traps to Watch
A classic trap: a TXV that is stuck open causes low superheat, frost on the suction line, and possible compressor damage; a TXV stuck closed causes high superheat and poor cooling with low suction pressure. Questions may describe an iced evaporator and blame the charge when the real cause is a stuck-open TXV or a faulty sensing bulb. Also watch for mixing up superheat and subcooling - read the question carefully. On orifice systems, do not attempt to adjust the tube; it is fixed.
Worked Rule of Thumb
Target superheat is typically in the range of about 8-12°F at the evaporator outlet for TXV systems (per manufacturer spec). If superheat is very low and the suction line is frosted, suspect an overfeeding TXV or low airflow. If superheat is very high with low suction pressure, suspect an underfeeding TXV, restriction, or low charge.