Expansion Valve, Orifice Tube, and Refrigerant Flow Control
Core concepts
The expansion device meters refrigerant and separates high and low sides. Know two families: the thermostatic expansion valve (TXV), which modulates flow using a sensing bulb/charge to maintain superheat, and the orifice tube, a fixed restriction used with an accumulator. Understand TXV internals - diaphragm, needle, spring, sensing bulb - and how superheat control protects the compressor from liquid slugging. Bus systems with multiple evaporators may use multiple TXVs or a distributor.
Behavior and diagnosis
A TXV that is stuck open floods the evaporator (low superheat, frost, possible slugging); stuck closed starves it (high superheat, low suction pressure, no cooling). A restricted orifice tube causes low suction and poor cooling with a characteristic pressure drop. Know that sensing-bulb contact and insulation are critical - a loose bulb misreads superheat. Understand that charge level, airflow, and expansion device interact, so diagnose with superheat/subcooling rather than pressure alone.
Traps and a worked rule
Traps: adjusting a TXV to compensate for a low charge; confusing a frozen evaporator (often airflow or low charge) with a bad valve; and forgetting that an orifice tube has no adjustment. Rule of thumb: measure superheat at the evaporator outlet - high superheat suggests starvation (restriction, low charge, stuck-closed valve); low superheat suggests flooding (overcharge, stuck-open valve, poor airflow).
Worked example
Suction pressure low, superheat 25°F, and the liquid line is cold after the receiver. The high superheat plus a cold restriction point to a blocked liquid line or stuck-closed TXV - not an undercharge alone.