The expansion device sits between condenser and evaporator. It works like the nozzle on a garden hose. A small opening holds back pressure and lets a controlled flow through.

Capillary tube: a long thin copper tube with a fixed bore. Cheap, no moving parts. It cannot adjust to changing load, so it suits small units with steady conditions, like fridges.

Thermostatic expansion valve (TXV): a mechanical valve with a sensing bulb clamped on the suction line. If superheat rises, the bulb pressure opens the valve more. If superheat falls, it closes. It self-adjusts within its design range.

Electronic expansion valve (EEV): a valve moved by a stepper motor. A controller reads suction temperature and pressure, calculates superheat and sets the valve position. It responds fast and works over a wide range. Most inverter heat pumps use an EEV.

Worked example — measuring superheat in the field.

• Suction gauge reads a pressure whose saturation temperature (from the refrigerant P-T chart) is 2 °C.
• Clamp thermometer on suction line near the compressor reads 8 °C.
• Superheat = 8 − 2 = 6 K. This is within a typical target of about 4–8 K.

Subcooling is measured the same way on the liquid line: saturation temperature at discharge pressure minus liquid line temperature.

Remember

• The expansion device creates the low-pressure side.
• Capillary is fixed; TXV and EEV adjust.
• EEVs control superheat using sensors and a controller.
• Superheat too low risks liquid return; too high wastes evaporator surface.