Air Conditioning System Components and Controls
Four functions, the hardware
Every system has a compressor (raises pressure and temperature), condenser (rejects heat), expansion device (drops pressure, meters flow) and evaporator (absorbs heat). Around them sit the accessories: filter drier for moisture and debris, sight glass with moisture indicator, receiver, accumulator to protect the compressor from liquid, solenoid valve, and crankcase heater.
Expansion devices and controls
A capillary tube is a fixed restrictor matched to the charge and application; an automatic expansion valve holds constant evaporator pressure; a thermostatic expansion valve holds constant superheat, typically 5–8 K, using a bulb clamped to the suction line. An EEV does the same job under electronic control, common in inverter units. Safety controls are the high-pressure cut-out, low-pressure cut-out, oil-pressure switch, overload and anti-frost thermostat.
Worked example
Suction pressure 4.8 bar gauge on R-22 gives an evaporating temperature of about −5 °C; the suction line reads +6 °C. Superheat = 6 − (−5) = 11 K. That is high for a TXV set at 6 K, so the valve is starving the coil - suspect a partial restriction, a lost bulb charge, or undercharge. Diagnose before adding gas.
Traps
Turning the TXV adjusting screw before measuring superheat; treating the HP cut-out as an operating control when it is purely a safety; and forgetting the crankcase heater must be energised before starting a cold compressor.