System Components
System Components
This domain tests how compressors, heat exchangers, metering devices, and accessories interact. The exam favors component-function questions and simple diagnostic deductions from component behavior.
Core rules
- A reciprocating, scroll, or rotary compressor moves vapor; liquid slugging damages it. Suction accumulators and crankcase heaters help prevent liquid return.
- The condenser desuperheats, condenses, and subcools. A dirty coil or fan fault raises head pressure and subcooling may rise if liquid backs up.
- The evaporator boils refrigerant; airflow and load determine its capacity. A starved evaporator has high superheat; a flooded one has low superheat.
- A TXV modulates to hold superheat using a bulb, external equalizer, and spring/adjustment. An EEV uses a stepper motor and controller.
- Filter driers remove moisture and debris; install in the direction of flow. A receiver stores liquid on the high side; an accumulator traps liquid on the low side.
- Pressure switches and safeties protect the compressor; never bypass them as a diagnostic shortcut.
Worked example
A TXV system has low suction pressure, high superheat, and a frost line at the evaporator inlet. The liquid line sight glass is clear, and the condenser is clean. Reasoning: a clear sight glass does not prove adequate flow. High superheat with a starved coil points to a restriction at the TXV inlet screen, a failed power element, or a bulb with lost charge. Check the screen and bulb contact before adjusting the superheat screw; adjusting a failed valve will not fix the starved coil.
Traps
Installing a filter drier backward; clamping a TXV bulb loosely or on the wrong line; ignoring an external equalizer; and diagnosing a failed compressor when the real fault is a pressure switch or contactor. Also, a receiver is not an accumulator.