Compressors, condensers, evaporators, expansion devices and accessories
What this domain tests
Construction and selection of compressors, condensers, evaporators, expansion devices and line accessories, and diagnosing their performance from pressures, temperatures, superheat and subcooling.
Core rules
- Compressors: reciprocating (hermetic, semi-hermetic, open belt), rotary vane for small room units, scroll for splits and VRF, screw and centrifugal for large plants. Clearance volume lowers volumetric efficiency; liquid slugging damages valves.
- Condensers: air-cooled, water-cooled (shell-and-tube, tube-in-tube) and evaporative. Approach = condensing temperature minus leaving cooling-medium temperature. A dirty air-cooled coil raises condensing pressure, current and discharge temperature.
- Evaporators: dry-expansion (DX), flooded and starved. Low-temperature coils need wider fin spacing so frost does not bridge fins; defrost may be off-cycle, electric, hot gas or air.
- Expansion devices: capillary (matched to charge, no adjustment), thermostatic expansion valve (superheat control, MOP charge, external equaliser on distributor coils), automatic expansion valve (constant evaporator pressure), electronic expansion valve (wide range, precise superheat), float valve for flooded systems.
- Accessories: filter-drier for moisture and dirt, sight glass (bubbles mean flash gas or undercharge), receiver, accumulator, oil separator, solenoid valve, HP/LP cut-outs, vibration eliminator.
Worked example
An R134a system shows a suction gauge reading of 1.9 bar; with atmospheric pressure 1.0 bar the absolute pressure is 2.9 bar. Saturation temperature at 2.9 bar is about 0 °C. The measured suction line temperature is 7 °C, so superheat = 7 − 0 = 7 K, inside the normal 5–8 K band. A reading of 15 K suggests a starved or undercharged system; 1 K means the valve overfeeds and liquid return is likely.
Traps
Calculating superheat from gauge instead of absolute pressure; calling condenser approach superheat; mismatching capillary bore and length; missing a blocked filter-drier as the cause of flash gas; ignoring condenser approach when diagnosing high head pressure.