Refrigeration Systems
Components and metering
Commercial refrigeration questions center on the metering device, the evaporator, and controls. A TXV maintains constant superheat across varying load; a piston/capillary is fixed and must be charged by superheat; an EEV is driven by a controller and often a sensor. Know the difference between a pump-down system with a low-pressure control and a simple off-cycle defrost cooler, and know the three defrost methods: off-cycle, electric, and hot gas.
Charging and service practice
Subcooling charges a TXV system; superheat charges a fixed orifice. Both are measured, not guessed. Evacuation targets a standing vacuum of 500 microns or lower (and a decay test to confirm no moisture or leak). Never pressurize with oxygen; use dry nitrogen through a regulator. HFC systems use POE oil, which absorbs moisture quickly - leaving a system open is a real defect, not a style issue. Venting refrigerant is prohibited; recovery is required under EPA Section 608, and technicians need certification.
Worked example
An R-410A system reads 118 psig suction, which corresponds to about 40°F saturation. The suction line measures 52°F. Superheat = 52 − 40 = 12°F. If the unit has a piston and the target superheat for the day's indoor wet-bulb and outdoor temperature is 8°F, the system is slightly undercharged or the evaporator load is low. If it has a TXV, check subcooling instead before adding refrigerant.
Traps: blend glide and fractionation (charge as liquid from a cylinder), and mixing up saturation temperature with line temperature.