Refrigeration Systems and Refrigerant Handling
Cycle fundamentals and the two measurements that decide everything
The vapor-compression cycle is compression, condensation, metering, evaporation. The core skill is converting gauge readings to saturation temperatures on a PT chart, then comparing those saturation temperatures to measured line temperatures. Superheat = suction line temperature minus saturation temperature at suction pressure; it tells you how much of the evaporator is being used and protects the compressor from liquid return. Subcooling = saturation temperature at liquid-line pressure minus liquid line temperature; it tells you how much liquid is stacked in the condenser.
Match the charging method to the metering device
A fixed orifice (piston) system is charged by superheat, using the manufacturer's chart that takes indoor wet-bulb and outdoor dry-bulb as inputs, because target superheat changes with load. A TXV or EEV system is charged by subcooling, typically 8-12°F, because the valve holds superheat roughly constant regardless of charge.
Handling essentials
Never vent refrigerant. Recover into a DOT-approved cylinder filled to no more than 80 percent of its water capacity by weight; never heat a cylinder above 125°F or with an open flame. Blends such as R-410A are charged as liquid. Zeotropic blends such as R-407C have temperature glide, so bubble and dew points differ and must be read separately. Evacuate to 500 microns or lower, pull Schrader cores, use large-diameter hoses, flow nitrogen while brazing, and remember POE oil is hygroscopic.
Trap
Questions love a superheat value calculated from a pressure that was never converted, or one implying a system can be charged accurately by pressure alone. Pressure substitutes for temperature only when the refrigerant is saturated.
Worked example
R-22, suction gauge 69 psig. The PT chart gives 40°F saturation. Suction line measures 52°F, so superheat is 12°F - normal for a fixed-orifice system. A 30°F superheat at that same pressure points to undercharge or a restriction.