Air Conditioning and Refrigeration
Refrigeration is a cycle, not a mystery: compress, condense, meter, evaporate. Know the four components and the state of refrigerant at each - low-pressure vapor into the compressor, high-pressure vapor out, liquid after the condenser, and a low-pressure liquid/vapor mix after the metering device. Understand superheat (vapor temperature above saturation) and subcooling (liquid temperature below saturation) as the two diagnostic windows into charge and metering.
Know the pressure-temperature relationship: for a given refrigerant, saturation pressure maps to saturation temperature. That is why a gauge set plus a thermometer tells you superheat and subcooling. Understand latent heat of vaporization, the role of the evaporator in absorbing heat, and why airflow problems masquerade as charge problems. SEER2 and HSPF2 are the current efficiency metrics; tonnage is capacity, not efficiency.
Traps: adding refrigerant to fix a low-suction problem caused by low airflow; confusing superheat with subcooling; and forgetting that refrigerant blends must be charged as liquid. Recovery, recycling, and leak detection are legal obligations, not optional.
Worked example: R-410A at 118 psig saturates near 40°F. If suction line temperature is 52°F, superheat is about 12°F - in a typical target range for a fixed-orifice system, suggesting charge and airflow are reasonable. High superheat with normal pressures points to undercharge or a restriction; low superheat points to overcharge or an overfeeding metering device.