System Components and Refrigeration Cycle Fundamentals
The four functions
Compression (compressor), condensation (condenser), metering (TXV, piston/fixed orifice, EEV, capillary), and evaporation (evaporator). Only the compressor adds energy to the refrigerant; the metering device drops pressure; both coils are heat exchangers, and the condenser rejects the evaporator load plus the heat of compression.
Reading the cycle
High side: desuperheating, condensing, then subcooling. Low side: flashing, evaporating, then superheating. For a near-azeotropic refrigerant, saturation temperature is fixed by pressure alone. Subcooling = liquid saturation temperature minus liquid line temperature. Superheat = suction line temperature minus suction saturation temperature.
Component interactions to know cold
- TXV: modulates to hold a constant superheat (typically 8–12 °F); verify charge by subcooling or weigh-in.
- Fixed orifice/piston: superheat varies with load; verify charge against a target superheat that depends on indoor wet bulb and outdoor dry bulb.
- Scroll vs. reciprocating: scrolls are quieter with fewer moving parts but do not tolerate liquid slugging.
- Accessories: filter drier (moisture and acid), accumulator (heat pump protection), receiver, crankcase heater (limits oil migration and dilution), reversing valve.
Traps
Trying to read superheat at the compressor service port - measure at the evaporator outlet, roughly 6–10 in. from the coil. Assuming low suction pressure always means low charge; it can be a restriction, low airflow, or a closed valve. Assuming a heat pump is charged identically in heating mode - it is not.
Worked example (target superheat, fixed orifice)
TSH = (3 × indoor WB − 80 − outdoor DB) / 2 = (3 × 63 − 80 − 95) / 2 = 7 °F. A measured 15 °F indicates undercharge after airflow is verified.