System Performance and Charging
What this domain tests
Performance and charging questions test how you prove a system is correctly charged and operating-using superheat, subcooling, condenser approach, and weigh-in-rather than guessing from suction pressure alone.
Core rules and concepts
- On a TXV system, charge to the manufacturer’s target subcooling. On a fixed-orifice or capillary system, charge to target superheat.
- Weigh-in is required for blends and critical charges. Charge zeotropic blends as liquid to avoid fractionation.
- Blends have glide. Use the bubble point for liquid subcooling and the dew point for evaporator superheat.
- Condenser approach = condensing temperature minus ambient temperature. A high approach suggests a dirty coil, poor airflow, or noncondensables.
- Measure evaporator superheat at the evaporator outlet, not at the compressor, because suction-line pickup adds superheat.
- Typical TXV evaporator superheat is about 8–12°F, but always use the manufacturer’s target. Subcooling targets vary by system and ambient.
Worked example
A system has 200 psig liquid-line pressure. The PT chart gives 100°F saturation. The liquid line is 92°F. Subcooling = 100°F − 92°F = 8°F. If the manufacturer targets 10°F, the system is slightly undercharged, provided the condenser is clean and there is no liquid-line restriction. Check the temperature drop across the filter-drier before adding refrigerant.
Traps that cost marks
Charging by suction pressure only; vapor-charging a blended refrigerant from the cylinder; using ambient temperature as condensing temperature; measuring superheat at the compressor; and ignoring glide when calculating superheat or subcooling.
Key points
- TXV systems are charged by subcooling; fixed orifices by superheat.
- Weigh in blends and critical charges; charge blends as liquid.
- Use bubble point for subcooling and dew point for superheat with zeotropic blends.
- Condenser approach reveals airflow and noncondensable problems.
- Measure superheat at the evaporator outlet.