How far a liquid's temperature is below its saturation (condensing) temperature at the same pressure.
Subcooling is the saturated condensing temperature minus the actual liquid line temperature. If head pressure corresponds to 110 °F saturation and the liquid line reads 98 °F, the liquid is subcooled by 12 °F.
Subcooling shows how much liquid has backed up in the condenser. Low subcooling points to too little refrigerant; high subcooling points to too much, or to a restriction that holds refrigerant back in the condenser. It is the usual charging method for systems with a TXV or electronic expansion valve, read against the manufacturer's target.
For zeotropic blends, use the bubble point pressure-temperature value when calculating subcooling.
Exam work areas
E · Commissioning and start-up F · Maintenance, service and troubleshooting