Subcooling
Also known as: liquid subcooling, subcooled liquid
Short definitionHow many kelvin the liquid leaving the condenser is colder than the condensing temperature at that pressure. Normal is 3–8 K; it is the best indicator of whether the charge is right.
After the vapour turns to liquid in the condenser, it travels a little further through the tubes and cools below the condensing temperature. If condensing is at 40 °C and 35 °C is measured on the liquid line, the subcooling is 5 K. This margin makes sure the expansion valve receives solid liquid with no bubbles. Each 1 K adds roughly 0.5–1% to capacity. For zeotropic blends the condensing temperature is read from the bubble column of the table.
In plants with long pipe runs or in hot climates, this margin is increased on purpose by adding a liquid-suction heat exchanger or a mechanical subcooler. In service, 0–2 K suggests a short charge; bubbles show in the sight glass. Above 10 K points to overcharge or liquid backing up in the condenser. If superheat is normal and subcooling is also between 3 and 8 K, the system is charged correctly.