Subcooling

The term subcooling (also called undercooling) refers to the intentional process of cooling a liquid below its normal boiling point.

The subcooling stage provides certainty that the refrigerant is fully liquid before it reaches the next step on the cycle, the thermal expansion valve, where the presence of gas can be disruptive.

Subcooling and superheating, which are similar and inverse processes, are both important for the stability and well-functioning of a refrigeration system.

[3] These may end up leading to behaviors similar to those observed with the flash-gas phenomena: problems in oil regulation throughout the cycle;[4] excessive and unnecessary misuse of power and waste of electricity; malfunction and deterioration of several components in the installation; irregular performance of the overall system and, if unmonitored, ruined equipment.

Superheating is analogous to subcooling in an operative way, i.e., occurring prior to a stage where refrigerant in a liquid-gas state would disrupt the cycle (uncompressible liquid-gas mixtures will destroy the compressor) and both processes can be coupled using an internal heat exchanger.

Normally, the fluid that is being subcooled is hotter than the refrigerant that is being superheated, allowing an energy flux in the needed direction.

[citation needed] SpaceX's Falcon 9 and Starship launch vehicles employ subcooling for propellants.

[1] This concept refers mainly to devices such as internal heat exchangers, independent subcooling cascades, economisers or boosters.

Subcooling is a key factor in improving the efficiency of refrigeration systems, which has led to extensive research.

These compressors can inject refrigerant from an internal heat exchanger, rather than the main evaporator, into the final stage of the compression process.

It is nearly impossible to avoid slight undercooling or overheating during this process, making superheating and subcooling inherent and unavoidable in conventional vapor-compression refrigeration systems.

Significant research is being conducted on this topic, focusing on multi-stage processes, ejectors, expanders, and various other devices and enhancements.

Gustav Lorentzen proposed modifications to the cycle, including two-stage internal subcooling, for these systems.