Subcooling

The term subcooling refers to a liquid existing at a temperature below its normal boiling point. For example, water boils at 373 K; at room temperature (300 K) the water is termed "subcooled". A subcooled liquid is the convenient state in which, say, refrigerants may undergo the remaining stages of a refrigeration cycle. Normally, a refrigeration system has a subcooling stage, allowing technicians to be certain that the quality, in which the refrigerant reaches the next step on the cycle, is the desired one. Subcooling may take place in heat exchangers and outside them. Being both similar and inverse processes, subcooling and superheating are important to determine stability and well-functioning of a refrigeration system.

Subcooling is normally used so that when the cycling refrigerant reaches the thermostatic expansion valve, its totality is in its liquid form, thus, allowing the valve to work properly. If gas reaches the expansion valve, in a refrigeration system, a series of usually unwanted phenomena may occur. These may end up leading to behaviors similar to those observed with the flash-gas phenomena: problems in oil regulation throughout the cycle; excessive and unnecessary misuse of power and waste of electricity; malfunction and deterioration of several components in the installation; irregular performance of the overall systems, and, in a totally unwatched situation, even ruined gear.

Another important and very common application of subcooling is its indirect use on the superheating process. Superheating is analogous to subcooling in an operative way, and both processes can be coupled using an internal heat exchanger. Subcooling here serves itself from the superheating and vice versa, allowing heat to flow from the refrigerant at a higher pressure (liquid), to the one with lower pressure (gas). This creates an energetic equivalence between the subcooling and the superheating phenomena when there is no energy loss. Normally, the fluid that is being subcooled is hotter than the refrigerant that is being superheated, allowing an energy flux in the needed direction. Superheating is critical for the operation of compressors because a system lacking it may provide the compressor with a liquid gas mixture, situation that generally leads to the destruction of the gas compressor because liquid is uncompressible. This makes subcooling an easy and widespread source of heat for the superheating process.

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