Photograph of an air conditioning thermal expansion valve for a heat pump Subcooling
Definition, measurement, use of subcooling in HVACR

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Defintion of Subcooling

Thermostatic expansion valve schematic (C) Carson Dunlop Associates

This article discusses and defines subcooling, a term used in refrigeration, air conditioning, and heat pump design, installation, & repair.

[Click to enlarge any image] Schematic of a thermostatic expansion valve courtesy of Carson Dunlop Associates.

Definition of Subcooled Refrigerant

A sub-cooled liquid refrigerant is at a temperature below (colder-than) the temperature at which the refrigerant would evaporate ("boil" or change from a liquid refrigerant to a gas - also called the refrigerant saturation temperature).

Subcooling is defined as the difference between condensing pressure/temperature and liquid temperature at the expansion valve inlet.

Subcooling of the refrigerant is necessary to avoid vapour bubbles in the refrigerant ahead of the expansion valve. Vapour bubbles in the refrigerant reduce capacity in the expansion valve and thereby reduce liquid supply to the evaporator. ubcooling of 4-5K is adequate in most cases. - Danfoss (2005) 

Measurement of Subcooling

Subcooling is measured in Kelvin (K) or °C.

Subcooling temperatures are measured on the refrigerant line just ahead of (supplying refrigerant into) the TEV.

Subcooling is measured in degrees of temperature (on any scale) and can be defined as the difference in temperature degrees between the liquid refrigerant's saturation temperature and the current or actual liquid refrigerant temperature.

Higher subcooling temperatures of a liquid refrigerant mean a more efficient HVACR system operation beause more heat is being removed per unit volume (or unit weight) of refrigerant circulating in the system.

Thus some HVACR technicians measure subcooling to take a look at the operating efficiency of the system and to compare it with the manufacturer's specifications. Higher subcooling numbers mean that the equipment will have to run less time to adequately cool the area being refrigerated or air conditioned.

Lower or too-low subcooling temperatures risk accidental conversion of liquid refrigerant to gas state within the piping system before it reaches the refrigerant metering device (capillary tube or thermostatic expansion valve (TEV)), reducing the efficiency of the system and possibly interfering with the proper operation of the TEV. - adapted from Emerson Climate Technologies (2005)

Danfoss (2005) TEV fitters notes provides details about subcooling on HVACR equipment, from which we quote this excerpt:


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Publisher - Daniel Friedman