Refrigerant coil heat transfer
Computair’s WinCoil and WebCoil selection software supports several types of coil: hot water, cold water, steam, run around, direct expansion / DX / evaporating and condensing. Most of these use water (or water with additives such as glycol) and do not involve any phase change. That is the water remains as a liquid or as a gas as it passes through the coil and doesn’t change state between liquid and gas. In evaporating coils and condensing coils, the circulating fluid (refrigerant) either boils from liquid to gas or condenses from gas to liquid. The associated release or absorption of latent heat forms a large part or most of the heat exchange rather than just a change of temperature. When Nigel Taffs wrote Computair’s first coil performance calculations in 1980 only a small number of refrigerants were in use and it was common to use simple graphs or tabulated data to determine heat transfer coefficients and changes in enthalpy. But a lot has changed since then, largely because of government regulation of the use of some of the fluids which have historically been used as refrigerants. Computair’s coil performance calculations now use the best available and proven mathematical models of refrigerant flow and heat transfer.
Composition (by mass for blends)
A few definitions
Regulatory changes affecting refrigeration:
In the 1980’s a relationship between the depletion of the ozone layer and the release of chlorofluorocarbons (CFC’s) and hydrochlorofluorocarbons (HCFC’s) was discovered and international treaties were agreed which eliminated the use of these compounds. That meant that in the 1990’s CFC refrigerants such as R12 became obsoleten the 21st century HCFC refrigerants such as R22 started to be phased out. Other classes of compound including hydrofluorocarbons (HFC’s) were used as replacements.
In the 21st century further international action took place limiting the use of compounds with a high global warming potential (GWP). The GWP of a gas is defined as its contribution to global warming relative to carbon dioxide. For instance, R410A has a GWP of 2088 meaning that 1kg of R410A released into the atmosphere contributes as much to global warming as 2088kg of carbon dioxide. This has led to the progressive phase out of HFC’s.