The molecules of refrigerants in the halocarbon refrigerant family are made up of some or all of the following elements: carbon, hydrogen, chlorine, and fluorine. Examples of the molecular structures of several classes of halocarbons are shown in Fig. 12.1. Because some of the halocarbons are considered environmentally safe and some others damaging, the industry has adopted a practice of distinguishing among them by the designations of CFC, HCFC, and HFC.
Figure 12.1a shows the structure of a chlorofluorocarbon (CFC) such as R-12, which is a nonhydrogenated (no hydrogen) halocarbon. The chemical is extremely stable, which is a desirable feature for a refrigerant, but when released to the atmosphere it ultimately diffuses to the. upper atmosphere. In the upper atmosphere it breaks down, and the chlorine combines with ozone that exists there, depleting the ozone concentration. While ozone has harmful effects at the earth’s surface, ozone in the upper atmosphere has the beneficial role of blocking some of the sun’s ultraviolet rays. Too high an intensity of ultraviolet rays could result in a greater incidence of skin cancer among the earth’s occupants.
The second group of halocarbons is hydrogenated, because it contains a hydrogen atom, as Fig. 12.1b shows. This group is called the hydrochlorofluorocarbons (HCFCs), of which R-22 is an example. Because of the hydrogen atom, the chemical is not quite as stable as a CFC, so when released to the atmosphere, most of it breaks down before reaching the ozone layer. An HCFC, therefore, is likely to be much less damaging to the ozone layer.
Yet another group of halocarbons, called the hydrofluorocarbons (HFCs), is illustrated in Fig. 12.1c. This chemical not only breaks down before reaching the stratosphere, it contains no ozone-depleting chlorine. The particular refrigerant shown in Fig. 12.1c is R-134a, which has similar saturation properties to R-12 and is the front runner for a replacement of R-12. Molina and Rowland (1974) first hypothesized that the presence of CFCs in the upper atmosphere contributed to depletion of stratospheric ozone (O3).
Further tests and analyses confirmed the perils of continued discharge of CFCs into the atmosphere. A further detriment of the CFCs in the atmosphere is the global warming potential due to the greenhouse effect. Perhaps the discharge of CO and CO2 represents a more signifcant risk than does the discharge of the CFCs, but the threat must be attacked on all fronts. Table 12.1 shows the ozone depletion potential (ODP) and the global-warming potential (GWP) of some CFC, HCFC, and HFC refrigerants. Ammonia, which is not listed, neither depletes ozone nor causes global warming.