Refrigerants

Refrigerants are used as heat exchange media in refrigerant cycles to absorb and reject heat. They usually accomplish this as a result of phase changes, which occur through evaporation and condensation processes. To be useful, a refrigerant must satisfy numerous requirements. Chemical stability, specific heat, and latent heat of vaporization are important, as well as cost, availability, flamma-bility, toxicity, and other factors. In a given application, the refrigerant used must be compatible with equipment, materials and operating characteristics. Finally, a refrigerant must be environmentally benign. Amongst the dozens of commercially available refrigerants, the need to satisfy a wide range of cost, safety, and thermodynamic requirements generally results in compromise.

The most commonly used refrigerants are ammonia, water, and a group of compounds containing fluorocar-bons. They are commonly referred to by commercial trade names and their refrigerant, or R, numbers:

• Ammonia (NH3), or R-717, has been in use the longest of the widely used refrigerants. Ammonia has a boiling point of about -28°F (-33°C), a freezing point of about -108°F (-78°C), and a liquid-specific gravity of 0.684 at standard atmospheric pressure. It is highly toxic in high concentrations and flammable and several building code restrictions apply. However, ammonia remains a valuable refrigerant when handled properly due to its low cost, high efficiency, low volumetric displacement and low weight of liquid circulated per ton or kWr of refrigeration. In addition to its excellent heat transfer qualities, ammonia has the ability to operate over a wide range of temperatures — often to -40°F (-40°C) or lower. An ammonia-water solution is the refrigerant used in certain absorption cycle refrigeration machines.

Distilled water (H2O), or R-718, is the most commonly used refrigerant in large absorption chillers. It has a very large latent heat of vaporization — 1,070 Btu/lbm at 40°F (2,488 kJ/kg at 4°C). It is also stable, non-toxic, readily available, and relatively inexpensive. Its principle limitations are that it is only capable of producing cooling at temperatures above 40°F (4°C) and must operate under a vacuum. In addition to the refrigerant, absorption refrigeration requires a second fluid, an absorbent. Most absorption equipment currently on the market in larger capacities utilize lithium bromide (LiBr) in solution with water. LiBr is a non-toxic salt that dissolves in the water vapor it absorbs.

Chlorofluorocarbons (CFCs) are a series of extremely stable fluorinated hydrocarbon compounds using ethane and methane as bases. The commonly used chlorofluorocarbons are trichloromonofluoromethane (CFC-11 or R-11), CO3F, which has a boiling point of about 75°F (24°C) at standard atmospheric pressure, and dichlorodifluoromethane (CFC-12 or R-12), CCl2F2, which has a boiling point of about -22°F (-30°C) at standard atmospheric pressure.

Hydrochlorofluorocarbons (HCFCs) are similar but less stable than CFCs due to the presence of at least one hydrogen atom. The commonly used HCFCs are chlorodifluoromethane (HCFC-22 or R-22), CHClF2, which has a boiling point of about -41°F (-41°C) at standard atmospheric pressure and diclorotrifluoroethane (HCFC-123 or R-123), CHCl2CF3, which has a boiling point of about 81°F (27°C) at standard atmospheric pressure. Hydrofluorocarbons (HFCs) differ from CFCs and HCFC due to the absence of any chlorine atoms. The most common HFC is tetrafluoroethane (HFC-134a or R-134a), CF3CH2F, which has a boiling point of about -15°F (-26°C) at standard atmospheric pressure.

• Azeotropes are mixtures of halocarbon compounds. A common azeotrope is R-500, which is a mixture of CFC-12 and HFC-152a. It has a boiling point of -27°F (-33°C) at standard atmospheric pressure.

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