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REFRIGERANT PROPERTIES
Refrigerants can be characterized by a number of properties. These properties are pressure, temperature, volume, density, enthalpy, flammability, ability to mix with oil, moisture reaction, odor, toxicity, leakage tendency, and leakage detection.
Freon refrigerants R-11, R-12, and R-22, plus ammonia and water, will be discussed in terms of the above-mentioned categories. Freon R-11, R-12, and R-22 are common Freon refrigerants. The number assigned to ammonia is R-717, while water has the number R-718.
Pressure
The pressure of a refrigeration system is important. It determines how sturdy the equipment must be to hold the refrigerant. The refrigerant must be compressed and sent to various parts of the system under pressure. The main concern is keeping the pressure as low as possible. The ideal low-side, or evaporating, pressure should be as near atmospheric pressure (14.7 pounds per square inch) as possible. This keeps the price of the equipment down. It also puts positive pressure on the system at all points. With a small pressure it is possible to prevent air and moisture from entering the system. In the case of a vacuum or a low pressure, it is possible for a leak to suck in air and moisture. Note the five refrigerants and their pressures.
Freon R-11 is used in very large systems because it requires more refrigerant than others—even though it has the best pressure characteristics of the group. Several factors must be considered before a suitable refrigerant is found. There is no ideal refrigerant for all applications.
Temperature
Temperature is important in selecting a refrigerant for a particular job. The boiling temperature is that point at which a liquid is vaporized upon the addition of heat. This, of course, depends upon the refrigerant and the absolute pressure at the surface of the liquid and vapor. Water (R-718) has the highest boiling temperature. Atmospheric pressure is 14.7 pounds per square inch.
Once again, there is no ideal atmospheric boiling temperature for a refrigerant. However, temperature-pressure relationships are important in choosing a refrigerant for a particular job.
Volume
Specific volume is defined as the definite weight of a material. Usually expressed in terms of cubic feet per pound, the volume is the reciprocal of the density. The specific volume of a refrigerant is the number of cubic feet of gas that is formed when one pound of the refrigerant is vaporized. This is an important factor to be considered when choosing the size of refrigeration-system components. Compare the specific volumes (at 5 degrees F) of the five refrigerants we have chosen. Freon R-12 and R-22 (the most often used refrigerants) have the lowest specific volumes as vapors.
Density
Density is defined as the mass or weight per unit of volume. In the case of a refrigerant, it is given in pounds per cubic foot (pounds/cubic foot). The density of R-717 (ammonia) is the least.
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