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Figure 6-30.—Pressure type cut-in, cutout control switch | Up
Utilitiesman Basic Volume 02 - Manual for electric, plumbing, water and other utilities | Next
R-22 Monochlorodifluoromethane (CHCIF) |
Figure 6-33.—A cutaway view of an oil separator.
Refrigerants are fluids that change their state upon
the application or removal of heat within a system and,
in this act of change, absorb or release heat to or from
an area or substance. Many different fluids are used as
refrigerants. In recent years, the most common has
been air, water, ammonia, sulfur dioxide, carbon
dioxide, and methylchloride.
Today, there are three specific types of refrigerants
used in refrigeration and air-conditioning systems—( 1)
Chlorofluorocarbons or CFCs, such as R-11, R-12, and
R-114; (2) Hydrochlorofluorocarbons or HCFCs, such
as R-22 or R-123; and (3) Hydrofluorocarbons or HFCs,
such as R-134a. All these refrigerants are
"halogenated," which means they contain chlorine,
fluorine, bromine, astatine, or iodine.
Refrigerants, such as Dichlorodifluoromethane
(R-12), Monochlorodifluoromethane (R-22), and
Refrigerant 502 (R-502), are called PRIMARY
REFRIGERANTS because each one changes its state
upon the application or absorption of heat, and, in this
act of change, absorbs and extracts heat from the area
or substance.
The primary refrigerant is so termed because it
acts directly upon the area or substance, although it
may be enclosed within a system. For a primary
refrigerant to cool, it must be placed in a closed system
in which it can be controlled by the pressure imposed
upon it.
The refrigerant can then absorb at the
temperature ranges desired. If a primary refrigerant
were used without being controlled, it would absorb
heat from most perishables and freeze them solid.
SECONDARY REFRIGERANTS are substances,
such as air, water, or brine. Though hot refrigerants in
themselves, they have been cooled by the primary
refrigeration system; they pass over and around the
areas and substances to be cooled; and they are
returned with their heat load to the primary
refrigeration system. Secondary refrigerants pay off
where the cooling effect must be moved over a long
distance and gastight lines cost too much.
Refrigerants are classified into groups. The
National Refrigeration Safety Code catalogs all
refrigerants into three groups—Group I – safest of the
refrigerants, such as R-12, R-22, and R-502; Group II –
toxic and somewhat flammable, such as R-40 (Methyl
chloride) and R-764 (Sulfur dioxide); Group III –
flammable refrigerants, such as R-170 (Ethane) and
R-290 (Propane).
R-12 DICHLORODIFLUOROMETHANE
(CC12F2)
Dichlorodifluoromethane, commonly referred to
as R-12, is colorless and odorless in concentrations of
less than 20 percent by volume in air. In higher
concentrations, its odor resembles that of carbon
tetrachloride. It is nontoxic, noncorrosive,
nonflammable, and has a boiling point of -21.7°F
(-29°C) at atmospheric pressure.
WARNING
Because of its low-boiling point at
atmospheric pressure, it prevents liquid
R12 from contacting the eyes because of
the possibility of freezing.
One hazard of R-12 as a refrigerant is the health
risk should leakage of the vapor come into contact with
an open flame of high temperature (about 1022°F) and
be decomposed into phosgene gas, which is highly
toxic. R-12 has a relatively low latent heat value, and,
in smaller refrigerating machines, this is an advantage.
R-12 is a stable compound capable of undergoing the
physical changes without decomposition to which it is
6-20
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