According to
ASHRAE standard 34, the R-number of a chemical refrigerant is assigned systematically according to its molecular structure and has between two and four digits. If there are
carbon-carbon
multiple bonds, there are four digits in all: the number of these bonds is the first digit and the number of carbon atoms minus one (C − 1) is next. If there is more than one carbon atom but no multiple bonds, there are three digits, and the number of carbon atoms minus one is the first digit. If there is only one carbon atom, then there are only two digits. The last two digits are always the number of
hydrogen atoms plus one (H + 1), followed by the number of
fluorine atoms. Any other atoms attached to the carbons are assumed to be
chlorine atoms. For example, R-22 has one carbon atom (1 − 1 = 0), one hydrogen atom (1 + 1 = 2), two fluorine atoms (2), and one chlorine atom (4 − 2 − 1 = 1), so it is
chlorodifluoromethane, while R-134 has two carbon atoms (2 − 1 = 1), two hydrogen atoms (2 + 1 = 3), four fluorine atoms (4), and no chlorine atoms (6 − 2 − 4 = 0), so it is one of the
tetrafluoroethanes. This basic scheme is modified as follows: • Capital letters "B" and "I" are suffixed, together with atom counts, if chlorine atoms have been replaced with
bromine or
iodine. • If there are two carbons, the
isomer without suffix is the most symmetrical; after this, lower-case suffixed letters "a", "b", ..., are added, moving forwards through the alphabet as symmetry decreases. • If there are more than two carbons, a more complex system of suffixed letters and possibly numbers is used to distinguish isomers, when necessary. • The suffixes "(Z)" and "(E)" may be used to differentiate
(Z)- and (E)- (
cis- and trans-) isomers. • For larger molecules, numerical "digits" exceeding 9 may occur; in this case, these numbers are separated by dashes. • For a cyclic carbon skeleton, the prefixed capital letter "C" is used. • For an
oxygen-containing refrigerant (an
ether), the prefixed capital letter "E" is used. There are separate numbering schemes for
zeotropic and
azeotropic blends, organic chemicals which don't fit into the scheme above, and inorganic chemicals: • Zeotropic blends are numbered starting with 400. Suffixed capital letters "A", "B", ... may be used to distinguish different blends with the same constitutents. • Azeotropic blends are numbered similarly to zeotropic blends, but starting with 500. • Other organic chemicals may be numbered starting with 600. Suffixed lowercase letters "a", "b", ..., may be used to distinguish isomers. • Inorganic chemicals are given numbers equalling 700 plus their
molecular weight, if the weight is less than 100, or 7000 plus the molecular weight otherwise. Suffixed capital letters "A", "B", ... may be used to distinguish different chemicals with the same molecular weight. == Columns ==