Interconversion of ASHRAE Refrigerant Numbers and Chemical Compositions A. A. Woolf Bristol Polytechnic, Faculty of Applied Science, Bristol BS16 1QY
Atwood ( I ) has pointed out that "there is no abun- Distribution of CFC Substitutes among Substituted Ethanes and dance of suitable candidate fluids awaiting discovConversion of ASHRAE Numbers to Compositions ery and development" to replace present refrigerants because requisite properties such as vapor Isomeric formulas pressures, heats of evaporation, low flammabilities and toxicities severely limit possibilities. New Empirical "Normal" a b ASHRAEa Add plants to manufacture the interim CFC replace- No. of CFC 90 Composition ments in the table have been or are to be conSubstitute structed. CFzCICHCIF CHFzCCIzF The code numbers of refrigerants (2)can confuse 123 213 CzHF3(CIz) CFsCHClz chemists more used to actual chemical formulas. 124 214 C2HF4(CI) CFICHCIF CHF&F?CI The usual conversion process, consisting of subtracting one from the first digit to give the number 125 215 C2HF5 of carbon atoms, adding one to the second digit for 134a 224 CzHzF4 hydrogens, and leaving the third digit unchanged for fluorines, can be simplified by merely adding 90 141b 231 CzH3F(CIz) to the code number to yield a def number and corre142b 232 CzH3Fz(CI) sponding Cd&Ffcomposition (see table). Ife + f < 2d + 2 additional atoms are reauired for saturation. T h e American Society af Heating, Refrigerating and Air-Conditioning Engineers Inc. Thus in the ethane series with six substituents the chlorine number in CFC's is found by difference. The "normal" isomer in any set, labelled purely numerically, has the smallest mass difference on each carbon. Extra a, b, or c labels are attached as the masses increasingly diverge from "normal". Only the CFC 132 set needs the c label. The relative position of the new products among the 55 possible ethanes, as well as the conversion between ASHRAE numbers (2) and composition, can be shown on right-triangular diagrams (see figure). Positions are given by x,y coordinates equal to the numbers of each atom within any of three possible pairs. Values for a third atom, with a z coordinate, are unnecessary since they can be found by difference. Limits set by flammability-too much hydrogen-and toxicitytoo much chlorine-effectively truncate the available area. Other property distributions over the area are irregular. Thus relevant properties, such as boiling points and vaporization heats in orthogonal directions (x or y = 0) exhibit maximum and minimum values, whereas boiling points in hypotenuse directions, as in the all-halogen substituted series (x + y = 6 ) , increase almost linearly Right-triangle diagram to show positions of potential new products as well-asconversion betwen ASwhen fluorine is replaced by chlo- HRAE numbers and mmpositon. (6.p. in parentheses: see shaded triangle for changes in ASHRAE rine. This is a result of intermo- number in the directions indicated). Volume 70 Number 1 January 1993
35
lecular interactions of oppositely directed dipoles C t H and C+X (X= halogen)present in some of the compounds, which accounts for the failure of simple group or atom-additivity schemes to predict properties correctly over the whole range of compounds (3).
36
Journal of Chemical Education
Literature Cited
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1992,30.
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