REFRIGERANTS Liquids Used in Refrigerating Machines.—Modern me chanical refrigeration (as well as other heat cycles) has undergone a great change during the last decade or two. The improved auto matic refrigerating machine and the advance in the use of comfort cooling have emphasized the need of a refrigerant that will be non toxic and non-inflammable. In Table Ha, column io, the co efficient of performance is reasonably the same for all refriger ants of the volatile liquid type when the temperature of liquefac tion is not too near the critical and where the properties of the refrigerant are well known, for example, "F-12" (C C12F2) whose evaporating pressure is given in the table as 2651b. per sq.in.abs., compared with Carrene (C H2C12) where the pressure at the same evaporating temperature is only 147lb. per sq.in.abs., and the piston displacement per ton of refrigeration 5-8i f per minute for "F-12" and per minute for Carrene. The large vol umes required for Carrene, "F-i 2" at low evaporating tempera tures, dieline and water vapour for comfort cooling can be handled satisfactorily by means of the centrifugal com pressor where the refrigerant may be compressed efficiently by careful design of the compressors so as to reduce friction in the impellers and diffusers. When the evaporating pressure is below atmospheric, as in the case of Carrene, dieline, ethyl chloride, sulphur dioxide, water vapour and butane air leaks are likely to be excessive, although where the refrigerant is nearly odourless ("F-12" and methyl chloride) leaks into the atmos phere, and loss of the refrigerant, may be excessive also.
the use of sulphur dioxide vapour. Carbon, dioxide has been used as a safety refrigerant for a long time, ag its presence in the con fined space of a ship's hold, even in large per cent by volume, is not a serious hazard unless the exposure is prolonged. Its con densing pressure is excessive for condenser water initially at 70° F. and higher, and with cooling water of about 80° F. it will not condense, so that with liquefaction near the critical tempera ture of about 87.8° F. the efficiency (coefficient of performance) is low (see Table lle). The triple point is —69.9° F. Sulphur di oxide is used almost entirely in fractional tonnage machines in which it lends itself to the air-cooled condenser. It is not in flammable, but is an irritant and a toxic gas, in spite of which it is practically harmless in the small household refrigerating machine. However, it is dangerous in multiple systems. Unless sulphur dioxide is anhydrous it will corrode the common metals used in the construction of pressure vessels. Methyl chloride, an anaesthetic in amounts of 5 to o% by volume, may be used in air-cooled condensers and is successful in both large and small sized units. It is miscible in mineral oils, and water in the sys tem will freeze in the expansion valve. Leaks may be detected by the use of acrolein. Dichlorodifluoromethane ("F-12") is tak ing the place of both ammonia and carbon dioxide, especially where the public is exposed seriously to the effect of heavy leaks. "Freon-12" is almost an ideal refrigerant inasmuch as it is not toxic, inflammable or irritant. The air-cooled condenser may be used, and leaks can be detected with a special torch. Ethyl chlo ride has as yet been used but little as a refrigerant, and the same is true of the hydrocarbons butane and propane. The use of dichloethylene (C2H2C12) and methylene chloride have been confined to the centrifugal compressors. Very low tempera ture refrigeration may use ethane, ammonia, or dichlorodifluoro methane. Other halides such as "F-is" (CC12F), "F-21" and "F-114" (C2C12F4) may be generally used to a greater ex tent in the future.