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American Society of Heating and Ventilating Engineers Guide, 1936 design follows closely that of any other centrifugal equipment, such as general service pumps and fans. Steam jet compressors which have recently entered the field are simple and compact and, having no moving parts, they produce practically no vibration but are not economical for water temperatures much below 40 F or where the cost of generating steam is higher than the cost of operation with other prime movers.: - Rotary compressors are generally used for methyl chloride and dichlorodifluoromethane because of their relatively low pressure and compression ratios. These compressors find widest use for fractional tonnage duty. The source of condensing water to some extent governs the type of refrigerant used. If condensing water is available at temperatures of not more than 70 to 75 F any of the refrigerants'mentioned can be used economically, but if the available condensing water temperature is above 80 F, carbon dioxide becomes uneconomical as its critical temperature is approximately 88 F. A condensing water temperature over 80 F makes the power required for compression high. All refrigerants have critical temperatures and pressures sufficiently high so that their efficiency is not materially affected by the condensing water temperatures, except in so far as this temperature affects the compression ratio. Steam jet cooling systems can use water up to 85 F, or even slightly warmer. The applicability of the various refrigerants is interesting. Carbon dioxide is limited by the condensing water temperature; the power con sumption is slightly higher than that of other refrigerants; and the pres sures are three to four times that of ammonia. - The condenser pressures of methyl chloride and dichlorodifluromethane are approximately one-half that of ammonia. Ammonia, probably the best known refrigerant, has the disadvantage of being toxic, and under certain circumstances explosive, corrosive, and irritating, even in small'q[uantities in the atmosphere. Ammonia is used exclusively in the larger indirect or brine cooling air conditioning systems. Sulphur dioxide is corrosive and irritating even in small quantities in the atmosphere and it is toxic under certain circumstances. . Dichloromethane operates at pressures below that of the atmosphere, and it is to some extent toxic. Dichlorodifluromethane under normal circumstances is non-toxic, nonirritating, and non-explosive, but under high temperatures it breaks down into several obnoxious, poisonous components. Methyl chloride, under certain conditions, is explosive and slightly toxic. . The steam ejector water vapor system has none of the disadvantages of toxicity, explosiveness and corrosiveness encountered in the other refri gerants, but the system operates at less than atmospheric pressure. This, however, is not an important factor as there are no moving parts in the compressor and the possibility of inleakage of air is remote as all. of the equipment can be welded air and water tight.. The supply of-water is inexhaustible, and as a refrigerant, the make-up cost is negligible. The same boiler equipment can be used for heating in winter and for cooling in summer. Chapter 10- -Cooling Methods Electric Motors ! The motors used for driving compressors can be roughly classified in three groups: synchronous, multispeed, or variable speed. Further infor mation on motors and their controls may be found in Chapter 42. Coolers The types of coolers used in connection with air. conditioning work fall into three general groups. The first is the direct cooling of water; the' snecaomncadio,, sdoeuidrcesic-yit.scvt-evo--mol,--in-wgowho-ivc-fh,a---ciroa;nna- -cnnodnotllheeiththeirrdww, aacltoPeorrlnionr-ga.oirf. brine One for circulation' method of the direct cooling of water is to install direct expansion coils in the spray chamber so that the water sprayed into the air comes in direct contact with the cooling coils. Another common and efficient method of cooling spray water is to use a Baudelot type of heat absprber where the waiter flows over direct expansion coils at a rate sufficiently high to give efficient heat transfer from water to refrigerant. Another type of spray water cooler is the shell and. tube heat exchanger, in which the refrigerant is expanded into a shell enclosing the tubes through which the water flows. The velocity of the water in the tubes affects the rate of heat transfer, and as the refrigerant is in the shell com pletely surrounding the tubes at all times, good contact and a high rate of heat transfer are insured. The disadvantage of such a system is that with the falling off of load on the compressor the suction temperature or the. temperature in the evaporator drops and there is a possibility of freezing the water in the tubes, which, of course, might split the tubes and allow! the refrigerant to escape into the water passage. This danger can be eliminated by automatic safety devices. Another system of cooling spray water is to submerge coils in the spray collecting tank, or in a separate tank used for storage. The heat trans mission through the walls of the coils, however, is low and a great deal more surface is required than for any Other type of cooler. However, with large storage tanks this type of cooling can be utilized to advantage: When direct cooling of air is employed; the refrigerant is inside the coii and the air passes Over it. Cooling depends upon convection and con duction for removing the heat from the air. The type of coil used can be either smooth or finned, the finned coil being more economical in space requirement than the smooth coil. The fins, however, must be far enough apart so as not to retain the moisture which condenses out of the air. The indirect cooler, where brine ,is cooled by the refrigerant and the resulting cold brine is used to.cool either air or water, introduces several other considerations. It is not the most economical from a .power ^con sumption standpoint, as it is necessary to cool the brine to a temperature sufficiently low so that there is a.ri appreciable difference between the average brine temperature and that of the substance being cooled. This requires that the.temperature-of the refrigerant must be still lower, and consequently the amount of: power required to produce a given amount of refrigeration increases due to the- higher compression ratio,.but there are Other considerations which make such a system desirable. In the firsf place, where a toxic refrigerant: is undesirable or cannot be arsed,'due:to fire or other risks especially in densely populated areas, the brine:can be 193