Document NNaJJp5wzGQe0mEEQXzO6kxww

American Society of Heating and Ventilating Engineers Guide, Chapter 10--Cooling Methods but the resulting loss in efficiency with the higher pressure gases linrits'jE centrifugal compressor to the two refrigerants named. At the preJ ... pnt cCan bu<e= w--eld~e--d---a--ir--a-nd--w--a--t-e--r--t-igo--ht.- The supply of water is equipmer |e anci as a refrigerant, the make-up cost is negligible. The tilmed the cenfrtirfiifiucrgna1lmcommnprrEeRsst^orrssaarrpelliimmiitteedd ttooaaiirrccoonnddiittiioonniinngg about 75 hp and more. Centrifugal compressors are usually built tn t or more stages where the compression ratio is high, and their toiler equipment can be used for heating in winter and for cooling ' same in summer follows closely that of any other centrifugal equipment, such as1' g--en^&j service pumps and fans. 1Steam jet compressors which have recently entered the field are . and compact and, having no moving parts, they produce practical]^ - electric Motors dors used for driving compressors can be roughly classified in 6 ,,,,ns. synchronous, multispeed, or variable speed. Further infor- Sn on motors may be found in Chapter 17. vibration but are not economical for water temperatures much beg| 40 F or where the cost of generating steam is higher than the cost) ^The types of coolers used in connection with air conditioning work fall operation with other prime movers. ; Rotary compressors are generally used for methyl chloride and dichlbl difluoromethane because of their relatively low pressure and compress^ ratios. These compressors find widest use for fractional tonnage du| -, ' to three general groups. The first is the direct cooling of water; the '^iond direct cooling of air; and the third, cooling of brine for circulation ' ^ 3 dosed system, which can cool either water or air. One method of the j-rect cooling of water is to install direct expansion coils in the spray The source of condensing water to some extent governs the type^ chamber so that the water sprayed into the air comes in direct contact refrigerant used. If condensing water is available at temperatures ofao with'the cooling coils. Another common and efficient method of cooling more than 70 to 75 F any of the refrigerants mentioned can be a| spray water is to use a Baudelot type of heat absorber where the water economically, but if the available condensing water temperature is ab|j flowlover direct expansion coils at a rate sufficiently high to give efficient 80 F, carbon dioxide becomes uneconomical as its critical temperature^ ; heat transfer from water to refrigerant. approximately 88 F. A condensing water temperature over 80 F mah the power required for compression high. All refrigerants have critic! temperatures and pressures sufficiently high so that their efficiency isf materially affected by the condensing water temperatures, except in? far as this temperature affects the compression ratio. Steam jet cootiij systems can use water tip to 85 F, or even slightly warmer. Another type of spray water cooler is the shell and tube heat exchanger 4-inlwhich the refrigerant is expanded into a shell enclosing the tubes & tlwough which the water flows. The velocity of the water in the tubes I affects the rate of heat transfer, and as the refrigerant is in the shell comtf pletely surrounding the tubes at all times, good contact and a high rate of beat-transfer are insured. The disadvantage of such a system is that with The applicability of the various refrigerants is interesting. Carbl the'falling off of load on the compressor the suction temperature or the dioxide is limited by the condensing water temperature; the power ri temperature in the evaporator drops and there is a possibility of freezing sumption is slightly higher than that of other refrigerants; and the ||,\ tliel.water in the tubes, which, of course, might split the tubes and allow sures are three to four times that of ammonia. If the/refrigerant to escape into the water passage. This danger can be The' condenser pressures of methyl chloride and dichlorodifluromethai ^.'eliminated by automatic safety devices. are approximately one-half thatTof ammonia. | Another system of cooling spray water is to submerge coils in the spray Ammonia, probably the best known refrigerant, has the disadvandj ^collecting tank, or in a separate tank used for storage. The heat trans of being toxic, and under certain circumstances explosive, corrosive, a it. mission through the walls of the coils, however, is low and a great deal irritating, even in small quantities in the atmosphere. Ammonia is ua |moje surface is required than for any other type of cooler. However, with exclusively in the larger indirect or brine cooling air conditioning systefe _|lk% storage tanks this type of cooling can be utilized to advantage. Sulphur dioxide is corrosive and irritating even in small quantity gv -.'When direct cooling of air is employed, the refrigerant is inside the coil the atmosphere and it is toxic under certain circumstances. .? \ and.ffie air passes over it. Cooling depends upon convection and con- Dichloromethane operates at pressures below that of the atmosph"e2 |f.."Ftln fr removing the heat from the air. The type of coil used can be and it is to some extent toxic. 1 pcette. smooth or finned, the finned coil being more economical in space Dichlorodifluromethane under normal circumstances is non-toxic, n# than the smooth coil. The fins, however, must be far enough irritating, and non-explosive, but under high temperatures it ^ aPrt.is as not to retain the moisture which condenses out of the air. down into several obnoxious, poisonous components; indirect cooler, where brine is cooled by the refrigerant and the Methyl chloride, under certain conditions, is explosive and sligljj - resulting cold brine is used to cool either air or water, introduces several toxic. . it-- K??nsi(*erations. It is not the most economical from a power con- The steam ejector water vapor system has none of the disadvantage standpoint, as it is necessary to cool the brine to a temperature toxicity, explosiveness and corrosiveness encountered in the other rffi W 80 t^lat t^iere *s an appreciable difference between the gerants, but the system operates at less than atmospheric pressure. L 6 {"lne temperature and that of the substance being cooled. This however, is not an important factor as there are no moving parts m. v-rconvvf5 the temperature of the refrigerant must be still lower, and compressor and the possibility of inleakage of air is remote as all |?1.. .;.-b?<l!Iently the amount of power required to produce a given amount of 176 177 J