Document zJ10Z4n9gO640X9pj7G4w35z

712 CHAPTER 39 1946 Guide which will permit placing thermometer wells in the suction connection and allowing sufficient room for free adjustment of the valve. Evaporators and 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 second, direct cooling of air; and the third, cooling of brine for circulation in a closed system, which can cool either water or air. One 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 absorber where the water 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 contaqt 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 Capacity Tons 0 to 5 5 to 25 25 to 50 50 to 400 400 and Over Table 11. Basis of Equipment Selection Majority Used Some Used Few Used Unit systems in con Unit central systems Built up central sys-. ditioned ^pace. using duct distribu terns. tion. Built up central sys tems using reciprocat ing compressors. Unit central systems using duct distribu tion. Unit systems in con ditioned space. Built up systems using absorption and adsorption systems. Built up central sys Built up central sys* Central systems tems using reciprocat tems using centri using adsorption ing compressors. fugal compressors. systems. Built up central sys tems using reciprocat ing compressors. Built up-central sys tems using steam jet and centrifugal com pressors. Built up central sys Built up central sys tems using centri tems using steam jet. fugal compressors. Refrigeration 713 more surface is required than for any other type of cooler. However, with large storage tanks this type of codling can be utilized to advantage. When direct cooling of air is employed, the refrigerant is inside the coil 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 an appreciable difference between the Fig. 11.. Compressor and Coil Performance 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. There are other considerations which make such a system desirable. In the first place, where a toxic refrigerant is undesirable or cannot be used because, of fire or other risks, especially in densely populated areas, the brine can be cooled in an isolated room or building and can then be circulated through the air conditioning equipment. This arrangement eliminates any possibility of direct contact between the air and refrigerant. Equipment Selection The selection of proper refrigeration equipment for any air conditioning job is of utmost importance for satisfactory results. The most important factors in the selection of the equipment are: 1. Loads (as determined by the conditions of the space to be cooled). 2. Economics (both initial and operating costs). 3. Codes (local safety codes must be adhered to and influence the type of system to be used).