Document grQaJY3rj9GyYj5azEOXnbGa

American Society of Heating and Ventilating Engineers Guide, 1937 place for the entire cooling period, regardless of what the outdoor tempera. . tures are. With the cooling tower, the temperature of the condensing water may rise to 80 or 85 F under maximum conditions, but under less than maximum conditions the temperature of the water off the coolings tower drops considerably, and it has been established that 50 per cent of'i the time the outdoor wet-bulb temperature varies from 60 to 70 F and the cooling tower water, therefore, for the same periods, varies from 65 to 75 F. ; When the outdoor wet-bulb temperature drops below 60 F, which occurs * approximately 30 per cent of the time, the condensing water temperature l is still lower. The cost of water used for condensing is. negligible, as the I only water required is that used to make up the loss by evaporation in the cooling tower itself. See also Chapter 12. Evaporative Condensers Due to the high cost of city water for condenser purposes and due to ordinances in some localities prohibiting the discharge of large quantities of such water into the sewage systems, there has been developed a con- Water thermostat- Water pressure Fig. 2. Diagram of Evaporative Condenser denser which uses a minimum amount of water on a finned surface, cooling it to approximately the wet-bulb temperature of the surrounding atmosphere. A .diagram of a typical evaporative condenser is shown in Fig. 2, which includes a propeller type fan that forces the air over a finned tube con denser coil to the outside atmosphere through a connecting duct. A fine jet of water from a nozzle located at the center of the coil is directed to a target on the hub of the. fan, where it is atomized and the spray is carried across the coil, wetting the surface. The hot refrigerant gases enter the top of the condenser coil and the liquid collects in the receiver below the condenser unit. A thermostat control valve and pressure regulator are arranged on.the condenser water supply for regulating and adjusting the water flow to the nozzle. It is particularly desirable that a condenser of this type be located as near the compressor as possible. Units of this design are available in sizes, ranging from 23^ tons of refrigeration up to about 40 tons. For larger capacities several multiple units may be used in parallel. About 10 per cent as much water is : required for this unit as is normally used in a shell and tube type condenser. 210 Chapter 11--Cooling Methods EVAPORATORS AND COOLERS The types of coolers used in connection with air conditioning work fall nto 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 oray 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 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 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 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 siich a system desirable. In the first place, where a toxic refrigerant is undesirable or cannot be used, due to fire or other risks especially in densely populated areas, the brine can be cooled in an isolated room or building and then be circulated through the air conditioning equipment in perfect safety because it is used to cool the water or air, without any possibility of direct contact between the air and refrigerant. When an indirect system of cooling is used, it will be found that the heat 211