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Heating Ventilating Air Conditioning Guide 1938 . with water for a long enough time to permit the wet-bulb temperature of the air and the temperature of the water to reach equilibrium, atmos pheric water cooling equipment aims to circulate only, enough air to cool the water to the desired, temperature with the least possible expenditure of power. In an air washer, humidifier or dehumidifier, the stir is first conditioned by water to change its moisture and temperature, and it is then sent to the place where it is to be used. In water cooling equipment the tem perature of the water is reduced by air, and the cooled water is carried to its point of usage. In the air washer, an excess of water is used to con dition a fixed quantity of air, while in water cooling equipment, an excess quantity of air is used to cool a fixed quantity of water. Both types of equipment have a common basis of design, however, in that the size of the equipment is determined by the quantity of air that must be handled. With the air washer, the size of the equipment is fixed by the quantity of air to be conditioned, and the amount of conditioning is controlled by the quantity and temperature of the water supplied and its method of application. With water cooling apparatus, its size and the quantity of air required bear no direct relation to the quantity of water being cooled, but vary through a wide range for different services and conditions. Sizes of Equipment Assuming a definite quantity of water to be cooled, the size and design of atmospheric cooling equipment are affected by the following factors: 1. Temperature range through which the water must be cooled. 2. Number of degrees above the wet-bulb temperature of the entering air to which the water temperature must be reduced. . 3. Temperature of the atmospheric wet-bulb at which the required cooling must be performed. 4. Time of contact of the air with the water. (This involves height or length of the apparatus and velocity of air.) 5. Surface of water exposed to each unit quantity of air. 6. Relative velocity of air and water. Items 1, 2, and 3 are established by the type of service and geographical location, while items 4, 5, and 6 depend upon the design of the equipment: The establishment of a proper cooling range-.depends upon: 1. Type of service (refrigerating, internal combustion engine and steam condensing). - 2. Wet-bulb temperature at which the equipment must operate satisfactorily. 3. Type of condenser or heat-exchanger used. Because the design of an entire plant is usually affected by the quantity and temperature of the cooling water supply, plants should be designed for cooling water conditions which can be most efficiently attained. The first consideration is usually the limiting temperature of the plant. For example, if an ammonia compressor refrigerating plant is to be designed , for 185 lb head pressure as a normal maximum, the limiting temperature of the ammonia in the condenser is 96 F. Should the ammonia tempera ture go above this figure the head pressure will exceed 185 lb and power consumption increases... To obtain this head pressure, the. temperature of 510 Chapter 25: Spray Equipment for Humidification & Dehumidification the circulating water leaving the condenser'must always be less than 96 F by an amount'depending upon the size.and design of the condenser, the quantity of water being circulated; and the refrigerating tonnage being produced. A condenser having a large surface per. ton of refrigeration may be designed to operate satisfactorily with the: leaving-hot water temperature within 3 or 4 F of the ammonia temperature corresponding to the head pressure, while a small condenser`might require a 10 F difference. Table 2 lists several gases with data as to the temperatures and pres sures for which commercial condensers are designed: Internal combustion engines have limiting hot water temperatures of 125 F to 140 F. The cooling of such fluids as milk or wort has variable requirements and is usually done in counter-flow heat-exchangers in. which the leaving circu lating water is at a much higher temperature than is the leaving fluid. Table 2. Condenser Design Data Maximum Pressure Gas Temperature Leavino Hot Water Temperature Gas Desired in in Condenser Deo F Condenser Deg F Best Condenser Design Average Condenser Design 1 Steam...-........ 28 in. vacuum Steam. 27 in. vacuum.... Steam. _ 26 in. vacuum Ammonia______ 185 lb gaee 101.2 115.1 125.9 97 110 120 93 105 114 head pressure. Carbon dioxide.. 1030 lb eaee 96.0 92 - 88 Methyl - chloride..... .. Dichlorodi- fluoromethane head pressure. 102 lb gage head pressure___ 117 lb gage .. head pressure.:.... 86.0 100.0 100.0 83 96 96 81 92 93 The temperature range, once the hot water temperature'-is approxi mately known, depends upon: diss1i.patMeda.ximum wet-bulb temperature at which the full quantity of heat must be 2. Efficiency of the atmospheric cooling equipment considered. . Design Wet-Bulb Temperatures The maximum wet-bulb temperature at which the full. quantity of water must be cooled through the entire range' is never, in commercial design, the maximum wet-bulb temperature ever:known to exist at the location nor the average wet-bulb temperature over any period. The former basis would require atmospheric cooling equipment several times greater than normal size, and the latter would result during a large part of the time, in higher condenser water temperatures than those for which the plant was designed. For instance, the maximum wet-bulb temperature recorded in New York City is 88 F, and the July noon average for 64 years is close to 68 F. Yet in the years 1925 to 1934, inclusive, there were but 8 hours per .year when the wet-bulb temperature reached 80 F or more, and there were 975 hours in the average summer (June to September, inclusive) when the wet-bulb, temperature was 68 F or above. As these 511