Document QkdBdQgOJ16Lnm5k0ByzE9VM6

650' 'CHAPTER 37 1946 Guide Sizes of Equipment Assuming a definite quantity of water, the size and design of atmospheric cooling equipment are affected by the following primary conditions: 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 i 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 condens ing). (2) wet-bulb temperature at which the equipment must operatesatisfactorily, (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 fpr 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 psi 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 psi and power' consumption increases. To obtain this head pressure, the temperature of 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 deg of the ammonia temperature corresponding to the head pressure, while a small condenser might require a 10 deg 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 for closed . Table 2. Condenser Design Data Gu Maximum Pressure Desired in 'Condenser ' Leating Hot Water Temperature Gas Temperattob - F Deo in Condenser . F Deg Best Condenser Design Average Condenser Design Steam................. 28 in. vacuum____ Steam................ 27 in. vacuum____ Steam...:..:_____ 26 in. vacuum___ Ammonia.......... 185 psia.... .............. Carbon dioxide. 1030 psia............ . . Methyl.chloride -102 psia................... Dichlorodi- fluoromethane 117 psia.............. __ 101.2 .115.1 125.9 96.0. 86.0 100.0 100.0 97 110 120 92 83 96 96 93 105 114 88 81 92 93 Head pressure. Spray Apparatus ______________ ' _________ ;______ 651 systems, and 110 F to 120 F for open systems,depending upon the quality of the cooling water. 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 circulating water is at a much higher temperature than is the leaving fluid. v The temperature range, once the hot water temperature is approxi mately known, depends upon: (1) the maximum wet-bulb temperature, at which the full quantity of heat must be dissipated, and (2) the effici ency 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 jyould 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 975 hours represent a third of the summer period, cooling equipment based upon the noon average July wet-bulb of 68 F would be inadequate. Commercial practice is to choose a wet-bulb temperature for air con ditioning design purposes which is not exceeded during more than 5 to. .8 per cent of the summer hours (75 F for New York City) with somewhat lower requirements for steam turbines and. internal combustion engines. This difference is made because the heaviest load on an air conditioning plant is coincident with high wet-bulb temperatures, whereas the heaviest electric power demand occurs either in the winter or after nightfall in summer, when the wet-bulb temperature is low. Table 2, Chapter 15, shows design wet-bulb temperatures which will not be exceeded more than 8 per cent of the time in an average summer. Knowing the hot'water temperature and the wet-bulb temperature for which the equipment must be designed, the cold water temperature must be chosen to place the requirement within the effectiveness range of the type of atmospheric water cooling apparatus to be used. This effective,ness is expressed as the percentage ratio of the actual cooling effect to the maximum possible cqoling effect. Since the wet-bulb temperature of the entering air is the equilibrium'temperature to which the water could be cooled, the effectiveness of water cooling apparatus can be indicated thus: (hot water temperature -- cold water temperature) X 100 hot water temperature -- wet-bulb temperature of entering air ' Magnitudes of this effectiveness ratio will vary through wide limits in accordance with construction and conditions of operation. Values indi cative of the commercial range of the effectiveness ratio are given in Table 3, although unusual designs may operate outside these ranges. From consideration of the factors which include the cooling range' and design wet-bulb temperature, the quantity of water required can be