Document ZJVmE8xX2B68vkvKnzXM5GXO8

American Society of Heating and Ventilating Engineers Guide, 1935: Sizes of Equipment Assuming a definite quantity of water to be cooled, the size and d- ' 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 the water temperature must be reduced.. 3. Temperature of the atmospheric wet-bulb at which the required cooling miJ performed. j . 4. Time of contact of the air with the water. (This involves height or length of5 apparatus and velocity of air.) 5. Surface of water exposed to each unit quantity of air. 6. Relative velocity of air and water. Table 2. Condenser Design Data Gas Maximum Pressure Desired in Gas Temperature in Condenser Steam. ............ 26 in. vacuum....... Ammonia.......... 185 lb gage head pressure..... Carbon dioxide.. 1030 lb gage head pressure..... Methyl 102 lb gage chloride......... head pressure..... Dichlorodi- 117 lb gage fluoromethane head pressure..... 99.7 114.3 126.0 96.0 86.0 100.0 100.0 Leaving Hot Water Temperate Best Design 97 110 120 92 83 96 96 Average De 9;T~ 105 114 88 81 92 93 , Items 1, 2, and 3 are established by the type of service and geograp'' location, while items 4, 5, and 6 depend upon the design of the equipm The establishment of a proper cooling range depends upon: 1. Type of service (refrigerating, internal combustion engine and steam condensi 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 quanli and temperature of the cooling water supply, plants should be desi_ for cooling water conditions which can be most efficiently attained, first consideration is usually the limiting temperature of the plant, example, if an ammonia compressor refrigerating plant is to be desr for 185 lb head pressure as a normal maximum, the limiting tempera of the ammonia in the condenser is 96 F. Should the ammonia,tempera, go above this figure the head pressure will exceed 185 lb and power ` sumption increases. To obtain this head pressure, the temperature of circulating water.leaving'the condenser must always be less than 96 by an amount depending upon the size and design of the condenser, _ quantity of water being circulated, and the refrigerating tonnage t: produced. A condenser having a large surface per ton of refrigera may be designed to operate satisfactorily with the leaving hot w< temperature within 3 deg or. 4 deg of the ammonia. temperature. responding to the head pressure, while a small condenser might req a 10 deg difference. 188 11--Humidification and. Dehumidification .. several gases with data as to. the temperatures and presrTable 2 bs omrnercial. condensers are designed. Internal combustion. ^sures'fpr,wh'C j;miting hot water temperatures of 125 F to 140 F. The 'erigi(ies "aV t. fluids as milk or wort has variable requirements and is Tooling o' su^ counter_flow heat-exchangers in which the leaving circu- .Tusuajlyo^ js at a much higher temperature than is the leaving fluid. oerature range, once the hot water temperature, is approxi- depend8 upon: i.v), wet-bulb temperature at which the full quantity of heat must be ^^Efficiency of the atmospheric cooling equipment considered. Wet-Bulb Temperatures The maximum wet-bulb temperature at which the full quantity of - must be cooled through the entire range, is never, in commercial 'ffiitn': ^e maximum wet-bulb temperature ever known to exist at the 'location nor the average wet-bulb temperature over any period. The fnneir 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 ' ;oIant` was designed. For instance, the maximum wet-bulb temperature recorded in New York City is 88 F, and the July noon average for 64 "wars iS close to 68 F. Yet in the years 1925 to 1931, inclusive, there were >biit'6 hrs per year when the wet-bulb temperature reached 80 F or more, -and?there were 9^5 hours in the average summer (June to September, Inclusive) when the wet-bulb temperature was 68 F or above. As these ;975'hburs represent a third of the summer period, cooling equipment rl&sed upon the noon average July wet-bulb of 68 F would be inadequate. `Commercial practice is to choose a wet-bulb temperature for refrigeration 'desigh'purposes which is not exceeded during more than 5 to 8 per cent ^of^theisummer hours (75 F for New York City), with somewhat lower ^requirements for steam turbines and internal combustion engines. ' This chjfereBce is made because the heaviest load on a refrigerating plant is coinddent with high wet-bulb temperatures, whereas the heaviest electric bogllpiemand occurs either in the winter or after nightfall, in summer, wet-bulb temperature is low. Table 1, Chapter 8, shows safe dgign-wet-bulb temperatures which will not be exceeded more than 8 per qenjfpfjjthe time in an average summer. ,, Knowing the hot water temperature and the wet-bulb temperature for wHjchythe equipment must be designed, the cold water temperature must b&chosen to place the requirement within the efficiency range of the type ojptmqspheric water cooling apparatus to be used. Efficiency of atmos- cooling apparatus is expressed as the percentage ratio of the |S!|k,c90l'nS range to the possible cooling range. Since the wet-bulb f the entering air is the lowest temperature to which the .water,could possibly be cooled this is: percentage cooling efficiency of atmospheric water cooling equipment =: (hot water temperature -- cold water temperature ) X 100 ji hot water temperature -- wet-bulb temperature of entering air 189