Document 7MLw8wBOKg8ZZ1zmJEXX5qgm8

ofAmerican Society Heating and Ventilating Engineers Guide, 1934 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 is 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. Table 2. Condenser Design Data Gas Maximum Pressure Desired in Condenser Gas Temperature m Condenser F Steam................ Steam. .......... Steam Ammonia........... 185 lb gage head pressure...... Carbon dioxide.. 1030 lb gage Methyl head pressure...... 102 lb gage chloride.......... Dichlorodi- 117 lb gage fluoromethane head pressure..... 99.7 114.3 126.0 96.0 86.0 100.0 100.0 Leaving Hot Water Temperature F Best Design Average Design 97 93 110 105 120 114 92 88 83 81 96 92 96 93 ' 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 temperature go above this figure the head' pressure will exceed 185 lb and power.con-' 152 11--Chapter Humidifying and Dehumidifying Equipment sumption 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 deg or 4 deg of the ammonia temperature cor responding to the head pressure, while a small condenser might require a 10 deg difference. Table 2 lists several gases with data as to the temperature and pressures 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. The temperature range, once the hot water temperature is approxi mately known, depends upon: 1. Maximum wet-bulb temperature at which the full quantity of heat must be dissipated. 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 plaiit 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 1931, inclusive, there were but 6 hrs 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 refrigeration 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 a refrigerating 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 1, Chapter 8, shows safe . design wet-bulb temperature 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 efficiency range of the type of atmospheric water cooling apparatus to be used. Efficiency of atmos- 153