Document RKo5D4J6qxVQVgZYJ6rQKKJz

HEATINC VENTILATING AIR CONDITIONING CUIDE 1944 for design, and when the plant is operating at full load. The apparatus must also, for best results, be located with its longest axis at right angles to the direction of the prevailing hot weather breeze. Table 1, Chapter 7, gives the average summer wind velocities and directions in representative cities. Natural draft cooling equipment should be designed to operate properly with not more than one-half of the average wind velocity, and in no case for a wind velocity of more than 5 mph. Natural draft equip ment must not be obstructed by trees, buildings, or other wind deflectors. Mechanical Draft Towers Mechanical draft towers usually consist of vertical shells, constructed of wood, metal, or masonry, in which water is distributed uniformly at the top and falls to a collecting basin at the bottom. The injide of the tower may be filled with wood checker-work over which the water drips, or the water surface may be presented to the air by filling the entire inside of the structure with spray from nozzles. Air is circulated through the tower from bottom to top by forced or induced draft fans. Since the air flows counter to the water, the air is in contact with the hottest of the water just before leaving the top of the tower, and each unit of air picks up more heat than a similar unit would on natural draft equipment, so the me chanical draft tower cools water by using less air than the other types of equipment need. As movement of the air through the towers is obtained by power-consuming fans, it is essential that the air used be reduced to a minimum so as to secure the lowest possible operating cost. The effectiveness of a mechanical draft tower is increased by increasing height, area, or air quantity. Increasing the height increases the length of time the air is in contact with the water without affecting seriously the fan power required, but it increases the pumping power needed. In creasing the area while maintaining constant fan power increases the air quantity somewhat and because of lowered velocities it increases the time this air is in contact with the water. The surface area of water in contact with the air is increased in both cases. Increasing the air quantity decreases the time the air is in contact with the waterrbutTsince a greater quantity is passing through, the average differential between the water temperature and the wet-bulb temperature of the air is increased, and this speeds up the heat transfer rate. Increased air quantities are obtained only at the expense of increased fan power, which increases approximately as the cube of the air quantity. Air velocities through mechanical draft towers vary from 250 to 450 fpm over the gross area of the structure. Mechanical draft water cooling equipment may be set up inside build ings, where it usually draws its air supply from the general space in which it is installed, and discharges its exhaust air through a duct to the outside. Indoor cooling towers may be either of the wood-filled or the spray-filled type. In many cases where little height but considerable area is available, water is cooled in a spray-filled structure similar to an air washer, with the air passing horizontally .through the apparatus and being discharged through a duct to the outside. Cooling Tower Design The method of design of equipment for energy transfer from water to an air-water vapor mixture is similar to that used for absorption equip- 536 CHAPTER 27. SPRAY EQUIPMENT ment. Details of this procedure are available4,6'6 and its application to the problem of the cooling tower operating at atmospheric pressure is illustrated by the following development. The nomenclature used is as follows: a = overall average wetted area (surface of water drops plus wetted tower surface) square feet per cubic foot of tower volume. ' c *= specific heat of liquid water, Btu per pound per degree Fahrenheit, e = effectiveness, e := natural base. G = weight rate of flow of air, pounds of dry air per hour. h -- enthalpy, Btu per pound of dry air. ha = enthalpy of air-vapor mixture, Btu per pound of dry air. /n >= enthalpy of saturated. air-vapor mixture at water tem perature, Btu per pound of dry air. K =. overall energy unit conductance, Btu per hour per square foot overall average wetted area per (Btu enthalpy dif ference per pound of dry air). Ka = overall rate coefficient, Btu per hour per cubic foot of tower volume per (Btu enthalpy difference per pound of dry air). L = water rate, pounds per hour. Im logarithmic mean. 5 average cross-sectional area of cooling tower for air flow, square feet. I = water-main body temperature, degrees Fahrenheit, fwb -- wet-bulb temperature, degrees Fahrenheit. V = tower volume, cubic feet. Note: Subscripts 1 and 2 when used in equations refer to water entrance and exit sections respectively, for the counter-flow tower. Conditions 1 Water flow L Lb per hr Energy exchange Lcdt.Gdh dV Air G flow Lb per hr Conditions 2 Fig. 4. Section of Typical CouNTER-Ff.ow Tower A section of a typical counter-flow tower is shown in Fig. 4. If the reduction in water rate due to evaporation within the volume is neglected, the energy balance for this differential section of the exchanger volume may be written as: Lcdt = Gdh (2) The potential for net energy transfer due to heat and mass transfer from the water to the mixture in contact with it may be expressed with reasonable accuracy as the difference between the enthalpy of saturated air at the water temperature, k", and the enthalpy of the main stream air vapor mixture7, h*. The rate of energy transfer is given by the expression: Ka (A" -- ha) dV (3) which equation defines the overall rate coefficient, Ka; the latter being the product of the overall energy unit conductance, K, and the ratio of the transfer surface to the exchanger volume, a. Equations 2 and 3 are conveniently illustrated by means of the tem perature-enthalpy diagram of Fig. 5. Equation 2 indicates that the succession of air and water states existing in the exchanger sections must combine to form a straight line (for L = constant) on the temperature `Principles of Chemical Engineering, byW. H. Walker, W. K. Lewis,W. H. McAdams and E. R. Gilliland (McGraw-Hill Co., 1937, p. 480). `Absorption and Extraction, by T. K. Sherwood (McGraw-Hill Co., 1937, p. 91). Performance Characteristics of a Mechanically Induced Draft, Counterflow, Packed Cooling Tower, by A. L, London. W. E. Mason and L. M. K. Boelter {AS.M.E. Transactions, January, 1940,VoL62,p.41). 'Determination of Unit Conductances for Heat and Mass Transfer by the Transient Method, by A. L. London, H. B. Nottage and L. M. K. Boelter {Industrial and Engineering Cherhistry, April, 1941, Vol. 33, p. 467). 537