Draft Cooling Towers

Plant layout and noise suppression material are two general noise abatement methods. Plant layout does not affect noise levels at any given point however, noise can be abated by screening off a section of the plant. An example of this is to orient cooling towers with their closed faces toward the critical location. This method must also consider wind direction to balance air draft. Tankage can be located to act as a noise screen. 

Fig. 8. Transverse cross-sectional view of double-flow induced-draft cooling tower. Courtesy of The Madey Co.

The thermal design of cooling towers follows the same general procedures already presented. Integration of equation 35 is usually done numerically using the appropriate software, mass-transfer coefficients, saturation enthalpies, etc. In mechanical-draft towers the air and water dows are both suppHed by machines, and hence dow rates are fixed. Under these conditions the design procedure is straightforward. 

Fig. 9. Natural-draft cooling tower (a) general tower drawing for countercurrent air—water dow arrangement (b) sectional drawing showing arrangement...

Natural-draft cooling towers are extremely sensitive to air-inlet conditions owing to the effects on draft. It can rapidly be estabUshed from these approximate equations that as the air-inlet temperature approaches the water-inlet temperature, the allowable heat load decreases rapidly. For this reason, natural-draft towers are unsuitable in many regions of the United States. Figure 10 shows the effect of air-inlet temperature on the allowable heat load of a natural-draft tower for some arbitrary numerical values and inlet rh of 50%. The trend is typical. 

Cooling-Tower Plumes. An important consideration in the acceptabiHty of either a mechanical-draft or a natural-draft tower cooling system is the effect on the environment. The plume emitted by a cooling tower is seen by the surrounding community and can lead to trouble if it is a source of severe ground fog under some atmospheric conditions. The natural-draft tower is much less likely to produce fogging than is the mechanical-draft tower. Nonetheless, it is desirable to devise techniques for predicting plume trajectory and attenuation. 

J. R. Singham, The Thermal Peformance of Natural Draft Cooling Towers, Imperial CoUege of Science and Technology, Department of Mechanical Engineering, London, 1967. 

Mechanics-draft cooling towers normally are designed for L/Q ratios ranging from 0.75 to 1.50 accordingly, the vSues of KaV/L vaiy from 0.50 to 2.50. With these ranges in mind, an example of the use of the nomograph will readily explain the effecd of changing variables. 

FIG. 12-14 Sizing chart for a coiinterflow induced-draft cooling tower, for induced-draft towers with (1) an iipspray distributing system with 24 ft of fill or (2) a flume-type distributing system and 32 ft of fill. The chart will give approximations for towers of any height. (Ecodyne Carp.)... 

FIG. 12-15 Horsepower chart for a coiinterflow induced-draft cooling tower. [Fluoi Coij). (now Kcodyne Cotj .)]... 

FIG. 12-22 Universal performance chart for natural-draft cooling towers. (Risk and Steel, ASCE Symposium on Thermal Power Plants, October 1958. )... 

Cooling towers are broadly classified on the basis of the type of draft natural draft (natural convection), mechanical draft (forced convection) and mechanical and natural. Further distinction is made based on (1) the type of flow i.e. - crossflow, counterflow, cocurrent flow (2) the type of heat dissipation-wet (evaporative cooling), dry, wet-dry and (3) the type of application-industrial or power plant. Each of the major types of cooling towers has a distinct configuration. The major designs are summarized in Figures 1 through 8 and a brief description of each follows. 

Lichtenstein, J. "Performance and Selection of Mechanical Draft Cooling Towers, " ASME Trans. (1943). 

A large natural-draft cooling tower collapsed in a 70-mph (110-km/hr) v/ind, probably due to imperfections in the shape of the tower, which led to stresses greater than those it was designed to take and caused bending collapse [10]. 

Natural and forced-draft cooling towers are generally used to provide the cooling water required on a site, unless water can be drawn from a convenient river or lake in sufficient quantity. Seawater or brackish water can be used at coastal sites, but if used directly will necessitate more expensive materials of construction for heat exchangers. 

Figure 9-101. Component parts of modem natural draft tower. Used by permission of Hamon Cooling Towers, Inc.

Figure 9-102. Cross-section of iow-head forced-draft tower showing fan housing arrangement, fiiling, water distribution spray system and spray eliminators. Used by permission of Foster Wheeler Corp., Cooling Tower Dept.

Figure 9-103A. Counterflow induced draft cooling tower. Used by permission of The Pritchard Corp. (now, Black and Veatch Pritchard).

The effects of wet bulb, approach and range on mecbanical draft cooling tower size is indicated in Figure 9-118. 

The economics of forced and induced draft cooling tower operation require a study of fan and water pump horsepower and usually dictate a fan static pressure requirement not to exceed 0.75-1.0 in. of water. For atmospheric and natural draft towers the economics of pumping water are still very important. This means that the ground area must be so selected as to keep the height dovm while not dropping the unit rates so low that performance becomes poor. This then, is a balance of ground area versus total deck height. Pritchard [16] presents an... 

Example 9-16 Wood Packed Cooling Tower with Recirculation, Induced Draft... 

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