Natural-draft towers

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. 

Data for determining the size of natural-draft towers have been presented by Chilton [Proc. Inst. Elec. Eng., 99,440 (1952)] and Rish and Steel (ASCE Swuposium on Thermal Power Plants, October 19.58). Chilton showed that the duty coefficient Df of a tower is approximately constant over its normal range of operation and is related to tower size by an efficiency factor or performance coefficient as follows ... 

To determine how a natural-draft tower of any given duty coefficient will perform under varying conditions, Rish and Steel plotted the nomograph in Fig. 12-22. The straight hne shown on the nomograph illustrates the conditions of Example 14. 

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

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... 

Data is given in Figure 9-129 for tvater-air system. Performance of Atmospheric and Natural Draft Towers... 

The evaluation of atmospheric and natural draft towers has not been completely presented in the detail comparable to mechanical draft towers. Some data are available in estimating form, but the evaluation of transfer rates is only adequate for estimating purposes [4]. The design of such towers by the process engineer must be made only after due consideration of this, and ample factor of safety should be included. Figure 9-130 presents general information on water loss due to wind on the tower. 

Having selected and purchased a cooling tower, it needs regular maintenance, as does any other part of the plant. This is true of every cooling tower, from the largest natural-draft tower to the smallest packaged unit. 

Figure 34.15 Ice on natural-draft tower (no de-icing ring fitted)...

Chimney-assisted natural draft towers are of hyperboloidal shapes because they have greater strength for a given thickness a tower 250 ft high has concrete walls 5-6 in. thick. The enlarged cross section at the top aids in dispersion of. exit humid air into the atmosphere. 

Evaporation losses are about 1% of the circulation for every 10°F of cooling range. Windage or drift losses are 0.3-1.0% for natural draft towers and 0.1-0.3% for mechanical draft. Usually the salt content of the circulating water is limited to 3-7 times that of the makeup. Blowdown of 2.5-3% of the circulation accordingly is needed to maintain the limiting salt concentration. 

The performance of a natural-draft tower is characterized in terms of a duty coefficient (Cd), which defines the overall capabilities of a tower under all operating conditions ... 

Operation Maintenance Costs Pumping head is less, so power cost for the circulating water pumps is less. Power cost for fans is considerable. Cost of maintaining fans and associated drives and transmissions is also significant. Total operating cost will favor natural-draft towers. 

Recirculation, Fogging These are major problems. Design accommodation, restrictions on tower dimensions, orientation with prevailing winds, and added capacity for recirculation can boost tower cost. Because of its elevated discharge, the natural-draft tower rarely has the trouble with recirculation and fogging. 

Figure 4.22 Indirect, dry-type cooling tower condensing system employing a natural-draft tower.

In natural-draft towers, excessive air flow means lower exhaust air temperatures resulting in larger stacks... 

Windage losses or drift vary with the type of tower and local conditions. Average estimates for normal tower operations are 0.3-1% of circulation for natural-draft towers and 0.1-0.3% of circulation for mechanical-draft towers. 

A natural-draft tower has no fans but usually has completely louvered sides and ends to allow wind to pass horizontally through the dripping water. These towers usually have wooden framing, wooden louvered sides and wooden fill or spray nozzles to break up the flow of water. Their framing and louvers may be of noncombustible material. 

The design of large natural draft cooling towers and analysis of their performance are complicated by the effects of variations in ambient air humidity. Often the effluent air from the tower is assumed to be at 100% relative humidity, to simplify calculations for design parameters. This study avoids the simplification, and proposes a procedure for determining the major design parameters for a natural draft tower. The theoretical and empirical relationships applicable to heat balance, heat transfer and transport, and tower draft and air resistance are given. 13 refs, cited. 

For counterflow natural-draft towers under normal operating heat loads and water loading , minimal ice formation can be expected in moderately cold environments, Figure 9.1 illustrates the variation in basin water temperature with inlet air wet-bulb temperature at different heat loads and water... 

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