Natural draft

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. 

Natural-draft, or hyperbolic-type, towers have been in use since about 1916 in Europe and have become standard equipment for the watercooling requirements of British power stations. They are primarily... 

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. 

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

Vapors (from drying) are removed at the feed end of the dtyer to the atmosphere through a natural-draft stack and settling chamber or wet scrubber. When employed in simple drying operations with 3.5 X 10 to 10 X 10 Pa steam, draft is controlled by a damper to admit only sufficient outside air to sweep moisture from the cylinder, discharging the air at 340 to 365 K and 80 to 90 percent saturation. In this way, shell gas velocities and dusting are minimized. When used for solvent recovery or other processes requiring a sealed system, sweep gas is recirculated throu a scrubber-gas cooler and blower. 

By viriue of its vertical construction, the turbo-type tray dryer has a stack effect, the resulting draft being frequently sufficient to operate the dryer with natural draft. Pressure at points within the dryer is maintained close to atmospheric, as low as 0.1, usually less than 0.5 mm of water. Most of the roof area is used as a breeching, lowering the exhaust velocity to settle dust back into the dryer. 

Vlanv oil biirnor.s arc do.SLgnod a.s cornbination ga.s/oil biirnor.s, An example of a modern low-N(), oil/ga.s forced-draft burner Ls. shown in Fig, 27-30, This is an air-staged design, with the air divided into pri-rnaiv, secondarv, and tertiarv streams. An air-staged natural draft process heater oil/gas burner is illustrated in Fig, 27-3L... 

Fuel-Staged Burners Use of fuel-staged burners is the preferred combustion approach for NO control because gaseous fuels typically contain little or no fixed nitrogen. Figure 27-36 illustrates a fuel-staged natural draft refineiy process heater burner. The fuel is spht into primaiy (30 to 40 percent) and secondary (60 to 70 percent) streams. Furnace gas may be internally recirciJated by the primaiy... 

FIG. 27-36 Low-NOj fuel-staged burner for a natural draft refinery process beater. (Callidus Technologies, Inc. )... 

Increased capacity in the event of fan failure, since the natural draft stack effect is much greater with induced draft. 

Low natural draft capability on fan failure due to small stack effect. 

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. 

For natural draft fire tubes, the minimum cross-sectional area oi liu-fire tube is set by limiting the heat release density to 21,000 Biu/lir nr. At heat relea.se densities above this value, the flame may become utr.ia ble becriuse of insufficient air. Using this limit, a rninimuin fire eiv diamelei is established by ... 

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]. 

The technology of kerosene burners is quite mature. The most popular kerosene heater is the perforated sleeve vaporizing burner or range burner (Figure 1). It consists of a pressed steel base with concentric, interconnected grooves and perforated metal sleeves, between which combustion takes place. Kerosene is maintained at a depth of about 1/4 inch in the grooves. As the base heats up, oil vaporizes from the surface, and the flame lights from asbestos wicks. Combustion air is induced by natural draft. The flame is blue, and the burner is essentially silent, odorless, and smokeless. 

Gas-Fired water heaters are also made more efficient by a variety of designs that increase the recov-ei y efficiency. These can be better flue baffles multiple, smaller-diameter flues submerged combustion chambers and improved combustion chamber geometry. All of these methods increase the heat transfer from the flame and flue gases to the water in the tank. Because natural draft systems rely on the buoyancy of combustion products, there is a limit to the recovery efficiency. If too much heat is removed from the flue gases, the water heater won t vent properly. Another problem, if the flue gases are too cool, is that the water vapor in the combustion products will condense in the venting system. This will lead to corrosion in the chimney and possible safety problems. 

This tower depends upon natural draft action the same as a chimney to draw cool air in at the bottom and expel it out the top as warm moist air (Figure 9-101). The action of the tower depends upon the atmospheric temperature therefore, on a hot day the action of the tower may be less than on a cool day. These towers are relatively large, and require power for pumping the water to a point in the tower which is usually lower than for an atmospheric tower. There are no fan costs. Units have been built 310 ft high, base diameter 210 ft and a throat of 120 ft, wdening to 134 ft in diameter at the top [30]. 

This type of tower uses fans at the base to force air through the tower fill or packing (Figure 9-102). Due to the relatively low oudet air velocity, there is a tendency for discharged hot air to recirculate into the fan intake and reduce tower performance. The fan handles only atmospheric air thereby reducing its corrosion problem when compared to the fan on an induced draft tower. The tower size for the forced as well as the induced draft unit is considerably less than for an atmospheric or natural draft unit due to the higher heat transfer rates. 

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. 

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