Natural-draft lowers

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. 

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]. 

In general, a fully modulating burner will be thermally more efficient, particularly if it maintains air/gas ratio accurately at intermediate rates. For natural-draft burners this may not be the case, as, in general, the air is not modulated, leading to progressively more excess air at lower rates. 

Natural draft is a low capital cost option, but the draft requirements tend to lead to high stack temperature with a low efficiency for the furnace. Forced draft and induced draft are higher capital cost options than natural draft but tend to lead to higher furnace efficiencies as the stack temperature can be lowered significantly. 

Natural draft. Natural draft cooling towers consist of an empty shell, usually constructed in concrete. The upper, empty portion of the shell merely serves to increase the draft. The lower portion is fitted with the packing. The draft is created by the difference in density between the warm humid air within the tower and the denser ambient air. 

In Fig. 20.6, we see a simple natural-draft heater with no convective-section tubes. The laws of hydraulics tell us that fluids flow from regions of high pressure to regions of lower pressure, and yet the draft readings in Fig. 20.6 seem to contradict this principle. 

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

Figure 9.18. Main types of cooling lowers, (a) Atmospheric, dependent on wind velocity, (b) Hyperbolic stack natural draft, (c) Hyperbolic assisted with forced draft fans, (d) Counterflow-induced draft, (e) Crossflow-induced draft, (f) Forced draft, (g) Induced draft with surface precooler for very hot water also called wet/dry tower. [(b)-(e) firom C3ieremisineff and Cheremisinoff, 1981).

The incompressible flow equation used here is accurate for natural draft devices where the resulting air velocity is much lower than the speed of sound. However, it is not a good assumption for fuel flow from a fuel orifice as fuel pressures are normally 25 psig or so and the exit velocity is sonic. For these cases, we must make use of a more inclusive form of the Bernoulli equation. 

The basic approach to improving simple biomass stoves is to enclose the combustion zone and vent to the ontside through a flue/chimney. Unfortunately, such designs can sometimes actually lower total efficiency because of the high airflow induced by natural draft in the flue reduce heat transferto the pot. Shutting back the airflow with dampers can lead to a rise in total efficiency, but at the same time may reduce combustion efficiency sufficiently to actually increase PIC (pollutant) emissions per meal. If vented out-... 

A furnace or fired heater can be classified as natural, induced, forced, or balanced draft. The pressure inside a warm furnace is typically lower because of buoyancy differences in the cooler outside air. A natural-draft furnace can operate using this approach however, when fans are used to push or pull the air through the furnace, greater heat transfer rates can be achieved. A natural-draft fired heater is severely limited in contrast to these systems. 

The advantages of forced draft air cooler are 1) tube bundle is easier to be removed for service, 2) fan and motor are easier to be installed and accessible for maintenance, 3) easier for warm air recirculation ftireold climate service, 4) use slightly less horsepower, since air to fan is cooler. Its disadvantages are 1) poorer air distribution over tube bundle, 2) tOip of tube bundle is exposed to ixmbient weather, 3) bottom of tube bundle is hard to reach for cleaning, 4) more chance for hot exit air recirciilation, due to lower exit air velocity and no stack, 5) less cooling in case fans are failed due to tow natural draft. 

The turndown ratio is an indication of the ability of the burner to maintain a stable flame at lower firing rates, and is a ratio of the maximum and minimum firing rates. Turndown can be low for average burners of both natural-and forced-draft burners, 3 1 being a typical figure with 5 1a maximum although up to 40 1 is possible with special burners. 

Note. The draft natural product rules [26] recommend that the new carbon atom takes the locant of the lower-numbered proximal atom. However, carbohydrate chemists regard the ring oxygen as formally originating from the non-anomeric (usually higher-numbered) position. 

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