For forced draft

The plenum chamber design may be a simple box shape, formed by flat sides and bottom, or curved transition sections may be used to obtain a tapered smooth transition from the rectangular bundle to the circular fan. Either design may be used for forced-draft or induced-draft air cooled heat exchangers. 

Normal recirculation in average installations for forced draft may run 3-10% of total inlet air, and 1-8% for induced draft towers, all depending upon the location and wind conditions during any day or season. Some towers can be arranged to have less than 1% recirculation. If conditions are suspected of being conductive to recirculation, it should definitely be allowed for in design of the tower. Recirculation increases the wet bulb temperature of entering air, increases the total air required (and hence size of... 

On the Minimum Size For Forced Draft Dry Cooling Towers for Power Generating Plants... 

Make appropriate assumptions about windage and evaporation losses and set out and solve an equation for blowdown. Windage losses will be about 1.0 to 5.0 percent for spray ponds, 0.3 to 1.0 percent for atmospheric cooling towers, and 0.1 to 0.3 percent for forced-draft cooling towers for the forced-draft towers in this example, 0.1 percent can be assumed. As for evaporation losses, they are 0.85 to 1.25 percent of the circulation for each 10-degree drop in Fahrenheit temperature across the tower it is usually safe to assume 1.0 percent, so E = AT/10, where AT is the temperature drop across the tower. Therefore, in the present case,... 

Fig. 18.—Characteristics of a multiblade casingless fan for forced-draft work.

Fa/T—mounted at the base of the flare and used for forced-draft operation Knockout drum with water seal Flare header... 

In a forced-draft air cooler, cool air is blown through the underside of the fin tube bundle. In an induced-draft air cooler, cool air is drawn through the underside of the fin tubes. Either way, road dust, dead moths, catalyst fines, and greasy dirt accumulate along the lower row of tubes. As the tubes foul, they offer more resistance to the air flow. However, note that for forced-draft fans ... 

Basically, there are two types of air cooler according to tube bundle location relative to air fan in the unit For forced draft air cooler, tube bundles are located at the discharge side of air fans. For induced draft air cooler, tube bundles are located at suction side of air fans. See Figure S for the difference of these two types of air cooler. Most air coolers are of forced draft type. 

Flare noise (roar of combustion) is the most serious because it is elevated and the sound carries. The flare can be located at a remote distance from the operating unit or surrounding community. Noise of steam injection into the burner can be reduced by using multiple no22les. Furnace noise from air intake, fuel systems, and combustion blower forced draft/induced draft (FD/ID) fans can be reduced by acoustics. The plot plan should be evaluated for noise generation and to find the means of alleviating or moving noise to a less sensitive area. 

The forced-draft design offers better accessibility to the fan for onstream maintenance and fan-blade adjustment. The design also provides a fan and V-belt assembly, which are not exposed to the hot-air stream that exits from the unit. Structural costs are less, and mechanical life is longer. 

Two types of mechanical-draft towers are in use today the forced-draft and the induced-draft. In the forced-draft tower the fan is mounted at the base, and air is forced in at the bottom and discharged at low velocity through the top. This arrangement has the advantage of locating the ran and drive outside the tower, where it is convenient for inspection, maintenance, and repairs. Since the equipment is out of the hot, humid top area of the tower, the fan is not subjected to corrosive conditions. However, because of the low exit-air velocity, the forced-draft tower is subjected to excessive recirculation of the humid... 

For inlet process fluids above 350°F, forced draft design should be used otherwise, fan failure could subject the fan blades and bearings to excessive temperatures. 

Forced draft The forcing of air by means of a fan into a closed chamber for combustion or other purposes. The pressurization of the chamber forces the air and combustion products up a stack. 

Gas A state of matter in which a substance completely fills the region in which it is contained, no matter how small the amount. Or any fuel in a gaseous form for use in an atmospheric or forced-draft burner. 

A boiler had been shut down for the repair of a forced draft fan. A blind was not installed in the fuel gas line, nor apparently was a double block and bleed in the fuel line utilized. Gas leaked into the firebox during the repair period and was not removed. A severe explosion occurred during the attempt to light of. 

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. 

Although the engine was not a success, Ericsson s trip to London allowed him to meet John Braithwaite, a machine manufacturer, who had the expertise to put Ericsson s ideas into practice. In 1828 Ericsson, with Braithwaite, patented the principle of artificial draft in steam boilers. The principle of forced draft was applied to a fire engine and a locomotive entered for the Rainhill locomotive trials of 1829. 

The majority of larger industrial burners, including furnace and boiler applications, are of the forced-draft type. These employ a combustion air fan to provide all the air needed for complete combustion. The burners are usually sealed into the combustion chamber so that there is no access to secondary air from the atmosphere as with natural-draft burners. Forced-draft burners may be of the premix type, where air and gas are mixed prior to the burner, or, more commonly, of the nozzle mix type, where the mixing takes place within the burner. 

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. 

Proprietary terminals are available up to 200 mm, but above this are not used, a Chinese hat terminal being considered adequate to keep out rain and larger foreign bodies. For large flues, particularly forced draft, terminals are not used. 

Forced-draft flues The above design parameters are relevant to natural-draft flues. With forced-draft flues, it is possible by choice of a fan - either forced or induced draft - to overcome system resistance so that the flue will still clear the products. A cmde mle-of-thumb is to allow 1 mm of flue area for each 2.2-3.7kW for natural draft and for 4.5-13.6kW for each forced draft. 

The range of satisfactory operation for a gas burner, defined by light-back, blow-off and incomplete combustion is limited. The variation in gas analyses, particularly higher hydrocarbons and inerts, can influence the range of operation. This has led to the definition of different groups of natural gas. A practical effect is that burners designed for the European continent may not be suitable for the UK without adjustment. This does not apply to forced-draft burners. 

Figure 23.8 is a schematic illustration of such a unit. It is desirable for each boiler to have its own economizer. Where one economizer is installed to take the exhaust gases from more than one boiler special considerations must be taken into account. These will include gas-tight isolation dampers. Consideration must be made of flue-gas pressures at varying loads and maximum and minimum combined heat load to match economizer and a pumped feedwater ringmain. Economizers may be used for both forced-draft and induced-draft boilers, and in both cases, the pressure drop through the economizer must be taken into account when sizing the fans. 

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