Fans, induced forced-draft

External means of mechanical circulation are induced draft fans and forced draft fans. Neither can do as thorough a job of in-furnace circulation as well-planned and strategically placed burner jets, but these draft fans or blowers do assist in overall transport or movement of gases out of and into a furnace. Induced draft fans have their inlet connected to the furnace, and fherefore create a suction or negative pressure forced draft fans and blowers have their outlet connected to the furnace, and therefore create a positive pressure. Large power boilers often have both induced and forced... 

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. 

Fig. 7. Ain preheaters where ID = induced draft and FD = forced draft fan. (a) Rotating metal basket or Lungstrom regenerative preheater and (b) hot oil or water belt (Uquid mnaround) used to move convection section heat to ain preheater in furnace retrofit.

Forced and Induced Draft The forced-draft unit, which is illustrated in Fig. 11-43 pushes air across the finnedtube surface. The fans are located oelow the tube bundles. The induced-draft design has the fan above the bundle, and the air is pulled across the finned tube surface. In theoiy, a primaiy advantage of the forced-draft unit is that less power is required. This is true when the air-temperature rise exceeds 30°C (54°F). 

Fan mounting should provide a minimum of one-half to three-foui ths diameter between fan and ground on a forced-draft heat exchanger and one-half diameter between tubes and fan on an induced-draft cooler. 

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... 

In addition to forced draft systems, induced draft systems are also used. The induced draft system is slightly more expensive but is recommended when particulate, or organic oils are present. Particulates impact upon a forced draft fan and will have a negative effect on the system performance. If the process stream is clean a forced draft system is appropriate. 

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. 

Firebox Overpressure - The firebox of a forced-draft furnace and boiler is designed to withstand the overpressure that can be generated by the fans with dampers in their closed position. This needs to be specially checked when both forced and induced-draft fans are provided to discharge combustion products through heat recovery facilities, since higher than normal fan pressures may be used to overcome pressure drop. In the case of high-pressure process furnaces, a tube rupture could also be the cause of firebox overpressure. 

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. 

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. 

Originally, the height of the chimney was designed to produce a draft sufficient to produce induced-draft air for combustion. With modern boiler plant, forced-draft and/or induced-draft fans are used. This allows for the greater degree of control of the air to be designed into the combustion appliance. The chimney is therefore only required to disperse the gases. 

Legend 1 = steam header, 2 = steam drum, 3 = attemperator, 4 = superheater, 5 = top header, 6 = riser and downcomer (note downcomer is outside the boiler), 7 = bottom header, 8 = water wall tube membrane (with radiant area inside membrane), 9 = burners, 10 = mud dmm, 11= boiler bank, 12 = economizer, 13 = dust collector, 14 = forced draft fan, 15 = air-heater, 16 = induced draft fan, 17 = stack... 

Fans direct air movement within the furnace or combustion area. The two primary fan designs are forced draft and induced draft. Draft is the difference between atmospheric pressure and the static-pressure of combustion gases. 

Most air coolers are either induced-draft or forced-draft, as shown in Fig. 14.1. The more common arrangement being forced draft. The air is moved by rather large fans. The tubes are surrounded with foil-type... 

Naturally, there is no reverse airflow on an induced-draft fan. That can occur only in a forced-draft fan. Reverse airflow can be observed with a forced-draft fan, by seeing which portions of the screen, shown in Fig. 14,2, will not allow a dollar bill to stick to the underside of the... 

Figure 8.4. Example of tubular heat exchangers (see also Fig. 8.14). (a) Double-pipe exchanger, (b) Scraped inner surface of a double-pipe exchanger, (c) Shell-and-tube exchanger with fixed tube sheets, (d) Kettle-type reboiler, (e) Horizontal shell side thermosiphon reboiler, (f) Vertical tube side thermosiphon reboiler, (g) Internal reboiler in a tower, (h) Air cooler with induced draft fan above the tube bank, (i) Air cooler with forced draft fan below the tube bank.

Figure 9.18. Main types of cooling towers, (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, [(fc)-(e) from Cheremisinoff and Cheremisinoff, 1981).

Air-moled h.Mlamxir—As used in the petroleum industry, air-cooled exchangers normally comprise two headers joined h> a horizontal bank ol tinned tubes. Usually two motor-driven fans located above (induced dralii or below (forced draft) the tubes are used lo circulate the air over the tinned surface. 

Air paths may be crossflow or counterflow. Fan placement can be ahead of the fill section (forced draft) or behind it (induced draft). Manufacturers use different criteria in sizing units. Figure 4.18 reviews one tower manufacturer s approach. Specific design details should be obtained from the tower manufacturer. 

Enclosed air space between drift eliminators in induced towers and between fan and fill in forced draft towers. 

Air-cooled exchangers are classified as forced-draft when the tube section is located on the discharge side of the fan (see Fig. 5.4). Air coolers are classed as induced-draft when the tube section is located on the suction side of the fan. 

Choose the fan size from the capacity table. Check the capacity table to be sure that it lists fans suitable for induced-draft (elevated temperature) service. Turn to the 11-in static-pressure-capacity table and find a capacity equal to 110,355 ft3/min. In the engineering data used for this fan, the nearest capacity at 11-in static pressure is 110,467 ft3/min, with an outlet velocity of 4400 ft3/min, an outlet velocity pressure of 1.210 U1H2O, a speed of 1222 r/min, and an input horsepower of 255.5 bhp. The tabulation of these quantities is of the same form as that given for the forced-draft fan (step 2). The selected capacity of 110,467 ft3/min is entirely satisfactory because it is only (110,467 — 110,355)/110,355 = 0.00101, or 0.1 percent, higher than the desired capacity. 

Plot the power-input curve AFED for inlet-vane control on the forced-draft fan or inlet-louvre control on induced-draft fans. The data for plotting this curve can be obtained from the fan manufacturer. 

Figure 7.1 shows a typical furnace in which fuel and air are introduced into the combustion zone. A control valve is shown on the air line, but this is just schematic. More typically the manipulation of the air flow is achieved by varying the speed of a forced-draft fan (on the inlet of the furnace). The pressure inside the furnace is controlled by varying the speed of an induced-draft fan (on the outlet of the furnace). 

Figure 105. Modern integrated single-train ammonia plant based on steam reforming of natural gas (Clide process) a) Sulfur removal b) Primary reformer c) Steam superheater d) Secondary reformer e) Waste heat boiler f) Convection section g) Forced draft fan h) Induced draft fan i) Stack k) TIT and LT shift converters ...

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