Forced draft

In petrochemical plants, fans are most commonly used ia air-cooled heat exchangers that can be described as overgrown automobile radiators (see HeaT-EXCHANGEtechnology). Process fluid ia the finned tubes is cooled usually by two fans, either forced draft (fans below the bundle) or iaduced draft (fans above the bundles). Normally, one fan is a fixed pitch and one is variable pitch to control the process outlet temperature within a closely controlled set poiat. A temperature iadicating controller (TIC) measures the outlet fluid temperature and controls the variable pitch fan to maintain the set poiat temperature to within a few degrees. 

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). 

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. 

Induced-draft units are less likely to recirculate the hot exhaust air, since the exit air velocity is several times that of the forced-draft unit. Induced-draft design more readily permits the installation of the aircooled equipment above other mechanical equipment such as pipe racks or shell-and-tube exchangers. 

In a service in which sudden temperature change would cause upset and loss of product, the induced-draft unit gives more protection in that only a fraction of the surface (as compared with the forced-draft unit) is ejmosed to rainfall, sleet, or snow... 

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. 

Air recirculation. Prevailing winds and the locations and elevations of buildings, equipment, fired heaters, etc., require consideration. All air-cooled heat exchangers in a bank are of one type, i.e., all forced-draft or all induced-draft. Banks of air-cooled exchangers must be placed far enough apart to minimize air recirculation. 

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... 

FIG. 27-30 Lo v -NO, combination oil/ as forced-draft boiler burner. (Todd Cojiihnsliort, Iric.)... 

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... 

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. 

Size Type Induced Forced Draft No. of Bays ... 

A forced draft thermal recuperative oxidizer consists of a fan which forces the... 

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. 

Forced Draft Air introduced at the bottom of the tower is forced to the top by a centrifugal blower. 

Furnaces and Boilers - Two potential forms of overpressure may apply to furnaces and boilers overpressure of the firebox by forced-draft fans or tube ruphire and overpressure of tubes due to loss of fluid flow or outlet blockage, with resultant overheating. 

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 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. 

Starkbrenner, m. a kind of burner with self-forced draft. 

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. 

Forced draft ducts Induced-draft flues and breeching Chimneys and stacks Water lines (max.)... 

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