Forced-draft fans

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. 

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.

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. 

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. 

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. 

Utilizing a forced-draft fan, the burner has a gas head arranged to mix the fuel and air in a blast tube which controls the stability and shape of the flame. Gas exits from nozzles or holes in the head and is mixed partly in the high-velocity air stream and partly allowed to exit into an area downstream of a bluff body. Behind the bluff body, a relatively quiescent zone forms which provides a means for flame stability. Many configurations exist, but the most... 

Remember that outside influence (for example, new building work in the vicinity of the installation) can increase air-based pollution, such as cement dust entering the tower at air inlet levels or via the forced-draft fan. 

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... 

Forced draft fans (FD fans) provide a positive air and combustion gas static pressure above that of atmospheric pressure. Almost all boilers have FD fans that force air from boiler inlets through the furnace and convection-pass sections. 

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... 

Consider a forced-draft boiler producing 600-psig steam as shown in Fig. 20.2. The fuel rate on this boiler is fixed and we are going to optimize the oxygen (02) content of the flue gas by adjusting the speed of the forced-draft fan. Do we simply adjust the forced-draft (FD) fan to give 2 percent 02 in the stack because someone once said that 2 percent 02 in the stack was a good number ... 

Turn on the forced-draft fan if there is one, to assist the smoke. 

May cause the forced-draft fan to operate out of character, requiring higher driver horsepower. 

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).

Express the required airflow in cubic feet per minute. Convert the required flow in pounds per hour to cubic feet per minute. To do this, apply a factor of safety to the ambient air temperature to ensure an adequate air supply during times of high ambient temperature. At such times, the density of the air is lower and the fan discharges less air to the boiler. The usual practice is to apply a factor of safety of 20 to 25 percent to the known ambient air temperature. Using 20 percent, the ambient temperature for fan selection is 75 + 75(0.20) = 90°F. The density of air at 90°F is 0.0717 lb/ft3, found in Baumeister—Standard Handbook for Mechanical Engineers. Converting, ft3/min = lb/h / 60(lb/ft3) = 296,000/60(0.0717) = 69,400 ft3/min. This is the minimum capacity the forced-draft fan may have. 

Compute the correction factors for the forced-draft fan. Commercial fan-capacity tables are based on fans handling standard air at 70°F at a barometric pressure of29.92 inHg and having a density... 

Obtain the engineering data for commercially available forced-draft fans and turn to the temperature and altitude correction-factor tables. Pick the appropriate correction factors from these tables for the prevailing temperature and altitude of the installation. Thus, in Table 6.30, select the correction factors for 90°F air and 5000-ft altitude. These correction factors are CT = 1.018 for 90 I air and CA = 1.095 for 5000-ft altitude. 

Enter the table at the nearest capacity to that required, 62,250 ft3/min, as shown in Table 6.31. This table, excerpted with permission from the American Standard Inc. engineering data, shows that the nearest capacity of this particular type of fan is 62,595 ft3/min. The difference, or 62,595 — 62,250 = 345 ft3/min, is only 345/62,250 = 0.0055, or 0.55 percent. This is a negligible difference, and the 62,595-ft3/min fan is well suited for its intended use. The extra static pressure, 6.5 — 6.3 = 0.2 inH20, is desirable in a forced-draft fan because furnace or duct resistance may increase during the life of the boiler. Also, the extra static pressure is so small that it will not markedly increase the fan power consumption. 

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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