Induced-draft fan

In wetted-wall units, the walls of a tall circular, slightly tapered combustion chamber are protected by a high volume curtain of cooled acid flowing down inside the wall. Phosphoms is atomized by compressed air or steam into the top of the chamber and burned in additional combustion air suppHed by a forced or induced draft fan. Wetted-waU. plants use 25—50% excess combustion air to reduce the tail-gas volume, resulting in flame temperatures in excess of 2000°C. The combustion chamber maybe refractory lined or made of stainless steel. Acid sprays at the bottom of the chamber or in a subsequent, separate spraying chamber complete the hydration of phosphoms pentoxide. The sprays also cool the gas stream to below 100°C, thereby minimising corrosion to the mist-collecting equipment (typically type 316 stainless steel). 

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. 

Induced-draft fans combined with collectors,... 

One design that is used for installation convenience, but can also allow higher efficiency, is an induced draft fan that pulls in extra air to cool the flue products to temperatures that can be safely vented through plastic pipe. Because of the fan, long horizontal venting runs arc possible with this system. 

For study purposes the effects of performance as related to fan horsepower may be patterned after Figures 9-121 and 9-122. The conditions for actual air inlet condiuons for an induced draft fan must be obtained from Equation 9-127 read from a diagrtim similar to Figure 9-109. 

Air density at oudet = 0.067 Ib/ft (close to 95% saturation at hot water temperature, induced draft fan condition). 

The advantages and disadvantages of forced and induced draft fan operation on the performance of the unit as presented by Hudson Products Corp. are used by permission in the following discussions. 

Figure 12-123B-2. Wear strip construction on induced draft fan for dirty gases. The inertia of suspended dust particles carries them toward the backplate or centerplate where the wear plate withstands the abrasion normally affecting the blade. (Used by permission Bui. 2-5100. The Howden Fan Co.)...

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. 

This is a very versatile stoker common in shell boilers and is used with modification in watertube boilers as the traveling-grate stoker. In shell boilers, the air supply is via forced- and induced-draft fans in combination with a balanced or slightly negative condition in the furnace. 

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... 

Induced draft fans (ID fans) are located near the end of the flue-gas path and provide a negative static pressure below that of atmospheric pressure (creating a partial vacuum). This induces outside air to infiltrate any openings in the boiler. 

A pneumatic conveyer system consists of a long tube or duct to carry air at high velocities, an induced draft fan to propel the air, a suitable feeder for addition and dispersion of particulate solids... 

A feed section, primary and secondary chambers, scrubber, and induced draft fan. 

The burners at the reformer s top are in an enclosure called a penthouse. The flue gas is collected at the bottom in horizontal fire-brick ducts called tunnels. Flue gas exits horizontally into a waste heat recovery (WHR) unit. Combustion gas is drawn through the WHR unit by an induced-draft fan and then discharged to the atmosphere through a stack. 

The vent gas from the ERH flows through an electrically heat-traced duct (to prevent condensation) into an electric heat exchanger that raises its temperature to 260°C (500°F) and then into a CATOX unit to destroy residual organic contaminants. The gas is then cooled to between 12°C and 15°C (60°F) in a scrubber/con-denser that removes additional contaminants. The gas then flows through an induced draft fan, discharging into the plant s HVAC system. 

The pressure in the ERH is continuously monitored at the vent gas exit duct of the ERH drum to ensure that it operates at subatmospheric pressure. In addition, the hydrogen gas concentration is measured in the ERH drum vent gas exit duct to ensure that hydrogen gas concentration remains below 50 percent of its LEL. Signals from these monitors are used to control the ventilation gas flow by changing the speed of the induced draft fan. 

The initial sampling site (runs 1-12) was an existing 3-in. port on the west side of the south duct leading from the induced draft fan to the stack. This duct was one of two parallel ducts between the electrostatic precipitator and the stack. Three dual tests (13-18) were performed in the north side of the same duct where a 6-in. port had been installed. 

An induced-draft fan (see Fig. 14.1) is a different story. As the tube bundle fouls... 

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.

At the ABCDF, vent gases from the neutralization reactors and the ton container effluent tanks pass through a scrubber, two carbon filters in series, and a fan, prior to entering the HVAC ducts that exit the three toxic/neutralization cubicles. These exhaust HVAC ducts lead to the final banks of activated carbon filters and to the induced draft fans. 

Vent gases from the hydrolysate storage tanks and the SBRs at the ABCDF are vented to a condenser, a knockout drum, two activated carbon filters in series, and an induced draft fan, prior to being discharged to the atmosphere through the plant stack. At the NECDF, the vents of the four hydrolysate storage tanks are directed through a reflux condenser to a common vent cubicle, then to the activated carbon filters and the induced draft fans of the HVAC system, and finally to the plant stack. 

Slightly lower horsepower due to cooler air temperature as compared to induced-draft fan motors. (Fan horsepower is directly proportional to air temperature.)... 

Focusing repair efforts to the source of the machine problems via vibration diagnoses so that repairs are effective and efficient. One service request for the vibration technicians involved a 200-H.P. motor-driven, induced-draft fan. An area maintenance crew considered sending the large motor to a repair shop because they were unable to get the system to operate smoothly. The vibration technician diagnosed the problem as a simple imbalance in the fan, which was corrected easily with balance weights. 

Combustion calculations show that an oil-fired watertube boiler requires 200,000 lb/h (25.2 kg/s) for air of combustion at maximum load. Select forced- and induced-draft fans for this boiler if the average temperature of the inlet air is 75°F (297 K) and the average temperature of the combustion gas leaving the air heater is 350°F (450 K) with an ambient barometric pressure of 29.9 inHg. Pressure losses on the air-inlet side are, in inFLO air heater, 1.5 air supply ducts, 0.75 boiler windbox, 1.75 burners, 1.25. Draft losses in the boiler and related equipment are, in inH20 furnace pressure, 0.20 boiler, 3.0 superheater, 1.0 economizer, 1.50 air heater, 2.00 uptake ducts and dampers, 1.25. Determine the fan discharge pressure and horsepower input. The boiler burns 18,000 lb/h (2.27 kg/s) of oil at full load. 

Compute the quantity of flue gas handled. The quantity of gas reaching the induced draft fan is the sum of the actual air required for combustion from step 1, air leakage in the boiler and furnace, and the weight of fuel burned. With an air leakage of 10 percent in the boiler and furnace (this is a typical leakage factor applied in practice), the gas flow is as follows ... 

Compute the induced-draft fan discharge pressure. Find the sum of the draft losses from the burner outlet to the induced-draft inlet. These losses are, for this boiler ... 

Induced-draft fans handle hot, dusty combustion products. For this reason, extreme care must be used to choose units specifically designed for induced-draft service. The usual choice for large boilers is a centrifugal-type unit with forward- or backward-curved, or flat blades, depending on the type of gas handled. Flat blades are popular when the flue gas contains large quantities of dust. Fan bearings... 

---