Forced-draft fans operation

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. 

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

Flue gas is supplied to the scrubbing system with a forced draft fan. Typically, the fan is designed for 20 inches of water pressure drop and is equipped with the typical draft control mechanism. A secondary damper is connected to the vacuum side of the fan. The secondary damper admits makeup air and thereby automatically maintains optimum gas flow in the scrubber during varying loads in the boiler operation. 

The ultimate users may have differing opinions about the size and drive of the force-draft fans. Boiler instrumentation has different requirements for attended and unattended operation. Different fuels have their impact on burner selection, soot blowers, design of heat recovery, etc. Feed water temperature and conditions influence the boiler design and consequently the price. 

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. 

Effluent air temperature should be limited to 220°F to prevent damage to fan blades, bearing, or other mechanical equipment in the hot airstream. When the process inlet temperature exceeds 350°F, forced draft design should be considered because high effluent air temperatures may occur during fan-off or low air flow operations. 

Air cooled induced draft preferred to forced draft to minimize hot gas recirculation. Include a water-cooled trim cooler . Ensure the exit tubes are flooded so that the vapor does not bypass condenser. If extreme cold conditions are expected, allow for fan to operate in reverse to counteract the overcooling by the natural circulation of cold air. 

In another process plant, the steam system operates with a large amount of LP vent. To prevent loss of valuable condensate due to LP vent, a condenser was installed to cool down the LP steam and return LP condensate back to the deaerator. Although this solution saves condensate, it did not resolve the LP long issue. It was later identified that a driver switch could help to reduce the LP vent In the boiler house, there are three forced draft (FD) fans currently run by MP-LP extraction turbines but fans have spare motor drivers. The operation policy acceptable to the plant was to use steam turbines for reliability reasons. The engineer wanted to establish the value of the driver switch to minimize the LP dump. 

Fan stack Cylindrical or modified cylindrical structure in which the fan operates. Fan stacks are used on both induced-draft and forced-draft axial-flow propeller fans. Also known as/on cylinder. Fill The section of the cooling tower consisting of materials which are placed within the coohng tower to effect heat and mass transfer between the circulating water and the air flowing through the tower. 

Air is circulated by multiblade propeller-type ns that provide forced or induced drafts. Fans can be supplied with either adjustable-speed or variable-pitch blades. The fen blade pitch can be changed to vary the air-flow rate to compensate for rising or falling air temperatures. Air coolers supplied with multiple fens may be operated with some of the fens shut down. Dampers, baffles, and bypasses can also be tised to further control liquid outlet temperature. For elevated installations, platforms are generally furnished for access to header boxes and motors. 

At the Tenneco Oil Refinery in New Orleans, 1 first learned how to optimize the excess oxygen content of the flue gas from a boiler. The fired boiler was equipped with a forced-draft (FD) fan. Due to mechanical deficiencies of the FD blower, the combustion air supply was limited, but constant. The boiler produced 650-psig steam, which was in short supply in the refinery. Flence, the boiler was operated to maximize steam production. 

The economics of forced and induced draft cooling tower operation require a study of fan and water pump horsepower and usually dictate a fan static pressure requirement not to exceed 0.75-1.0 in. of water. For atmospheric and natural draft towers the economics of pumping water are still very important. This means that the ground area must be so selected as to keep the height dovm while not dropping the unit rates so low that performance becomes poor. This then, is a balance of ground area versus total deck height. Pritchard [16] presents an... 

A furnace or fired heater can be classified as natural, induced, forced, or balanced draft. The pressure inside a warm furnace is typically lower because of buoyancy differences in the cooler outside air. A natural-draft furnace can operate using this approach however, when fans are used to push or pull the air through the furnace, greater heat transfer rates can be achieved. A natural-draft fired heater is severely limited in contrast to these systems. 

Cooling towers may operate on natural draft, as in the case of wind-cooled towers for small home airpower generating stations (Fig. 8.12). In process plants, the towers are more often aided by fans, either forced or induced draft operations, to improve the cooling capacity (Fig. 8.13). 

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