Barometric damper

Personnel working around hot furnaces must be protected from bums near hot flues. Best practice is to position lightweight, insulated, vertical ducts (open at both ends with a 1 ft high gap between their open bottom ends and the floor to admit cooling air) so that all poc exiting the furnace are drawn up into these ducts by their own chimney effect. This barometric damper also tends to minimize excessive draw by flues that get too hot, which could otherwise snowball into a very uneven temperature situation within the furnace chamber. Likewise, failure to clean scale or other blockages from flue entrances can cause uneven heating because nonblocked flues will get hotter and pull more draft by natural convection. 

Furnace pressure or draft is normally controlled by a damper in the stack, thus choking off the outfiow of gases and pressurizing the furnace. (See sec. 6.6.3.) If negative furnace pressure is needed, use a speed control on an induced draft fan, a pressure (volume) control on an eductor jet, or a barometric damper. (See sec. 6.7.1 on Turndown Devices.)... 

In-the-wall flues or tall flue systems are not generally recommended unless barometric dampers or air breaks (see Glossary) are used to counteract the resultant changeable draft. 

Using a long shaft to operate many dampers in parallel at the tops of in-wall stacks presents a balancing-problem nightmare. Air dampers (sec. 6.6.3) also may be difficult to balance with multiple flues. A better way to protect personnel is to simply erect open-bottomed stacks as barometric dampers at each flue, positioned to shield anyone from the hot flues. 

Fig. 7.5. Back-wall-fired in-and-out furnace. Stacks without bottom openings (without barometric dampers) must have automatic furnace pressure control.

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. 

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