A chimney is a vertical flue, usually of iron or brick, for conveying the heated air and combustion gases from the fire to the outer air. It usually extends some distance above the tops of buildings. The height of the chimney determines the intensity of the draught. The capacity of the chimney depends upon its height and area. A draught may be natural, induced, or forced.
A natural draught is produced by a chimney alone, and is due to the difference between the weight of a column of the hot gases inside the chimney and an equal column of air on the outside. To illustrate: The air entering the furnace may-have an average temperature of 62° F., while that in chimneys often has a temperature of 500° F. A cubic foot of air at 62° weighs .0761 lbs., and at 500° it weighs .0413 lbs. The heated air is therefore .0348 lbs. lighter than the average air. Hence its rapid passage to the smoke-stack and the consequent draught. The length of stack or passageway has much to do with the rapidity with which the smoke travels. On every square foot of the cross-section of a 100-ft. stack, there is at the bottom an upward pressure of 100 times .0348 lbs., or 3.48 lbs.
Induced draught is obtained by placing a fan-blower at or above the boilers. The uptake from the boiler is connected to the inlet of the blower and the outlet is carried to the chimney, discharging the gases and heated air into the chimney.
Forced draught is obtained by conducting the discharge of a powerful blower to the ash pit, the air being forced through the fire.