Chimneys release combustion by-products into the air, consisting of carbon dioxide, water vapor, nitrogen, oxygen, and pollutants. Smokestacks were originally used to improve combustion, and natural draft is necessary for effective operation. Taller chimneys were used to reduce pollution, but fans are now more commonly used.
Chimneys are a type of flue, often used in industrial environments, which release combustion by-products into the air. The gases emitted through the chimneys consist largely of carbon dioxide and water vapour, although nitrogen and oxygen are typically present, along with a range of pollutants. A factory or industrial furnace is often distinguished by its chimneys, often built high enough to allow for a wider and therefore less concentrated dispersion of pollutants.
Smokestacks were originally used not to disperse pollutants but to increase the airflow into a furnace, thereby improving its combustion. Because the air inside a smokestack is warmer than the air outside, it is also less dense. This leads to a deferent pressure between the bottom air of the chimney and that outside the flue, causing outside air to be drawn into the chimney and the air moving through the connected firebox at a higher velocity. This phenomenon is known as natural draft or pile effect.
Chimneys must be built to exacting specifications to provide adequate natural draft for a furnace. The outside air pressure, the temperature and composition of the gases leaving the furnace, and also the materials used to construct the chimney are all significant factors. A fair amount of trial and error is often required to build an effective smokestack.
Smokestacks were first used in the 17th century and became ubiquitous during the Industrial Revolution of the 18th and 19th centuries, often emitting noxious fumes into the city’s air. Taller chimneys helped remedy this problem, though not absolutely, in the 20th century when fans replaced chimneys as a way to increase air draft in furnaces. Today, some chimneys exceed 17 meters (18 feet) in height in an effort to maintain healthy air quality while complying with government regulations.
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