Smoke Stacks and the Stack Effect
Smoke stacks, whether fueled by incinerators, boilers or fireplaces, function on the concept of the stack effect. The stack effect relies on the movement of air through the pressures of buoyancy. As its name implies, the stack effect relies on an exhaust, or stack, which facilitates the release of hot air and contaminants to the exterior of the structure.
The stack effect is based on the principle that higher-pressure hot air, surrounded by lower-pressure cool air, rises. Since air does not move steadily by itself, the stack effect involves the use of a heat generator by which to expand the air. Once the air has been heated and increased in pressure, it is expelled out of the smoke stacks and replaced by cooler air, causing the cycle to repeat. The stack effect is stronger in colder climates than in warmer ones, and also has an increased effect when applied to taller rather than shorter buildings.
Home and Business
In the stack effect created by residential chimneys, a fire is lit at the base of the chimney in order to heat the air. Once the air is heated and expelled, colder air is drawn in from beneath doors and through the windows toward the fireplace in order to replace the expelled hot air. Fireplaces are one of the poorest facilitators of heat via the stack effect, often drawing in more cold air than can actually be heated.
In buildings, (taller buildings being the most optimal source of stack effect air circulation) the ground-floor entrance or parking garage provides an excellent source of returned air. Once again, the air is heated through an artificial source, usually a boiler or furnace, and continues to rise until it exits at the top of the building through the smoke stack. Stairwells and elevator shafts contribute to the optimal performance of the stack effect, especially in the case of fires.
Both noise and polluted air intake are concerns when relying on the stack effect, This is of special concern for inner-city office buildings which receive air with a large amount of exhaust emissions from uptakes such as their front entrance and attached parking areas. The best solution for outside noise that is filtered-in with air uptake, is to rely on mechanical sources of ventilation. In the case of contaminants, it helps to construct a protected courtyard with natural vegetation near the site of uptake, by which to filter out contaminants.
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