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A furnace flue discharges harmful gases created during combustion, such as carbon monoxide, sulfur oxide, or nitrogen oxide, to a ventilated area. Buoyancy draws the hot, light gases up and away from the furnace, while industrial systems use fans to push harmful gases up the chimney. The flue vents noxious gases outside human-occupied areas, with larger industrial systems using taller vent piles to keep the gas away from people. Home systems may use the heat emitted by the chimney to create a secondary heating zone.
A flue is the part of the furnace that discharges harmful gases. Depending on the type of furnace, this can be a small pipe, a home fireplace or even an industrial chimney. The gases created during combustion, such as carbon monoxide, sulfur oxide or nitrogen oxide, are harmful to humans. The furnace flue moves these gases to a ventilated area, allowing them to dissipate to the atmosphere without causing harm.
Most furnace flues operate on a system called buoyancy. All combustion processes create a wide variety of harmful gases and particulates. These gases are almost always lighter and hotter than the surrounding air. This means that as the furnace draws in oxygen to continue the combustion process, the hot, light gases try to move up and away from the furnace. The chimney provides them with an escape route.
Some industrial processes do not use standard combustion methods. If the industrial furnace creates a substance that is heavier than the surrounding air or that burns so fast that the gases don’t have time to escape, the system typically uses a fan. The blower pushes harmful gases up the chimney and out of the system before they can move into inhabited areas or stop the combustion process. Some smaller ovens also incorporate a fan, but this is often more of a safety feature than a requirement.
The furnace flue generally vents noxious gases outside the human-occupied areas around the furnace. In a residential situation, it’s usually right outside the home. Because the vented gases are so light, they dissipate almost immediately when released into the air.
Industrial systems download much more material at any given time. As a result, they use much larger and taller vent piles. The amount of noxious gas they expel can be large enough to harm people at the point of release, which is why they use such high stacks. Higher piles keep the noxious gas away from people and blow it into higher wind streams, where it will dissipate faster.
The gases that a chimney emits are often very hot. Some home systems use that heat to create a secondary heating zone. Instead of moving the gases directly out of the house, the kiln flue travels into the wall or a specially designed area, where it heats the bricks. These bricks absorb heat from the gas as it passes, transferring heat through the house. Even if the gas loses a small amount of buoyancy due to heat loss, the underlying gases continue to push it up.
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