Why are some industrial chimneys so high, like what’s the difference if it’s 150 or 300 meters?

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Why are some industrial chimneys so high, like what’s the difference if it’s 150 or 300 meters?

In: Engineering

The chimney is there to generate draft. As hot air rises up through the chimney it will leave a void behind where fresh air is pushed into. The taller the chimney the more hot air is contained within it and the more draft it creates. So having a tall chimney means there will be a lot more draft which may be needed for bigger furnaces.

First reason is that industrial chimneys, especially flare stacks that are used to burn off excess chemicals that for whatever reason cant be safely stored at the moment, release stuff into the environment which may be toxic. So the higher you make the chimney, the more the wind will scatter it and wider an area those chemicals will cover. Initially that might seem bad, but it means the concentration of the chemical will be lower in the affected environment which minimizes harmful impact.

Second is that taller chimneys are just more effective at moving air thanks to the Stack Effect. Chimneys act as insulation to the outside air, meaning the air inside is (typically) warmer than the air outside and warmer air rises. The more chimney you have, the more warm air you have rising up it, which causes more suction to draw air into the bottom of it. This might be desirable for a number of reasons, like say if you have a furnace at the bottom you’d otherwise need to pump air into to maintain operating temperatures.

Industrial chimneys are (or at the very least should be) built tall in order to emit their gasses above the inversion layer.

Inversion layers are layers in the atmosphere where the temperature stops decreasing with altitude, so when toxic gasses (like ones from industrial chimneys) reach it, they get trapped beneath, thus reducing air quality.

One of the ways to counteract this is to build chimneys above these layers.

I’m sure there are also other reasons to build industrial chimneys tall.

Chimney height is directly proportional to how strong draft it will create (if there is a temperature difference and if inside and outside pressure is the same). The lower the temperature difference and the longer sideways the smoke will have to travel the taller the chimney has to be to create a sufficient draft to clear out smoke&gasses.

Also, the taller the chimney is the further the smoke will travel before it hits ground level so the more time it will have to disperse (and less likely to get caught in wind effects created by other buildings).

Note that if you want to disperse pollutants the height of the chimney has a double effect, both allowing the smoke to travel further but also sending it up the chimney with greater force so that it’s already more diluted in the chimney (due to pulling in other gases with greater force).

Efflux velocity, chuck the waste products upwards with sufficient velocity that they are carried away and diluted by the winds.