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.

I know that it helps disperse the crap that’s being expelled from them. the higher it starts dropping, the larger the area it covers and so, less concentrated. knowing big money companies, it probably helps avoid law suits.

Industrial processes produce exhaust gases, which contain pollutants (toxic substances).

Having too large concentration of a pollutant in the near-ground air is bad, because people will get poisoned. So concentrations are limited by laws and other documents (depending on country), and factories need to comply.

Fortunately, when there is a flow of air and a pollutant is placed into it, it gets dispersed (actually, true for any gas or liquid).

If we imagine a “chimney” exhausting gases, and if we move in the direction of the wind at level of the chimney – pollutant concentration will diminish.

At the ground level, however, pollutant concentration will go up to some maximum concentrations, then go down.

And the higher the chimney is, the further away from it the max. ground-level concentration will be achieved, and the smaller that concentration will be.

So, to NOT exceed the limit of the concentration, a factory needs to either purify the exhaust, or to build a tall chimney. Or both. And the state may require factories to always have at least certain level of purification system.

(Also some answers here state tall chimneys are to generate draft – well… In a lot of processes exhaust is pumped by blowers, so draft may be used, but main reason is to disperse the pollutants.)

Haven’t seen cooling mentioned. Cooling means a lot more stable gases are released and farther from people.

Also, people are overstating the effect of draft. There will be a temperature gradient from low at the top to high at the bottom. Flow is driven by the pressure delta in the ends, among other things. Longer stack doesn’t affect pressure at the base or flow that much

If you want to pollute, you can make your chimney so tall nothing terrible lands in your state and you won’t have to worry about it.

Speaking of industrial chimneys, sometimes some have a corkscrew spiral in metal near the top. Is that for air disruption? Strengthening?

Like this?

https://imgur.com/pnx9NL3

When I was a teenager, we were building a house from scratch. My dad pored over engineering books to figure out how wide and tall the chimney needed to be for the fireplace we made. It turns out you have to have the proportions just right or it won’t work.

Going to come at this from a slightly different angle, but industrial chimneys or stacks may create a large amount of noise, depending on the exit speed of the gas/particulate matter that it is emitting. The noise acts as a point source, and the higher it is from the ground the less noisy it is to the people around it. There are specific guidelines in some jurisdictions that regulate how loud a stack can be, and what noise levels are allowed in different noise zoning areas may affect a company’s decision to build a different height of stack. For example, here is the Ontario noise guideline for stacks: https://www.ontario.ca/page/environmental-activity-and-sector-registry-limits-and-other-requirements-activities-air-emissions#section-3

To add to this question, why do some chimneys esp those over plants using steam, like a nuclear reactor so wide as well as being tall?

Industrial chimneys like the ones on powerhouses must be a certain height depending on emissions. It depends on what kind of plant and where it is. Different states have different guidelines and there are federal guidelines for some things. Remember most of the things coming out of smokestacks are very bad for humans that is why they are released usually over 300 ft up so it can mix with the wind.

“The solution to pollution is dilution” so the further away from the monitoring equipment it is released, the more thinned out it gets by the time it comes back around … or so the theory goes. Sudbury Ontario is a great case study if your interested.

Have power burners and cleaner burning fuels made the old tall chimneys obsolete?

The large ‘superstack’ in Sudbury dispersed the output into a strata of air current that moved it far down wind (major dick move)

Back before scrubbers, dilution of industrial waste gas was the gold standard.

Taller stacks reduce the concentration of regulated gases at ground level.

That’s why smelter stacks are the record holders – smelters produce more tons of sulfuric gases than metal.

Even in the pre Clean Air Act days there were rules about killing all the vegetation downwind of the smelter.

I work at a place currently undergoing expansion. A chemical we emit is dangerous to humans, but *rapidly* decomposes to something harmless when exposed to the enviroent. If our exhaust was only 1′ tall, there is a legitimate risk that downdraft could cause local exposure. If it were 10′ tall, rain might pull some of it down (even though it is decomposed quickly in water). Our exhaust is 50′ tall- eliminating any risk to humans in the area. If we worked in an area with skyscrapers or other humans living above us, we would need to go higher. But OSHA and the engineers who know more than me, the lowly chemist, have determined that this is safe.