Why are some industrial chimneys so high, like what’s the difference if it’s 150 or 300 meters?
In: Engineering
It’s the same reason big trucks have exhaust pipes elevated above the truck cab. The marginal benefit of exhausting pollutants at a higher elevation. It’s easier for manufactures to do so than to actually have to develop clean engine tech. (IMO EV when the whole life cycle is considered may not be any better than today’s gas feed engines)
One, A chimney is a “zero-energy” vaccum cleaner.
Two, The more smoke you create, the higher suction you need. The sides are keeping it in and the height provides a pressure differential. The higher you go, lesser is the air pressure around compared to ground.
Three, Some pollutants require to be let off higher up. It gives more time for sulphur and nitrogen compounds to burn up.
The solution to pollution is dilution – The mantra that was hammered into everyone in my Chem eng degree.
Pollution is seemingly not a thing if you dilute it enough, and the higher the chimney the wider the dispersion amongst the local area. The chimney will be calculated from a factor of the offgas (waste gasses) and local regs around the release of these gasses.
Hopefully this is no longer a phrase that people in the industry still use today, but it was about 7/8 years ago 🙁
Something I’ve not seen mentioned in other replies: It can depend a lot on local landscape and microclimate/airflow
e.g. a factory in a valley might need a taller stack to avoid issues with local smog.
Industrial chimneys allow the reclamation of gasses that will seperate and condense based on altitude and cooling as the exhaust rises. Some of these smokestacks are built to channel those chemicals back for reuse. Happens a lot on Petro chemical factories.
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.
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.
The large ‘superstack’ in Sudbury dispersed the output into a strata of air current that moved it far down wind (major dick move)
Have power burners and cleaner burning fuels made the old tall chimneys obsolete?
“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.
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