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Why are smokestacks so tall?

https://practical.engineering/blog/2025/6/3/why-are-smokestacks-so-tall
115•azeemba•13h ago

Comments

ErrorNoBrain•6h ago
i always assumed it was so the factory (and the neighbors and roads) weren't covered in smoke
rkagerer•6h ago
Some shorter, ELI5 answers: https://www.reddit.com/r/explainlikeimfive/comments/p3m9fp/e...

I like the backgrounder about Sudbury.

lloeki•5h ago
That's basically what I remember: the leading reason is that a steel furnace needs a lot of heat to build up a lot of pressure and push carbon in, and higher chimneys help provide that.

Others like Japan found another way to achieve the necessary temp/pressure, but it hardly scaled as it needed to during the industrial revolution.

TBH the "let's avoid smoke" aspect sounds like a retcon, the mythical London smog is a testament of that.

jodrellblank•2h ago
> TBH the "let's avoid smoke" aspect sounds like a retcon

Yes, that’s what the article says:

“ When you look at all the pictures of the factories in the 19th century, those stacks weren’t there to improve air quality, if you can believe it. The increased airflow generated by a stack just created more efficient combustion for the boilers and furnaces. Any benefits to air quality in the cities were secondary. With the advent of diesel and electric motors, we could use forced drafts, reducing the need for a tall stack to increase airflow. That was kind of the decline of the forests of industrial chimneys that marked the landscape in the 19th century. But they’re obviously not all gone, because that secondary benefit of air quality turned into the primary benefit as environmental rules about air pollution became stricter.”

Mistletoe•45m ago
People hate on Reddit but this is why I love Reddit, I got the answer in a few seconds as opposed to the original article pontificating and padding forever about it.
saagarjha•4h ago
Huh, I always assumed it was because wind speeds would typically be faster higher up, creating lower pressure to draw up air.
potato3732842•50m ago
At first that's true. That's why chimneys all have a more or less minimum height above the roofline (and people can get away with little to nothing for a house on a ridge line or like an ice fishing shack or something).

Beyond the minimum the effect tapers off and what TFA is talking about starts mattering.

eulgro•3h ago
The amount of AI generated imagery in the video is baffling.
Kye•2h ago
I didn't see any.
michaelt•1h ago
Except for the sci-fi city at the 40 second video mark, I'm pretty sure it's almost all real video, just brought from a big stock video provider.

If you want video of a drone flying over a power plant or hot air balloons taking off, you can license them from stock providers, just like with stock photos.

Of course, it does share some of the cues of AI-generated content - but I suspect a lot of these AI companies buy a lot of stock content for their training datasets.

geerlingguy•20m ago
Some of the stock content providers are also polluting the waters a bit as well, allowing AI generated stock clips to be added :(
einpoklum•2h ago
I leafed through that page, and it still seems like the answer is: "To make sure the pollutants are dispersed and/or carried away enough to reduce exposure of people around the base."

Am I wrong?

dweekly•2h ago
You're right, but the less intuitive part is that the stack makes the air rise much more quickly; the exit velocity is higher the taller the stack.
kortilla•1h ago
That’s secondary. Smoke stacks were tall long before people cared about pollution (1800s).
pfdietz•5m ago
There's a related technology that creates downdrafts by cooling air. In a region with warm air near cold water (like, say, Los Angeles, with cold ocean water), injection of the water at the top of a large tower can cool the air, causing it to descend.

This was proposed to be used, again in Los Angeles, as a way to not only generate power (via turbines at the bottom of large hyperboloidal towers) but also clean pollutants from the air. I don't think it ever went anywhere (probably too expensive) but it would work at least in principle.

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https://blog.paavo.me/radiant-ai/
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https://practical.engineering/blog/2025/6/3/why-are-smokestacks-so-tall
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