Why are smokestacks so tall?

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

by azeemba • 1 year ago

195 30 1 year ago

practical.engineering

ErrorNoBrain• 1 year ago

i always assumed it was so the factory (and the neighbors and roads) weren't covered in smoke

e402• 1 year ago

Nah, they care not at all about the neighbors. They built the factory in the bad part of town for a reason.

wahern• 1 year ago

More often poor people moved near industry because the land was much cheaper on account of it being less desirable. There are some high-profile modern examples where industry moved into existing communities, but that's historically atypical.

Of course by the 3rd of 4th generation it becomes a distinction without a difference. But understanding patterns of development is important. If today you want to prevent poor people from tomorrow living in polluted areas, rich people have to make it easier to build affordably in nicer areas--e.g. allow increasingly dense development so poor people don't get pushed toward industry.

rkagerer• 1 year ago

Some shorter, ELI5 answers: https://www.reddit.com/r/explainlikeimfive/comments/p3m9fp/e...

I like the backgrounder about Sudbury.

lloeki• 1 year 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• 1 year 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• 1 year 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.

kelnos• 1 year ago

That's a bit uncharitable towards the article. If you're just looking to answer a question as simply as possible, you're going to want a different source than if you're curious about the background and history of something.

tracerbulletx• 1 year ago

It's not an article. Its a transcript of an entertaining educational video on one of the best engineering youtube channels in the world and a gift to society. It says that it's a transcript at the top of the text for goodness sake.

fortyseven• 1 year ago

Why watch a 2 hour film when you can read a quick summary? Sometimes it's the storytelling and presentation that's the entertainment.

saagarjha• 1 year ago

Huh, I always assumed it was because wind speeds would typically be faster higher up, creating lower pressure to draw up air.

potato3732842• 1 year 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• 1 year ago

The amount of AI generated imagery in the video is baffling.

Kye2• 1 year ago

I didn't see any.

michaelt• 1 year 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• 1 year ago

Some of the stock content providers are also polluting the waters a bit as well, allowing AI generated stock clips to be added :(

Kye2• 1 year ago

I assumed the city came from the same stock series as this meme which predates generative AI: https://knowyourmeme.com/memes/the-world-if

einpoklum• 1 year 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• 1 year 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• 1 year ago

That’s secondary. Smoke stacks were tall long before people cared about pollution (1800s).

Anthony-G• 1 year ago

Strangely, your comment shows up in light grey, indicating that you’ve been downvoted – for asking a genuine question! Upvoting your question (and its answers) to mitigate that.

pfdietz• 1 year 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.

h1fra• 1 year ago

as always, this channel makes your watch 20minutes of something you couldn't care less and you always end up amazed

beAbU• 1 year ago

He is one of the few truly gifted communicators on youtube. He manages to convey complex topics in a way that makes it easy to understand, and his optimism is truly infectious.

I want to also commend him for never criticizing when covering disasters/accidents. He's objective, points out the facts, and explains it all in such a way that at the end if feels like a positive teachable moment for everyone.

mcthorogood• 1 year ago

Calculating smokestack height was in my undergraduate chemical engineering curriculum that I completed in 1976. Height is required so that the National Air Quality Standards in the U.S. Clean Air Act are not violated at the base of the stack.

a3w• 1 year ago

"politics of high smokestacks" was when e.g. Germany got higher smokestacks, since we initially killed the local plant life when burning coal. Now, we can kill the whole planet at once, but only a tiny bit. Problem solved, until we later said "actually, use filters, not (only) high chimneys".

Thanks for having been to my Ted talk.

Next up: Why climate change made the filter solution not work, either — with cutting edge science claims back from 1856.

(Damn, the actual timeline for 1950-1980 and 1856 mixes these two issue non-chronologically. Sorry, to be fair: we were completety certain of climate change in 1990, when we saw that the 1970s era cooldown was not a new trend, but just a decade of a brighter albedo due to particle emissions.)

nemo44x• 1 year ago

Important consideration when building an offset smoker for BBQ. Many of the cheaper ones have stacks that are small and not very high. Taller stacks make a better cooker because it pulls the air faster creating better convection and therefore better bark; desirable characteristics.

owenversteeg• 1 year ago

I didn't see anyone go into detail here, so I figured I'd explain:

The traditional reason was that a taller chimney will contain a taller column of hot air which will generate more draft, which is handy for the design of the furnace. Why will a taller column of hot air generate more draft, why does height matter and not volume? Well, because draft is determined by the difference in atmospheric pressure at the bottom of the chimney. Atmospheric pressure is the weight of the column of air above the ground - 1 cm2 from the ground to space is about 1kg - but warmer air, being less dense, will weigh less; air is 1.18 kg/m3 at STP and 0.95 kg/m3 at 100C. So, a tall column of hot air will weigh significantly less, meaning the bottom of the chimney will have substantially lower pressure, and thus cause substantial draft.

This is less common these days, with modern systems that have mechanically induced draft. Tall chimneys are still built, though - why? Well, for pollution dispersal. Pollutant limits are rarely absolute; they are typically measured per m3 of air or soil or water. If you can disperse the same pollutant over a wider area, your factory can release the same amount of pollutant and still meet regulatory standards. Higher smokestacks are very effective at dispersing pollutants over a wide area because average windspeed significantly increases with altitude. That, itself, is another fascinating effect, and is how soaring birds fly: https://en.wikipedia.org/wiki/Wind_gradient

And now a fun aside. Many years ago I designed and built my own woodstove from scratch, welded up out of scrap steel. It was a pretty good, modern design but it only worked half the time. Why? Lack of draft!

Why are smokestacks so tall?

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