from Hacker News

Why are smokestacks so tall?

by azeemba on 6/7/25, 1:06 AM with 46 comments

• by owenversteeg on 6/8/25, 2:55 AM

  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!

• by rkagerer on 6/7/25, 7:58 AM

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

  I like the backgrounder about Sudbury.

• by h1fra on 6/7/25, 4:05 PM

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

• by pfdietz on 6/7/25, 2:20 PM

  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.

• by saagarjha on 6/7/25, 9:43 AM

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

• by a3w on 6/7/25, 6:32 PM

  "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.)

• by ErrorNoBrain on 6/7/25, 7:42 AM

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

• by mcthorogood on 6/7/25, 5:36 PM

  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.

• by nemo44x on 6/7/25, 7:22 PM

  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.

• by einpoklum on 6/7/25, 12:00 PM

  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?

• by eulgro on 6/7/25, 10:41 AM

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