Why are smokestacks so tall?
practical.engineering
practical.engineering
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!
I like the backgrounder about Sudbury.
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.
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.”
I didn't think so; I also tried to read the article, but spreading out a 20 word answer over what seemed like 2000 words of navel-gazing got me out of there in a hurry.
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.
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.
Beyond the minimum the effect tapers off and what TFA is talking about starts mattering.
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.)
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.
There is a long history of this sort of thing.
A video about this: https://youtu.be/3VJT2JeDCyw?si=lzbBWQjXk-o0cblD
Am I wrong?
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.