Why does the smokestack of old steam-powered locomotives have that odd narrow-wide-narrow shape? Why isn’t it just straight like a pipe?
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The smoke stack is a venturi to draw more air through the firebox.
The steam exhaust from the pistons is what powers the venturi. This is why the stack “chugs” like a choo-choo.
The more steam that is exhausted, the more air is pulled through the firebox, which means the fire burns hotter, which means the engine can make more steam.
People are likely right about spark arrestors as well, but the primary function is that it’s a venturi.
Here’s a [diagram](https://qph.fs.quoracdn.net/main-qimg-a410cad5f3eba37ec6910885702ce123-lq)
Fig. 115 shows it the most clearly, the smoke can escape through the shallow mesh fish while the sparks/cinders fall into the conical bucket
This thread is why I’m on Reddit. I came to ELI5 for something else entirely, and before I even got around to my goal, I just learned a ton of cool shit about something I had never even thought about. Thanks, all!
Wood burning steam engines have that shape and it contains a spark arrestor so it doesn’t set everything on fire. Coal and oil burning engines have a straight pipe
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Spark arrestors. So, when you burn things, you can end up with flaming little embers escaping with the steam. Those that exit out the chimney could land on brush near the train and start fires. So chimneys had these little mesh grates called spark arrestors to catch the embers and hold them in place preventing them from escaping and causing fires. But that causes a build up of residual material in the chimney, reducing pipe area, which reduced the exhaust and subsequent power of the engine.
So those wide brimmed areas were designed to funnel the smoke, and the embers with them, to the side where the mesh grating would catch the embers and deposit in in the wider brim. It’d have to be emptied out occasionally but prevented buildup in the main chimney chamber.
It’s basically a bucket to hold stuff to prevent fires while also not clogging the main exhaust.
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