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Author Topic: "Crown Thing" on Older Steam Engine's Stacks?  (Read 1557 times)
Jake of All Trades
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« on: April 14, 2007, 02:10:05 am »







I'm terribly embarrassed to say this, as I clearly use them frequently, but what are they?  What are they called?  Do they have a purpose other than aesthetics?  I've looked and looked for an answer, and even posted on a railroading forum, but no one seems to know...
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Mr. Pickles
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« Reply #1 on: April 14, 2007, 02:26:36 am »

I don't know for sure, but I think I remember them being a primitive spark arrester.  Or maybe they thought that it help the draft of the smoke stack?  Could just be aesthetics too like you said?   Huh

Mr. Pickles
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La Bricoleuse
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« Reply #2 on: April 14, 2007, 02:33:01 am »

I don't know if it's the official term for it, but i'd refer to it as a crenelated stack, or a smokestack with a crenelated aperture, or if i were being droll, a dagged-cap stack.
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Gomez_Addams
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« Reply #3 on: April 14, 2007, 04:01:47 pm »

I believe the correct term for the decorative top of early stacks is "bonnet", but please don't write that in a textbook or museum plaque based solely on my opinion.  Smiley

The petals of the earliest bonnets were solely decorative, and absent a wire screen (included with later versions of the bonnet stack) could not have arrested sparks.  Bonnet stacks were fairly primitive and were soon eliminated in favor of an "inside smoke box" design which included a steam ejector nozzle to create a relative vacuum in the smoke box and thus increase draft through the firebox.  I apologize if this is too much of a digression from the original question.
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HAC
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« Reply #4 on: April 14, 2007, 06:48:51 pm »

Steam ejector design gets pretty complicated, and as you suggest, is critical to boiler efficiency (That and firebox design, grate area, etc), Chapelon did some good work there, as did Gresley. If you ever have to climb into the smokebox to clean it out, its very easy to give your shins a good whack on the blastpipes
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Harold
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Jake of All Trades
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« Reply #5 on: April 14, 2007, 10:43:59 pm »

So, are we all agreed on "Bonnet"?  Thanks for the information, all!
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