[modeleng] Re: Smokebox Innards
- From: "Clif Walker" <clif.gwr@xxxxxxxxxxxxxx>
- To: <modeleng@xxxxxxxxxxxxx>
- Date: Fri, 12 Nov 2004 10:06:11 -0000
Hi Tim and All,
The correct design of the petticoat bellmouth and the taper in the chimney
can make a heck of a lot of difference to the performance of the action of
the blast pipe.The actual chimney should not be parallel but after the
tapering in of the bellmouth to a choke diameter it should then taper out
again to the top of the chimney.The diameter of the Blast Pipe also comes
into the calculation.
I usually work to dimensions which were scaled from GWR practice and called
the Swindon Draughting.
If you want them I shall have to trawl through a huge pile of old ME's
because it's in there somewhere.
I usually machine the bellmouth in the lathe because you need some sort of
spigot to fix it up inside the chimney.Sometimes I just do it by guess work
or other times by drawing a template,fixing it to the bed and having a
pointer fixed to the cross slide.By juggling the feeds the pointer then
follows the template and the bellmouth is machined to the correct shape.
Let me know if you need the dimensions.
Regards
Clif
----- Original Message -----
From: "Tim Rickard" <the_viffer@xxxxxxxxxxx>
To: <modeleng@xxxxxxxxxxxxx>
Sent: Thursday, November 11, 2004 9:26 PM
Subject: [modeleng] Smokebox Innards
> Look just ignore the last subject line of mine and read this one OK?
>
> OK here is a chance for you chap(ettes) to wet yourselves once again at my
> ignorance.
>
> I'm comfortable with the idea that the purpose of smokeboxes is to
generate
> a partial vacuum thereby to draw the fire. As I understand it this is done
> using Bernoulli's principle in which the exhaust is accelerated as it goes
> up the chimney.
>
> So the loco I'm just finishing has according to LBSC's drawings a
> bellmouthed petticoat pipe which terminates in a parallel portion equal to
> the chimney id. Now I'm sure I'm being slow on this one but does that
work?
> Seems to ignorant me that the bell mouth is useless and the chimney would
> work just as well all parallel. Tell me if I'm wrong please. If I need a
> bellmouth I was going to spin it unless anyone has a better idea.
>
> What I think I might need is choke in the chimney bore but if so how do I
> calculate how much to choke it and where?
>
> Oh and blastpipe nozzles. Is there any way apart from the really empirical
> (and here comes the scientist in me who hates all things empirical) in
> deducing how big the nozzle should be?
>
>
> MODEL ENGINEERING DISCUSSION LIST.
>
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