[modeleng] Re: Smokebox Innards
- From: "Phill Smith" <steam@xxxxxxxxxxxxxx>
- To: <modeleng@xxxxxxxxxxxxx>
- Date: Fri, 12 Nov 2004 23:55:43 +0800
Alan,
The petty coat is the convergent section of the nozzle (which is what you
were decribing as a venturi), and the chimney should always get larger in
diameter as you progress up (along) the chimney. This alows for the decrease
in velosity of the gas stream moves up the chimney. A straight chimney will
work (by all appearances quite nicely) but it will not work as well as a
expanding taper one, and hence cannot move as much air without the use of a
smaller nozzle (and hence higher back pressures).
Also, the steam, as it exits the nozzle, has 2 expansion cones. The first is
a cone of 1 in 6 (diameter to height), which must touch the sides of the
chimney (preferably an inch or 2 before the top, or longer) to create the
main "pull" up the chimney. The second is a cone of 1 in 3, which should
just touch the throat of the peticoat (smallest diameter section). The
second cone is mostly caused by the interaction with the surrounding gasses
(on the external surface of the first cone), which slows down the jet stream
and causes the increase in the jet streams area, and hence the second cone.
I really recommend that you read Henry Greenly's book "Model Steam
Locomotives". (edited by E.A. Steel in later editions) E.A. Steel was his
son inlaw.
Actually, I am surprised that you don't own a copy. He was a brilliant
engineer, and model engineer to boot.
He was also a POM, so you POM's should all have copies of his books. <GRIN>
If you want to know more about the workings of blast nozzles and chimneys
(including single, and double, and tripple pettycoat pipes) you know where
to find me.
Cheers,
Phill.
----- Original Message -----
From: "alanjstepney" <alan@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx>
To: <modeleng@xxxxxxxxxxxxx>
Sent: Friday, November 12, 2004 5:45 PM
Subject: [modeleng] Re: Smokebox Innards
> Tim,
> Firstly, what loco is it?
> Not really relevant, but I am inquisitive!!
>
> The steam emerges from the blast pipe and forms a cone. That cone should
be
> fully encompassed by the flare at the bottom of the petticoat pipe.Once it
> is "in" the pipe, the pipe then reduces in diameter.
> Really, it should be formed as a venturi, basically a convergent-divergent
> nozzle, but back in the days when LBSC
> was writing, the number of people who could have formed that was minimal.
> In fact, I gather you dont lose much efficiency by having a straight pipe,
> and "inefficient" is better than not having a working loco at all.
>
> As for the bell mouth, spinning it is an excellent way of making one,
> although I have seen (and done) them made by beating (well annealed) tube
> over a former.
>
> The actual blast nozzle size can be calculated. I have seen the formula
> somewhere.
> However, it was usually empirical (suck-it-and-see?) plus some experience.
> My 1952 copy of one of the locomotive design manuals states "it is not
> possible to lay down any definite diameter for a blast nozzle exhausting
> from a given size of cylinder, as the size of the nozzle is determined
> having due regard to the whole of the boiler layout".
> Not much help there! (But see below.)
>
> As I am sure you are aware, the size is a compromise.Too small and it will
> increase back pressure, too large and there wont be enough draught.
> Of course, small will also give a nice throaty bark to the exhaust when
> under load, and some people prefer that. The loss of efficiency in our
> scales is not worth worrying about unless you are entering IMLEC.
> Of course, that "bark" is wasted energy.
> Jos Koopmans wrote an article ( at,
> http://www.alansteopney.info/page14.html) about improving exhaust, and his
> arrangement gives a very soft exhaust, almost soundless under normal
running
> conditions.
>
> In full size, the designers had difficulties in calculating the exact size
> of the nozzle, and many engines had subsequent modification to improve
> steaming. The unofficial one was to poke a "jimmy" in the blast pipe, to
> slightly reduce the area of the nozzle. That also splits the steam which
may
> then fill the end of the bell mouth where it didn't before.
>
> In models, I would follow what the designer says, and make a couple of
> spares, one with a slightly smaller hole (you can always drill it out) and
a
> blank one in case a complete re-think is required.
>
> One loco design book I have gives the following:
>
> One greatly simplified formula for calculating blast nozzle size is:
> area of orifice - (1.1 * A1) over 8M + 4K (A1/R) sqrd.
> Where
> A1 = area of chimney choke
> K = expression for total gas resistance through boiler
> R = grate area
> M = 0.5 * (A1/a0)sqrd
> A0 = Area of chimney cap
>
> Then there is a whole lot more on the subject.
> If you are interested, I will copy the pages and email them to you.
> However, I should tell you that at the end, it STILL doesnt tell you
exactly
> what size to make it !
> Also, I don't know how it would relate to model sizes.
>
> The more important thing is to ensure that the nozzle is directly under
the
> centre of the chimney,
> and the usual 1:3 and 1:6 tapers are about right.
>
> alan@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
>
> www.alanstepney.info
> Model Engineering, Steam Engine, and Railway technical pages.
>
> ----- 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?
>
>
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