[modeleng] Re: Smokebox Innards
- From: "alanjstepney" <alan@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx>
- To: <modeleng@xxxxxxxxxxxxx>
- Date: Fri, 12 Nov 2004 15:58:10 -0000
Phil,
I DO have a copy.
However, it is old information and even Steels re-write didn't update much
of it.
The work that Clif mentioned by the GWR, was by Sam Ell, and was based upon
work by Prof Goss of Pardua University.
That resulted in many improvements to UK loco's, and also showed that the
one big problem was that our loading gauge was too small, thus restricting
the area of chimney to below the ideal size. The answer as, of course, dual
or multiple chimneys-blastpipes.
Since GWR times others have also worked on draughting, including Port, and
Wardale and, Jos Koopmans, among many others.
Prof Bill Hall did some work on it, and showed that provided the area of the
entry to the petticoat pipe is twice that of the chimney, a straight chimney
only loses some 15% of efficiency compared to a properly constructed
venturi. The entry acts as the convergent part, thus increasing the gas
velocity, and creating a partial vacuum..
Of course, 15% can make the difference between a good and a mediocre
performance.
Rather than calculate the size of the blast pipe orifice it is probably
easier to sue the software he designed and try out different sizes, whilst
checking the vacuum produced.
As you say, the 1:3 and 1:6 angels are important, and I did mention this on
my previous email.
alan@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
www.alanstepney.info
Model Engineering, Steam Engine, and Railway technical pages.
----- Original Message -----
From: "Phill Smith" <steam@xxxxxxxxxxxxxx>
To: <modeleng@xxxxxxxxxxxxx>
Sent: Friday, November 12, 2004 3:55 PM
Subject: [modeleng] Re: Smokebox Innards
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.
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