[modeleng] Re: Blaastpipe calculations

Andy
I found this on A.M.E.magazine's Web site. I hope it's some help.
http://www.ameng.com.au/article.htm

Lee

Sizing the blast nozzle from the cylinder diameter, as recommended in some
books on model engineering, is unsound because it takes no account of the
piston speed. By this method, one could have the same nozzle for a shay and
an eight-foot single!
The work of many investigators is summarised by E. Young (1936) in "A Study
of the Locomotive Front End" and the following points seem to be well agreed
for a single blast pipe: (a) Minor changes in dimensions can make a major
change to the draught. e.g. the LMS Class 2, GWR "Manor" & LNER V2 had steam
output increased by 60% to 100% by minor changes made at Swindon.
(b) The slimmer the chimney down to some unspecified limit the better the
draught.
(c) Chimney and blast pipe together form an ejector and should be designed
as such,
(d) Chimney taper should not exceed 1 in I4 on diameter.
(e) The longer the chimney up to seven diameters the better the draught.
(f) There is little conflict in earlier work with that of S.O. Ell at
Swindon. Since he obtained results with the single blast pipe which were
little inferior to more sophisticated arrangements like the Giesl ejector
there cannot be much wrong with the proportions set out by him. (fig. 1)
Unfortunately the formula given in Ell's paper to determine the size of the
blast nozzle cannot be directly used on miniature engines.


----- Original Message ----- 
From: "AndyA" <AndyA@xxxxxxxx>
To: <modeleng@xxxxxxxxxxxxx>
Sent: Tuesday, May 24, 2005 8:56 AM
Subject: [modeleng] Blaastpipe calculations


> Hi All
> What calculations do you use to calculate the blast pipe diameter? The
cylinders are 1.6875" diameter by 2.4375" stroke. This scales up very
closely with the full size engine dimensions of 19" diameter  by 28" stroke.
Someone suggested that I scaled the full size blast pipe which at 5.125"
would give me a dimension of 0.454" diameter however this seems a little
large. What does everyone think? The model design calls for 0.25" but that
gives a fair bit of back pressure and a very strong blast. How about using
the 0.25" designed diameter with a multiple jet nozzle? Would that offer any
advantage?
> Andy
>
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