Double Chimney Draughting

[tony9f]

I am looking for advice on whether there are any general rules for the draughting of double chimneys. All the information that I have found so far seems to relate to single chimneys.

My build is a double chimney 9F and I have made the guts of it to scale size which tallies up fairly closely to Greenlys' 1:3 and 1:6 ratios, also when checked against other noted worthies formula, it does appear to be somewhere in the ballpark.

The question is, does the ratio between cylinder size and blastpipe nozzle size still apply as for a single chimney, or does this alter?  If so, how does this affect the proportions of the petticoat pipes or are the rules the same regardless.

This has always been a bit of a conundrum to me and having run it on air, the draught seems to be effective and likely to be more so on steam but any thoughts or advice would be appreciated.

[oldnorton]

When I wrote my article for ME on draughting theory I didn't come across any literature for double chimneys on models.

You need to get the diameter of each chimney choke correct, and my guess would be to use a value that gave one-half the cross sectional area of the single chimney equivalent. Then the exhaust velocities up each chimney correspond to the single equivalent. Similarly the blast nozzle diameter. If the piston size for your 9F is similar to a Britannia you could use those dimensions to re-scale for cross sectional area. Just ask and I will have a look for those.

If you have got the BR original sizes from full size drawings, or perhaps from one of the Locomotive Profile books, then you could re-scale those. The only difference is that a 5" blast nozzle would be smaller than the scale calculation from full size.

[tony9f]

Many thanks for your reply. I find it strange that considering the amount of material on this subject that there should be so little on the proportions of double chimneys, and this includes full size.

Fortunately for me the 9F bore and stroke is the same as the Brits and used a lot of the same parts. The drawings I have are dimensioned so it was an easy job to scale them and build a chimney and blastpipe to suit. The blastpipe nozzles in full size are 4" bore, this would equate to around 3/8" diameter on the model but using the 1/6-1/7 ratios comes out at 1/4" or 9/32" diameters which accommodate the 1:3 and 1:6 triangles quite closely.

I would be grateful if you could have a look for any information pertaining, it would give me some idea if I am on the right track.

Thanks