Belpaire boilers

Hi Threadmates

Does anyone have a set of design principles for Belpaire boilers for gauges 3.5" and smaller?

I'm a bit stumped about the relationship between the roof of the inner firebox and the centre of the main boiler tube (and, to avoid nitpicking arguments, assuming that the outer firebox roof is parallel to the inner firebox roof and the main boiler tube, which is not tapered).

Regards

Martin F. Slater

Hello Martin,

My general approach, when starting from scratch, is to assume the top edge of the flue nest is located at or just under the center line of the barrel. This isn't an absolute, but rather a starting point. This will put the upper surface of the firebox crown sheet a short distance above the barrel centerline, equal to whatever allowance you want to make for the radius of the formed head, plus two layers of copper. Does this help?

Harry

hi martin,

there is a list of recommended heights in martin evans' book 'model locomotive boilers'.

this is only a guide and was published many years ago.

the late jim ewins always favoured a higher firebox crown. jim was quite critical of the martin evans recommended heights.

there are lots of factors to consider...

exact type of belpaire boiler whether tapered barrel or round, whether wide firebox or narrow, water space, where the steam is collected from, free gas flow through the tubes, and whether decent superheaters are fitted etc

there is a tendency to have the firebox crown too low IMHO in order to have a longer length of gauge glass visible.

some of martin evans' designs such as Rob Roy have a ridiculously low firebox crown.

a good starting point is to have the centre of the superheater flues (if on the top row) the same centre line of the barrel. everything else is worked out from this.

most of my locos have a high firebox crown, a generous free gas flow through the tubes, and quite a small length of glass showing in the water gauge.

cheers,

julian

I think the vid belonged in another thread.

Neil