What gauge boiler tubes and expanders?

Ibe given up on the large tube aproach - no one seems to think its a good idea so I will opt for 7/16 copper tubes (I know this also carries its own cons in a steel boiler but lets not go there for now). Wil I get a better seal with 16g or 20g (I assume wall/rigidity plays a part in this? No fancy tube rollers im afraid, it will either be a hammer it home tapered mandrel or a tighten and squash rubber gromit jobby, again, any advice welcome...

Hi again ,

A lot depends on whether the tubes just act as flues or whether there is substantial dependency on them as load carrying stays as well .

Full size tubes were fitted in lots of different ways early on but the favourite was 'clunking' (to use R Stephensons delightful term ) . Basically tubes were made with a bit sticking out each end and after expanding into tube hole the ends were rivetted over - giving a very sound joint .

There are two designs of simple expanded fitting :

(a) Expanded into the hole but no more - will seal against pressure but not reliably hold against end loading .

(b) Expanded so that two 'larger than the hole' circular bulges form - one inside and one outside . This is a very strong fitting and will seal against pressure and take end loads .

Other methods use rammed in ferrules and (just rarely ) screwed on ring nuts .

Notes :

To do expanded tube joints properly they have to be dry fitted . No sealant and no solder .

There is no certainty that either thickness of tube that you mention is better than the other for making expanded joints . My best guess would be 18 swg but you would have to do some test assemblies to be sure .

The tubeplate holes have to be properly designed to avoid the tube expanding process from simply shearing the tubes off .

Thats it for now .

Michael Williams .

How does one design the right hole?

Most of the tube expander suppliers give the spec for the hole for a given dia and thickness of tube.

This thread on HMEM has some chat about expanders that may be handy

Edited By JasonB on 10/07/2012 19:05:48