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Gauge of fire tubes

Effects of tube thickness

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David Haynes02/05/2015 22:12:55
168 forum posts
26 photos

Hi Folks,

What is the effect of boiler tube thickness on steaming properties? If you have a 3/8" dia tube with 0.055" wall thickness, the heat will be a little slower getting across to the water than if you have 0.045" thick tube walls. However, I think the effect must be negligible and as most of the heat transfer is in the firebox, this makes the difference even more negligible.

What are your thoughts on this?

julian atkins02/05/2015 23:31:40
1254 forum posts
353 photos

hi david,

both those thicknesses are far too thick for 3.5"g and 5"g stuff. 22 and 24 SWG being standard, with 18 and 20 SWG for larger superheater flues.

there is no point having tubes any thicker than required. i recently cut up for scrap a 5"g boiler made by Tom Goodhand of Gillingham in the late 1950's. the boiler had seen considerable use since the early sixties and i quite expected the tubes to be thin as a result of scouring and cleaning, but they were not at all. after 40 years use the 22 SWG tubes were pretty much spot on original thickness.

incidentally on the same boiler the sifbronzed barrel joint was as perfect as when first brazed up. i mentioned this to Peter Rich at the time with a sigh of relief from Peter as all his boilers had sifbronzed structural joints!



Neil Wyatt05/05/2015 19:06:25
19033 forum posts
734 photos
80 articles

I won't contradict Julian's sensible "there is no point having tubes any thicker than required" but I recall Tubal Cain pointed out that the thermal resistance of copper is three orders of magnitude less than that of water so for a given bore a thicker tube would increase the heating area and actually carry more heat into the boiler (as the extra copper would conduct heat a thousand times faster than the water it displaced, though naturally the reality will be confused somewhat by convection).


JasonB05/05/2015 19:11:51
22747 forum posts
2653 photos
1 articles

But with the thicker wall tube at the same bore you would not be able to get as many tubes in the same space. Or if you kept the OD the same the bore would be smaller so less area for the combustion gasses to heat in the first place not to mention making it harder for the fire to breathe and they would dirty up faster.

Edited By JasonB on 05/05/2015 19:13:56

julian atkins05/05/2015 22:17:34
1254 forum posts
353 photos

hi neil,

the late Jim Ewins found that very little heat was transferred via the tubes in a miniature loco anyway, with the only heat transfer taking place in the first 1/3rd of tube length.



David Haynes06/05/2015 07:51:19
168 forum posts
26 photos

Hi All,

Well, I now have 3/8" OD tubes with .035 wall thickness, so that must be an improvement on .045" or .055".

I think...

There is also of course the 'rule' mentioned by Martin Evans in 'The Model Steam Engine' :-

'Examination of the most successful boilers shows that length of the tubes (between tube and smokebox tubeplate) divided by the square of the internal diameter works out between 50 and 70.  Similarily, it has been found that most successful model locomotive boilers seem to follow the same rule.

… the following formula can be recommended:-  d= sqrt(L/65)' [I shall call this Tube Index]

For my tiny boiler, the 17 SWG tubes (ID .263" gave a tube index of 65, whilst the thinner 20 SWG tubes (ID .319) gave a tube index of 49.  This is even lower for 22 and 24 SWG.

I have thought about how long the hot gases are in such short tubes and knew about the comments about how little the tubes contribute.  This had to balanced against the above relationship between ID and length, with the wall thickness brought it as well.  It also seemed interesting to bet more tubes in.  On balance, I felt the thinner tube seemed the way forward, after all, it may be academic, making very little difference anyway. 

All the best,


Edited By David Haynes on 06/05/2015 08:31:19

Martin Kyte06/05/2015 09:14:42
2751 forum posts
48 photos

Might not the dominating factor be draught to the fire. In which case the bore and the number of tubes is more important. Larger bore tubes would tend to create a slower gas velocity and thus more heat transfer time, this with the increased draughting should result in a brighter fire. Just a thought experiment really.


Neil Wyatt06/05/2015 09:48:55
19033 forum posts
734 photos
80 articles

Yes Julian, I remember reading about Jim's work, I recall the same is true of full size locos as well.

Jason's point about limiting the number of tubes/restricting also well made.

Tom Walshaw was no doubt right, but all he proves is that the wall thickness of the tube is irrelevant (as long as it's strong enough).

One day I'll build something more ambitious than a cross-tube centre-flue boiler and have some real experience to relate


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