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Design of boilers

Can the model loco or traction engine boiler be improved?

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Bob Worsley18/01/2021 11:14:56
121 forum posts

Been reading books again. Going through my collection of old engineering books looking to see what to keep and what to chuck. There is quite some interesting reading in the later half of the 19C in treatises, encyclopaedias and single volumes.

How they made boilers in those days, for marine, locomotive and factory type uses. Commonly all steel shell with brass fire tubes and copper firebox.

It is a recurrent comment that the boiler has too little space around the tubes, there needs to be a gap of at least 1/2” between the tubes. The tubes can be only 1.5/8” outside diameter. This gap won’t allow the ebullition (what a wonderful word!) or boiling of the water to pass upwards. Priming also seems to be a problem, possibly due to the poor steam flow past the tubes. The flow comes in fits and starts, and carries water into the steam space.

In a model boiler since scale water is unobtainable then perhaps this minimum spacing of 1/2” should also be used? Looking in Model Locomotive Boilers by Evans and the 3.1/2” Jubilee boiler has 3/8”diameter flues on 1/2” centres for example, all the other boilers in the book are pretty similar. This leaves just 1/8” space to allow both the steam to rise and water to descend. Simple observation in a boiling kettle points out that the steam bubbles produced on the tube, or element, are much larger than 1/16”. Whether the size of the bubble reduces as the steam pressure increases I have not been able to determine, but rather doubt it. If the metal is too hot then putting water on it just makes the water turn into spheres and skate around. This could be a problem where the firebox is really hot, the sphere floats on a layer of steam?

Even worse is the space given to the circulation of steam and water on the sides of the firebox. Again from the book this gap is only 3/8” on 3.1/2” Jubilee, 1/2” or 7/16” for a 5” 2-6-4 and 3/4” on a 7.1/4” Highlander. From the old books it is reckoned that 3/4 of the steam raised is done by the firebox. And that shortening the tubes by a 1/4 actually increases the amount of steam raised.

So perhaps the firebox needs to be made quite a bit smaller to allow at least 1/2” water space, even on a 3.1/2” loco? This would also apply to traction engines since their boilers are very similar?

The problem is that the fire has to burn red hot, so that the coal starts to gassify and then burn. A black fire achieves nothing, lots of practise with coal fires and Rayburns! This means that the fire is around 700C, and the copper conducts this to the water almost immediately. It is very necessary for the eater to wet the firebox in order to accept this heat and then a very fast flow of boiling water taken upwards. A vigorous constant circulation of water down and steam up is essential.

Perhaps the cramming in of lots of tubes also needs to stop? Why not just put a very few, three, large diameter tubes at the top of the firebox and one row of smaller ones under if room? So for the 2-6-4 you would have three 1.1/4” or even 1.3/8” flues with perhaps eight 3/4” flues in a zig-zag pattern underneath? There are no flues in the bottom two rows as currently described. For the Jubilee with a much smaller barrel just three large flues at the top, none at all underneath. Also reduce the length of the flues to give room in the smokebox.

What this gives is plenty of space for both a superheater and secondary circulation heater in the flues.

Two of these large diameter flues have a type of stainless steel superheater. Can’t immediately find an example to quote but called a spear superheater? The firebox end is formed form a block of stainless shaped like a spear point. But these heaters are plumbed so that one end goes into a bush at the bottom of the tube plate, the other end a similar bush right at the top of the boiler in the steam space. If these flues are 1” diameter then can fit a 3/8” heater, if 1.1/4” diameter then 1/2” heater. These heaters are the normal there and back type with a return block brazed or welded on. These can be continued inside the firebox, possibly with the tubes arranged vertically to minimise the shadowing of the crown by the heaters.

There will be a mix of water and steam in these tubes, and I would assume that as the water is turned into steam then is would flow upwards to emerge in the steam space. It is unlikely that the steam have a choice of upwards against steam or downwards against water will always choose the easier option. This is why they need to be as large as possible, reduce the flow friction. Perhaps smaller boilers, 3.1/2”, would only need one very large flue tube to get 3/4” secondary tubes in. The superheater is different in that there is a pressure difference due to the flow of steam so the diameter isn’t so important.

Bob Worsley18/01/2021 11:16:44
121 forum posts

Too long, had to split it.


The point of these is to change the heating area from the fire tubes to a radiant type heater at the top of the firebox. I can find no information about the water temperature gradient within the boiler from above the firebox to just behind the tube plate. Whilst the pressure has to be constant, the rate of boiling doesn’t. Imagine a boiler three feet long, the water temperature will quite definitely drop substantially from one end of the fire tubes to the other.

The third flue has a similar stainless tube arrangement but this is a superheater. If a superheater isn’t wanted then can either have all three flues with the secondary circulation heater, or even just two larger flues.

The secondary circulation heaters are in circuit all the time, the superheater is only used when drawing steam from the boiler. The tube plate will need moving backwards to give room for the plumbing.

Another possibility is to fix solid copper bars to the front of the firebox so they run along inside the bottom of the barrel. If of reasonable size, 1/2” or more square, then conduction will give as much heating as the flues they replace would. Similarly a combustion chamber is just a complicated nuisance, enlarge the flues inside the barrel. With the internal secondary circulation system and the much larger flues then should still heat ok. It is all to do with water circulation around the firebox.

If that wasn’t enough, consider the Stefan-Boltzmann law. This is the power lost due to radiation from surfaces at higher temperatures than the environment.

This is power loss in watts = area in sq cm x 5.735.10^-12 Stefan constant x (work temperature ^4 - ambient temperature ^4). The important detail here is that the temperatures are in K, and that they are to the fourth power. This means that at high temperatures the losses are enormous.

To give an example, at 730C, 1000K, and an environment at 27C, 300K, what size plate can just be kept at that temperature with a 10kW gas torch?

10,000 / (5,735E-12.9.919E11) = 10,000 / 5.69 = 1757 sq cm = 42cm sq

So 10kW will maintain a 16” square copper plate at 730C, assuming the coke bed it is resting on is at the same temperature so no loss on the other side.

This of course assumes that 100% of the heat from the gas torch actually heats the plate, in reality it will be somewhat less.

This is why soldering a boiler 24” long is such a struggle. I am still working on using induction heating, also using a 10kW gas radiant heater under the boiler as background heat. Possibility of resistive heating but the high amperage connection is a puzzle.

More importantly it suggests that it would be better to reduce the size of a boiler by 1 cm all round and replace the missing size by insulation. Also that making a taper boiler is a waste of time, fill the taper under the cladding with insulation.

If really keen then can work out the heat flow from an incandescent fire through the firebox walls and what is then available for the water to absorb. If the water gap around the firebox is increased to allow better circulation then the foundation ring could become a source of weakness. Might need to stop using a solid bar and flange instead, give a bit more flexibility.

A firebox for a 3.1/2” loco obviously can be built and works, what needs testing is whether that size fitted in a 5” loco is an improvement, or not, or can’t tell the difference.

Something to think about at the moment?

I quite realise that this isn’t how it has always been done, but I have never seen any of these type of calculations in the magazines and books. The radiant heat loss equation I had seen before, but the real effect of it had never become apparent, it is a killer.


Edited By JasonB on 18/01/2021 11:29:16

Bazyle18/01/2021 11:46:07
6377 forum posts
222 photos

Too long - fell asleep after the first section!

Two things to consider. a) In the 19th century they thought for a few years that longer boilers would be better at giving the chance for more heat to pass from the gasses to the water but found they were worse steamers.
b) the ME author 'Chuck' wrote some articles about building a loco specifically for passenger hauling and was particular in this about reducing the number of tubes to just a couple of large diameter ones.

J Hancock18/01/2021 12:06:39
843 forum posts

How much convected or conducted heat do we receive from the Sun ?

So, that just leaves 'radiant heat' and that is from 96million miles away.

That's the importance of radiant heat in any boiler.

Former Member18/01/2021 12:44:50
1085 forum posts

[This posting has been removed]

noel shelley18/01/2021 13:32:47
1433 forum posts
23 photos

An interesting essay if a little long !

The issue of tube spacing stopped me at considerable cost from building the Reeves 6" verticle boiler. I had bought all the materials and drawings- cost neck end of £300 30 years ago. I started to build it only to be told by the club boiler inspector that he would not even look at it as the tubes were too close together for proper circulation, he also commented that that with so many tubes there was in suffient water space. I at that time knew little about this sort of thing and took it up with Reeves. If I deviated from the drawing and used larger tubes and bigger spacing it was not to design and would need all the calculations - I lost heart put it all in a box and it's still there.

Chuck - the muddle engineer, Terry Aspin well known for his foundry work !


Edited By noel shelley on 18/01/2021 13:33:33

JasonB18/01/2021 14:05:17
23005 forum posts
2757 photos
1 articles

Must be a boiler inspector thing, there was a very good build write up of the RV6 on MEM forum and it passed inspection without problem., maybe time to dig yours out from storage. Don't think the Stanley steamers had too many problems with lots of close fitting tubes eithersmiley

Howard Lewis18/01/2021 16:34:34
6297 forum posts
15 photos

In full scale practice, there used to be formulae for the proportions of boilers, length of barrel : diameter, tube areas etc.

Churchward made very good use of taper boilers on the Great Western Railway, and they stuck with taper boilers to the end of British steam. So maybe their proportions and top feed arrangements were not too far away.

In the latter days of steam, North America and Bulleid on the Southern used Thermic Syphons. Have never read anything to suggest that Battle of Britain or West Country locos were bad steamers; keeping their feet, yes,;but not shy on steam.

You can't scale physics, and good water circulation is needed to transfer heat to water, which is the purpose of the exercise.

Wonder how well a fairly accurate scale model of a Swindon boiler would behave, bearing in mind that you can't get water in scale sizes?


SillyOldDuffer18/01/2021 16:53:41
8859 forum posts
1994 photos

I agree it's an interesting subject, but I think it would need a serious programme of testing to get to the bottom of it.

Several compromises going on at once, for example:

  • Volume of water and the thermal load due to the metal work vs the amount of heat available from the fire. (The boiler mustn't be too big for the fire or vice versa!
  • Boiler pressure (effecting steam temperature and boiling point.) vs strength. High pressure steam is best for locomotion but the mechanical build is harder to do, more weight & stress etc.
  • Surface area of the tubes relative to the amount of heat. (Lots of small tubes good.)
  • Sufficient tube volume and flow so as not to throttle the fire (Lots of small tubes bad.)
  • Flow allowing time for hot gas to maximise heat transfer from tube to water as it heads for the funnel. Excessive flow rate wastes heat. (Big tubes bad)
  • Small tubes are more likely to choke with ash. (Big tubes good)
  • Low temperatures at the smoke box end means there must be an optimum tube and boiler length for a given boiler diameter and heat source. The purpose of the tubes is to boil water, not to make the smoke-box hot, or to have the smoke-box end remove heat from the boiler.
  • Tubes shouldn't stop water in the boiler from circulating freely. Heat can't be transferred efficiently to the water if they do. A few small tubes are best.

Several contradictions! My feeling is careful design and research might produce a better boiler, but there are so many competing factors that improving one would be counter-balanced by worsened problems with the others. And undermining all attempts to do better is the unavoidable heat loss that means a small boiler can never be thermally efficient.

I wish someone would investigate though. Almost 100 years since LBSC shook a complacent hobby in the Battle of the Boilers. Reading old ME mags I get the impression boilers improved slowly in terms of construction until about 1970, and since then designs are just copied. Apart from discussions like this no-one has given thermal or physical design a good seeing to since. I've a sneaking suspicion model boilers are a good as they can be, but it would be nice to know. Not impossible: Tornado outperforms the original A1 due to modern design tweaks.


duncan webster18/01/2021 17:39:58
4100 forum posts
66 photos

OP is making the mistake of trying to apply full size design rules to models. If the water spaces around our size fireboxes were too small and restricting release of bubbles then we'd get overheating of the plates, burning and bulging, similarly for gaps twixt tubes. I've not read of any such problems, apart from when spaces are full of scale. Simple way to look at it our firing rates are much smaller than full size, so heat fluxes are smaller. Flow in tubes in our size is laminar, full size is turbulent, so any design rule for full size is unlikely to be helpful. Article by Martin Johnson in ME was most illuminating. Jim Ewins showed many years ago that model boilers are typically 70% efficient, which is comparable to full size. OP seems determined to show that long established practice of both design and construction of model boilers is wrong without any practical or theoretical backup.Let him make a boiler with 2" water spaces around the firebox and a couple of 1" firetubes, not sure how he'll get any coal in!

Bob Worsley21/01/2021 12:55:22
121 forum posts

What a dismal response!

More people put to sleep than commented, and as for waking up the brain cells, forget it. It is perfectly obvious that the scale boilers are not very good, been made for years, like the Ford model T, but things do move on. Simply reading books on physics and suggestions fly of the page, steam bubble size for instance. Discussion in another thread about boiler staying, well, just read books on bending of beams, side stays could be regarded as a long girder, or as an edge restrained disc. All it needs is a bit of maths.

Ok, how much would someone charge to make four 1.1/2" Allchin boilers, all to different specifications of tube count, dimensions etc? I will provide materials other than silver solder and flux.

I don't have any facilities to make them, or I would, and with current happenings not certain when I would get facilities organised.

Former Member21/01/2021 13:01:58
1085 forum posts

[This posting has been removed]

noel shelley21/01/2021 13:21:38
1433 forum posts
23 photos

A VERY generous offer ! I for one would be very interested in the results. Call Helen Veral/Geoff Stait ! Noel

duncan webster21/01/2021 13:29:17
4100 forum posts
66 photos
Posted by Bob Worsley on 21/01/2021 12:55:22:......It is perfectly obvious that the scale boilers are not very good, been made for years, like the Ford model T, but things do move on. .........

Perfectly obvious to who? Model boilers have been tested and shown to be ~70% efficient. You can't get 100% because the flue gas coming out is bound to be hotter than the air going in. Come on Bob put your money where your mouth is, get a boiler made to your ideas and test it against one made to more conventional practice.

Former Member21/01/2021 13:33:21
1085 forum posts

[This posting has been removed]

JasonB21/01/2021 13:34:53
23005 forum posts
2757 photos
1 articles

Bob while looking at a thread to check something related to the stay question yesterday I cam across this by a very competent builder who has actually made a number of boilers, this was referring to a 7" dia copper Traction engine boiler. Something to consider as it's the opposite to you thoughts on larger tubes and ties in to what Duncan has just said

Doug Said

"I have gone with 1/2" diameter tubes in lieu of 5/8 as specified on the drawings, as using 1/2" OD tubes gives a ratio of 70 (which is supposedly ideal) from K. N. Harris' book "Model Boilermaking". It also means I can get more tubes in and hence some more heating surface. One thing I have noticed with some engines that use bigger tubes, is that the flue gases are still so hot when they hit the smokebox door that the handles turn blue! This implies to me, that there is a heck of a lot of waste heat going straight up the chimney and doing nothing useful."

Former Member21/01/2021 13:45:50
1085 forum posts

[This posting has been removed]

Dave Halford21/01/2021 13:54:15
2091 forum posts
23 photos
Posted by Tony Wright 1 on 21/01/2021 13:35:59:

What was dismal about my response ? Tony.

Bob likes to flame post

Roderick Jenkins21/01/2021 14:15:20
2200 forum posts
616 photos
Posted by br on 21/01/2021 13:45:50:

Totally off topic , apologies, but just had this thought

A £3000 boiler with a £600 deposit .

You do not need many orders to have a tidy amount in the bank earning interest for you for 2 years

Morgan cars used to operate on those lines.


Edited By br on 21/01/2021 13:52:13

£600 earning 0.1% per annum isn't going to buy many lunches!


Former Member21/01/2021 14:19:20
1085 forum posts

[This posting has been removed]

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