Is a 3″ Cornish coal fire possible?

[Christopher Jones 9Participant @christopherjones9]

Hello.

I've been researching Cornish boilers after my recent visit to Beamish museum. I really want to build my first coal fired boiler and want it to be a Cornish type.. I currently have a 3inch copper tube but should I use this for the fire flue or the outer boiler? I don't want a huge boiler but also not a under powered one.

Also does anyone have or recommend a plan for a coal Cornish boiler?

Thanks chris

[Christopher Jones 9Participant @christopherjones9]

I’ve always had an interest in live steam but now I need it

[Martin KyteParticipant @martinkyte99762]

Well there's an interesting build. My first thoughts are that they were designed for low pressures driving atmospheric engines for pumping mines. As cornwall was far from coal fields they would heve been quite efficient so flue temperatures and gas flow rates would have been quite low. I would say your main issue would be ensuring enough draught to keep the fire bright. These boilers usually had quite tall chimneys to get that to work. I seem to remember a series in Model Engineer of a chap building a chimney for his boiler. Don't remembre too much in the way of detail but I think he built a Lancashire boiler. Have a dig around for the issues. Don't quote me as a boiler expert just someone who has taken notice over the years.

regards Martin

[Christopher Jones 9Participant @christopherjones9]

I do love the stationary models. I have a model in mind of a working mine head.. "mine shaft" and pump house.. hense the smaller boiler size to keep size down over all..

[Howard LewisParticipant @howardlewis46836]

The boilers for Cornish Beam Engines tended to use what Richard Trevitick called Strong steam" i.e at 40 psi compared to the low pressures favoured by James Watt

Often, the single flue was corrugated, to increase heating area.

As a model, possibly you could use the 3" copper tube as the outer shell, and a piece of 15 mm corrugated copper tube as the flue. I think that such tube should be available as a "flexible" pipe for plumbing in water taps.

It would be even better if available in 22 mm.

At a push, you could make up a set of rolls and corrugate 22 mm yourself..

Since the copper pipe available for domestic plumbing has a bursting pressure of several hundred PSI, you should be fairly safe working at a prototypical 40 PSI, given good brazing!

Howard

[Nick Clarke 3Participant @nickclarke3]

There have been many O gauge live steam locos with coal fired boilers so I am tempted to suggest that one could work using your 3" copper tube as the flue in a Cornish boiler – but this is only a guess and not based upon experience. Using it as an outer tube in a boiler would imply a flue 1" or less and a grate 1/2" to 3/4" wide which is probably too small.

[Nick Clarke 3Participant @nickclarke3]

There have been many O gauge live steam locos with coal fired boilers so I am tempted to suggest that one could work using your 3" copper tube as the flue in a Cornish boiler – but this is only a guess and not based upon experience. Using it as an outer tube in a boiler would imply a flue 1" or less and a grate 1/2" to 3/4" wide which is probably too small.

[Nigel Graham 2Participant @nigelgraham2]

Corrugating the flues did increase the heating area slightly but that was not the main reason.

It was for strength!

Some manufacturers relied on single, circular embossings, especially at radial joints, for the same reason. I believe this was sometimes called a Bowling Ring, but my memory may be shaky on that detail and it might have been one builder's name for it.

It also eased the expansion / contraction strains: some 19C Cornish and Lancashire boilers burst because ironically they were structurally too stiff.

The main problem with a flue several times longer than its diameter is that it will fail at far lower external pressure than the same tube under internal pressure, and in very complicated ways with many variables at play; although if the boiler is supplying a model Cornish mine pumping-engine it won't need work at a very high pressure at all. Scaled-down strong steam!

The corrugated pipe sold for plumbing is to allow easy bending from rigid pipe to (usually) the tap tail; and I would not like to guess how it would cope with being used in a boiler. From my catalogues, it would not be suitable at all. Such pipes are too small for a start, at 15mm and 22mm bore; and are short lengths of corrugated stainless-steel with both ends terminated by copper end-pieces to allow easy jointing with conventional plumbing methods.

Marshalls patented a stayless firebox for their portable-engines, in which strength was given by profiles pressed in the crown-plate. The principle is of course common throughout engineering: think of all the wiggly bits in the thin steel of your car.

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As for efficiency, they were probably not ever so efficient; but the engine was not much better, even after James Watt's major improvements to the operating principles. Cornwall is not all that far from the nearest coal-fields in Somerset and South Wales, especially after the railways arrived. The metals' main buyers were probably much further away.

[noel shelleyParticipant @noelshelley55608]

I think there are many stumbling block to using coal as a fuel, in a boiler of this type and size, draughting being one. To keep the fire burning in such a small firebox would I fancy be all but immposible, it might work with charcoal. Never the less an interesting idea, good luck. Noel.

[Mark RandParticipant @markrand96270]

I'm guessing that one could build an induced draught fan into a small building at the bottom of the miniature stone/brick chimney. 25 or 40 mm fan, rechargable battery, stone 'shed'

[Christopher Jones 9Participant @christopherjones9]

Although it could very well be a massive failure regards a coal fire.. I think it's at very least going to be worth a try just to see if I would work… I like the idea of a fan in the chimney base! That's a great idea.

[Christopher Jones 9Participant @christopherjones9]

Would it be worth testing the firebox size in steel? I have a good source of. Steel offcuts

[Nigel Graham 2Participant @nigelgraham2]

Nicke gives a fair assessment of proportions: the flue is about or slightly above 1/3 the shell diameter, and is placed so its highest point is on a bit below the shell's centre-line.

The furnace section seems typically about 1/3 total length, and the flue is of constant diameter for its full length to the smokebox. That was always given an access hatch for cleaning.

The grate is slightly narrower than the flue's inner diameter so it rests below the flue axis, and the "ash-pit" segment is closed by a door containing a damper.

The firebox, as such, is therefore rather like the "marine" type used on some narrow-gauge locos – I think the 'Polly' range exemplifies them although those boilers have firetubes, not a single flue.

Taking up Nick's point about coal-fired O-gauge locomotives, I would endeavour to find how their boilers are arranged, since on the face of it your proposal is for a boiler somewhat larger than those. They evidently work, but I don't know their secret!

If you use a hidden fan as Mark suggests, I would think it better to make it induce the draught in the chimney by driving air through a simple ejector in the chimney base, rather than pulling the hot gases through the fan.

Cornish boiler-houses generally had the chimney right up against the house wall, and the chimneys I've seen didn't seem to have the pedestal common to many factory chimneys. The fan etc, could be concealed in an adjoining coal store, with a discreet tube within the model's base to feed the ejector.

'

An interesting challenge! Do keep us posted!

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As an aside, some may recollect Ron Jarvis' magnificent models of particularly significant, early steam-engines, including Newcomen's Atmospheric Engine. This, complete with all-correct lead pipes and square nuts, works as it should (I wonder where his models are now – Ron died quite some years ago), but for the boiler. About the shape and size of an orange, and with an orange-peel finish to simulate the original's hand-forged wrought-iron plates, it uses an electric heater to produce steam at the prototype's not-so-strong 2psi. Ron used to joke to us, his fellow society members, that the heater's electronic controller made this the only 18C engine with 20C microprocessor control!

[HopperParticipant @hopper]

Posted by Christopher Jones 9 on 23/05/2022 13:03:46:

Hello.

…. I currently have a 3inch copper tube but should I use this for the fire flue or the outer boiler? I don't want a huge boiler but also not a under powered one.

A 3" diameter boiler will power a model steam engine. My dad made one when I was a kid. But we fired it with one of those old petrol blowtorches. But it was underfired with a 3/4" diameter return flue through the length of the boiler drum and a piece of 3/4" copper pipe for a stack at the front of the boiler joined to the flue with an elbow. You could consider something similar for your coal firing. That way you could have a larger grate area under the boiler that would be easier to stoke on coal.

[duncan webster 1Participant @duncanwebster1]

Posted by Hopper on 23/05/2022 23:52:16:

Posted by Christopher Jones 9 on 23/05/2022 13:03:46:

Hello.

…. I currently have a 3inch copper tube but should I use this for the fire flue or the outer boiler? I don't want a huge boiler but also not a under powered one.

A 3" diameter boiler will power a model steam engine. My dad made one when I was a kid. But we fired it with one of those old petrol blowtorches. But it was underfired with a 3/4" diameter return flue through the length of the boiler drum and a piece of 3/4" copper pipe for a stack at the front of the boiler joined to the flue with an elbow. You could consider something similar for your coal firing. That way you could have a larger grate area under the boiler that would be easier to stoke on coal.

GHThomas describes something like that (but bigger) in his Model Boiler book. It has fire underneath but firetubes inside the shell, a return chamber at one end to divert the gasses from the fire space underneath into the firetubes and then a smokebox on the other end. In full size this concept was used on mill engines in the USA. It wouldn't look much like a Cornish boiler tho'. I think something like this has every chance of working, not too sure about a coal fire inside a pipe about 1.25" diameter. You'd need space underneath the grate for air to get in, then the thickness of the grate, plus fire, then some space for the flames to get away over the top.

I can scan the relevant page if you want.

[Paul LousickParticipant @paullousick59116]

Have a look on this site for a set of drawings to make a 90mm dia horizontal boiler. (may give you some ideas)

**LINK**

**LINK**

[AdrianRParticipant @adrianr18614]

Before investing a lot of time and expensive materials into making it, why not model it out of tin cans first. I am not suggesting making steam, just make the firebox/tube and flue and cool it with running water.

For a fan look on ebay under Computer Components and Parts, you can get very tiny radial and centrifugal fans

[Christopher Jones 9Participant @christopherjones9]

I have obtained some steel tubes.. ranging from 25mm up to 33mm.. I'm going to mock up the fire side of the boiler.. the fire grate size isn't to much of a problem as if I really have to I can hide a bigger firebox. Below the boiler in the "brickwork"… Would a Lancashire boiler have the same problems?

[Nigel Graham 2Participant @nigelgraham2]

A Lancashire Boiler would, yes. In fact for the same shell diameter the flues would probably be smaller. They might be amenable to gas-firing but harder to make and operate as efficient coal-burners in such a small size.

[Christopher Jones 9Participant @christopherjones9]

Back to the drawing board… Coal in my current design is a no go… Whats everyone's smallest coal fire box?

[Nick Clarke 3Participant @nickclarke3]

The cornish boiler design with the very limited air space under the grate would probably need to be larger to burn coal successfully than a locomotive boiler or a Babcock boiler where the whole area of the grate can have draught through.

[duncan webster 1Participant @duncanwebster1]

Posted by Christopher Jones 9 on 25/05/2022 20:42:34:

Back to the drawing board… Coal in my current design is a no go… Whats everyone's smallest coal fire box?

Tich firebox is 2.125"*2.375" (I think)

[norm nortonParticipant @normnorton75434]

I think you would have success with a 2.5" or 3" diameter fire tube inside a 5" Cornish style boiler tube. The grate could then support a 2" x 4" firebed which would produce more than a couple of kilowatts if 1" deep AND supported by an adequate draught. That should generate enough steam for a 2" stroke mill engine, perhaps bigger? Just my 'guesstimates' from running small steam locomotives.

I would use a compressed air blower jet, up the chimney, to start and stabilise the fire until there was enough steam to take over.

The problem will be getting a new design of boiler approved for use in public, but there is little doubt you could make a massively strong boiler from two tubes and two end plates.

[Nigel Graham 2Participant @nigelgraham2]

Norm –

Christopher doesn't say where he is or if he's in a model-engineering society affiliated to one of the MELG Federations, but if he is, the boiler-testing problem should not too difficult. Depending on its boiler examiner… who can decline to test a boiler of personally-unfamiliar design, but should not do so out of hand.

The now-white-with-orange-lettering book explains the test requirement and process; a club may have some additional requirement such as the owner providing an adaptor to fit its test-pump hose.

The point would be to talk to the club's boiler-examiner first. He or she is likely to want to see the shell-strength calculation (a simple formula), and to inspect the work as it progresses, but there is nothing inherently novel about the Cornish Boiler design.

The sticking-point might be the flue strength at that diameter – heavily dependent on diameter, length, roundness and pressure, and very hard to calculate – and I would suggest no more than about 30-40psi WP and a wall thickness no less than 3mm as a start.

I though the idea is to run a Cornish-style engine from it. They used very modest steam pressures, and anyway a model version is not likely to be doing any significant work needing anything like even one kilowatt (well over 1HP!) of power.

[norm nortonParticipant @normnorton75434]

Yes you are right Nigel, boilers can be approved, but the stress calculations are a science that only some seem to enjoy.

Two kW of fire energy has to get through perhaps only 1% or 2% efficiency so it might be only 20 watts of engine power?

Edited By norm norton on 27/05/2022 15:58:23

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