Design of boilers

[Jeff DaymanParticipant @jeffdayman43397]

+1 for Duncan Webster's statement on reading Martin Johnson's article. My own dismal thoughts would encourage that, as well as reading the Alan J Haigh book "design, construction and working of locomotive boilers". This book has a deep description of heat transfer, structural design, and operation of full size boilers, and is fascinating reading from a professional boiler designer. Well worth a look. As to Mr Worsley's inflammatory and unproven statements, I'd just say "don't feed internet forum trolls."

[Former MemberParticipant @formermember12892]

[This posting has been removed]

[Anonymous]

Posted by Bob Worsley on 05/02/2021 10:35:41:

All my books say that the Stefan-Boltzmann law is the radiated loss between two temperatures, the boiler and the environment.

I'd be inclined to recycle the books as they're apparently wrong.

The Stefan-Boltzmann equation gives the total radiated power (across all wavelengths) per unit square of a black body. The radiated power is proportional to the thermodynamic temperature (in Kelvin) of the black body, to the fourth power. The temperature of any surrounding objects is irrelevant.

Andrew

[JasonBModerator @jasonb]

martin's articles was spread over 4 issues 4584, 87, 89 & 91. No links available but you may be able to buy digital back issues from Pocketmags.

Regarding increasing the water space I suppose it's a two edges sword, any increase in the space between wrapper and firebox will reduce the grate area so you will have less heat available to put into your increased space and it's hard hard enough to keep the fire going in these small engines to start with without making it even smaller..

[Dave Smith 14Participant @davesmith14]

If it is the article I remember, Martin's article is about predicting the performance of a boiler from first principles. The calculations take the form of an excel spread sheet. Martin was quite happy for people to have a copy of his excel file and you get a paper he wrote explaining all calculation method involved. By the way he uses the Stefan Boltzman equation. I have a full copy of all of it, however the copyright is Martins and so cannot let you have a copy. If you talk nicely to Jason or Neil they may be able to help with contact details or someone else who has access to a paper copy, which I don't. The articles are in the flash player digital copies which I can no longer read.

Dave

[Anonymous]

Posted by Dave Smith 14 on 05/02/2021 16:14:58:

…….By the way he uses the Stefan Boltzman equation.

The difference vis-a-vis another poster is that Martin understands it.

Andrew

[Dave Smith 14Participant @davesmith14]

Posted by Andrew Johnston on 05/02/2021 16:23:31:

Posted by Dave Smith 14 on 05/02/2021 16:14:58:

…….By the way he uses the Stefan Boltzman equation.

The difference vis-a-vis another poster is that Martin understands it.

Andrew

Dave

[duncan webster 1Participant @duncanwebster1]

Posted by Dave Smith 14 on 05/02/2021 18:41:33:

Posted by Andrew Johnston on 05/02/2021 16:23:31:

Posted by Dave Smith 14 on 05/02/2021 16:14:58:

…….By the way he uses the Stefan Boltzman equation.

The difference vis-a-vis another poster is that Martin understands it.

Andrew

Dave

and another

[ManofkentParticipant @manofkent]

I am reading this thread with great interest, although the maths is a bit over my head!

The OP does raise a good question – are we settling for model boiler designs that could be improved.

Many if our clubs are after all model and experimental engineers!

[Bob WorsleyParticipant @bobworsley31976]

The maths is over most people's heads, but it is easy to use what others propose, such as Reynold numbers, Stefan-Boltzmann laws etc. This is where the Schaum books are quite good, worked examples which can be adapted.

I too have been reading, on to Clark's The Steam Engine now. What is apparent from all the tests done years ago is just how little of the heat actually goes through the fire tubes. Is this why modern boilers are water tube? Read any book on heat transfer and in my experience you won't get far, they are not very well written. But all of them discuss Stefan-Boltzmann and the fourth power between TWO environments, the boiler and the world.

My initial comments were about water spaces, that bubbles are not scaled. Again, any book on heat transfer will describe the stages of boiling with the difference in temperature between fire and firebox. It is intriguing that there is a period of negative resistance in the heat transfer, tunnel diodes if you know about them. The firebox has about 15% of the heating surface and contributes about 60% of the steam raising, read the tests done. But, my point, to repeat, is that the bubble formation on the firebox is limited by the flow of water to replace the steam, pressure is irrelevant. With a 1/4" water space then this does, or doesn't, happen? Even in full size there are comments about the too close spacing of tubes, so putting them 1/16" apart in a model boiler is? sensible? not when the bubbles are larger than that.

Go into the kitchen, put some water in the kettle, and boil it. Look at the bubble formation, they are not 1/16" in diameter.

Clamp a sheet of glass to a steel sheet with a gap between, fill with water, get the torch out and boil it.

It is worth pointing out that the word experimental means that you have an open mind.

[Dave HalfordParticipant @davehalford22513]

Posted by Bob Worsley on 20/02/2021 10:49:39:

Go into the kitchen, put some water in the kettle, and boil it. Look at the bubble formation, they are not 1/16" in diameter.

Clamp a sheet of glass to a steel sheet with a gap between, fill with water, get the torch out and boil it.

It is worth pointing out that the word experimental means that you have an open mind.

That will only give you the bubble size at atmospheric pressure, what happens at 50+psi? Do the bubbles squeeze through narrow gaps? They form round, but do they have to stay round?

[Howard LewisParticipant @howardlewis46836]

As a non steam man, may I offer a few thoughts?

The purpose of a boiler is to transfer as much heat from the fuel into the water within the boiler.

In this context, the consideration is a locomotive type boiler where the steam requirement, for a loco or a road engine are not constant, resulting in varying heat inputs and pressures..

A marine boiler will have a less variable steam requirement.

The heat would seem to be extracted from the firebox and from the firetubes passing through the waterspace within the boiler. There used to be formulae giving the ideal ratio between diameter and total are of firetubes and barrel length.

Ideally the surface area to transfer heat to the water needs to be maximised, and at the same time the ability of water to circulate in the water legs around the firebox and within the boiler barrel needs to minimise hot spots which might damage the boiler.

But improving the thermal efficiency could could come at the cost of mechanical reliability. There will be a themal gradient between the inner firebox and the firebox wrapper, and between the firetubes and the barrel., so the stays and tubeplates will be subjected to stresses caused by expansion, plus the tensile stress imposed by the internal pressures to which inner and outer plates are subjected.

To this must be added the fatigue stresses induced by varying firing rates, steam pressures and steam off take.

Boiler design for efficiency and mechanical safety is not simple, and to add to our problems, "Ye can't scale Physics"

Howard  WHY do the typos only come to sight as "Add" is ticked?

 

Edited By Howard Lewis on 20/02/2021 12:57:36

[OldironParticipant @oldiron]

Posted by Howard Lewis on 20/02/2021 12:56:06:

Howard WHY do the typos only come to sight as "Add" is ticked?

I think an Irish gentleman by the name of Murphy has something to do with it Howard.

regards

Edited By Oldiron on 20/02/2021 13:13:37

[Anonymous]

Posted by Bob Worsley on 20/02/2021 10:49:39:

The maths is over most people's heads………………..

My maths is usually more than good enough; but I'm stumped as to how electron tunnelling in diodes is applied to boiler design? On another forum it was stated that the analysis of a steam engine governor needed to be done with the z-transform. I wonder if that would help here, especially as the coal is fed in discrete lumps?

Andrew

[Howard LewisParticipant @howardlewis46836]

We do tend to overthink things.

We worry about the change in specific heat vs temperature when we ought to be opening up the blast nozzle, or putting on more coal!

Howard

[Howard LewisParticipant @howardlewis46836]

EXAMPLE

An engine was rejected for excessive oil consumption.

When stripped for examination, all the pundits stood around pondering on what was wrong. Cross hatch angle, wall pressure, ring groove angle, surface finish on rings or grooves, clearances?

Fairly junior engineer then asked if anyone had noticed that the piston rings had been fitted upside down!

As the Radio fraternity put it K I S S!

Howard

[noel shelleyParticipant @noelshelley55608]

I love it, Howard ! Spot on ! I got caught like this recently, the cabinboy had the answer !

Dante Porta had the answers to many poorly running steam engines, and David Wardale proved some of them. For practical purposes steam is a gas, so to work properly it needs to be gas flowed. What's the point of lots of steam if it can't get to the cylinders when needed ? Putting pressure gauges on the steam chests (9F) was quite a revelation, it showed the driver what was REALLY happening. You put exhaust gas temp gauges on a big diesel ! Noel.

[Howard LewisParticipant @howardlewis46836]

Yes, Noel!

Big diesels were harmonised by adjusting the individual jerk pump on each cylinder until the exhaust temperature at each port was the same.

WHEN happy times return, try and visit The WaterWorks Museum in Hereford. They have a 5 cylinder Blackstone, and if you ARE allowed inside the safety railings, (try asking nicely ) you will be able to see a pyrometer in each port. The temperatures will be low because the engine running off load.

I was involved in a major uprate of a smaller engine for marine use. It involved utilising parts from two different versions of the engine. Owing to a misunderstanding, the timing was set the exact opposite of what I wanted. Having seen the low power and HIGH exhaust temperatures, I had the timing advanced half way. The exhaust temperature fell, and the power increased.

Since the engine was a prototype, for an external contractor to develop, I left it there. With telephone conversations, the contractor advanced the timing to where I had originally wanted it. One tiny nudge, and within the first week, we had achieved the objective, and had another five months in which to tie up any loose ends, even the late "Can you just"s.and "If only……"

It took a long time for some of the other British railways to adopt the methods used by Churchward and his successors. In British railways days, one of those who had worked on GWR was sent to LNER. Using Swindon methods he cured the fault that had beset their 3 cylinder locos from the outset.

It used to be said that Freddie Dixon, a great exponent of tuned Rileys, prewar, could add 5 mph to the a car's top speed by stripping and rebuilding the rear axle.

As always, the devil is in the detail!

Howard

[Bob WorsleyParticipant @bobworsley31976]

Umm

"As to Mr Worsley's inflammatory and unproven statements, I'd just say "don't feed internet forum trolls.""

Please provide a list of these, and as to why they are both inflammatory and unproven.

I used the exaple of tunnel diodes as a comparison, different bits of engineering might be useful to other bits.

Why not give us some calculated examples of thermal expansion and stresses rather than just stating that it happens? If you read Babcock & Wilcox's book on steam you will see that they use a spiral shaped tube to provide thermal flexibility. This is also in the Clark book from over 100 years ago. Clark also mentions joints in the fire tube that also gives thermal flexibility. The question for experimental engineers is why hasn't this been tried?

Clark mentions notching which is caused by thermal changes, often caused by feed water introduced into the very hot boiler. Even opening the fire door introduces a gale of cold air. There is thermal expansion, but what seems to be the killer is uneven thermal expansion, hence feed water and fire door air.

Stefan-Boltzmann is THERMAL transfer of energy, and as such does not cover all wavelengths of electromagnetic radiation for the simple reason that only around the infra red is heat transmitted. Perhaps you need to read a few books and then apologise.

[DrDaveParticipant @drdave]

Posted by Bob Worsley on 24/02/2021 17:49:26:

Stefan-Boltzmann is THERMAL transfer of energy, and as such does not cover all wavelengths of electromagnetic radiation for the simple reason that only around the infra red is heat transmitted. Perhaps you need to read a few books and then apologise.

That's an interesting statement! I dug out one of my old text books, "Heat Transfer" by JP Holman to see what he might have to say. "When the energy density is integrated over all wavelengths, the total energy emitted is proportional to absolute temperature to the fourth power.

Eb = (alpha) x T^4

This equation is called the Stefan-Boltzmann law."

There then follows more than 20 pages of theory before the equations for heat transfer are derived. These are a function of (T1^4 – T2^4), but also include emissivity factors and shape functions. Note that this difference in temperature to the fourth power is for heat transfer: it is not Stefan-Boltzmann's law.

I recall from my Heat Transfer course many years ago that calculating the shape functions is fiendishly difficult, except for trivial examples. Even the finite element program that I was using last year, and which has a thermal analysis capability, is not brave enough to attempt it!

Dave

[Nigel Graham 2Participant @nigelgraham2]

No arguing, please…

Shall we try a little perspective?

People have tried "improving" miniature boiler efficiency more or less since the days when it was realised a scaled down version of full-size but with fewer and proportionately larger tubes was feasible.

More than 100 years ago, a Dr. J. Bradbury Winter built a magnificent 1:12 model of an LBSCR 0-4-2 tender locomotive called 'Como'. (I don't know if that was his or the original name for it; nor if the model-standard 5" gauge existed then – Como would be about 4-5/8" g.) It would be interesting to know its performance, given the photograph of the fully girder-stayed firebox shows the model apparently carrying the full complement of tubes of right size and presumably correct lack of super-heater. More to the point perhaps, does 'Como' still exist? I do hope so.

In the same 1917 volume of Model Engineer & Electrician describing how Dr. Bradbury-Winter built 'Como', we have a photo of a traction-engine to perhaps 4" scale (freelance) with a novel boiler. A water-tube coil forms an inner wall to an otherwise dry-wall firebox partially overlapped by a stepped portion of the multi-tubed barrel. The article carries the boiler drawing. The engine's builder, photographed driving it, was a Mr. R. Briggs…

'

Since then, a good deal of accumulated experience had led to hundreds of well-functioning miniature railway locomotives and traction-engines to a very wide menu of designs. LBSC set out to give designs that may sometimes raise purist eyebrows, but actually work.

Competitions like IMLEC have encouraged examining how the engine uses the steam.

Presently, engineers like Doug Hewson, Peter Seymour Howell and Luker are showing us we can have locomotive fittings that may conceal compromises necessary for proper function, but whose appearances are as near scale and prototype as possible.

'

Yet we still have the constraints: The models have to be driveable and their "works" reasonably accessible for servicing. We "can't scale Nature". Most importantly though, we are building scaled-down replicas of machines that became commercially obsolete partly (though not only) because they were just so thermally inefficient!

'

So do we "need" delve into the intricacies of Thermodynamics to try to screw an extra % or so from something fundamentally inefficient? Especially as the gain may be a matter of diminishing returns, and the engine's performance on the day depends heavily on the coal available, the driver's skill, to some extent on the weather; and for traction-engines, very much on the ground conditions?

By all means experiment if you wish, bearing in mind that what the engine does with and to the steam, counts at least as much as what the boiler does with the coal and water.

Water is a constant, apart from hardness, but coal isn't very constant and is not going to improve. "Full-size" tests on the railways, steam-ships and power-stations realised that, and would analyse the coal's own efficiency. Perhaps IMLEC needs a calorimeter and chemical-balance as well as the dynamometer-car, but rather as its title implies, it is comparing locomotives all using a given fuel on a given day. It is not an absolute test of individual engines.

I am all for we model-engineers trying to make our engines built and operated to the conventional pattern, perform as well as possible; and for us to try to fire and drive them as skilfully as we can. Nevertheless, I very much doubt out-and-out thermal-efficiency chasing will really achieve very much more than the best available now.

+++

Thermodynamics – One of my managers gained his PhD in Very Cold Things… like liquid helium , which "is fun", he said. He told me his Physics Professor claimed that "God invented Physics – then invented Thermodynamics".

'

Howard Lewis raises an interesting point with the engineers baffled by the engine. I have seen the Can't Be That syndrome several times. I have caught it myself. It is a common trait.

The simplest cause is too simple to see when you have years of deep knowledge and skill in your head. It is worse in a group, where respect for one's peers inhibits suggesting an error too elementary for their undoubted sagacity.

If I had a fiver for every scrappy Reauleaux Diagram and dimensions-calculation that littered the club workshop bench holding a newly-built loco chassis for months. The problem? Strangely-strangulated running (on air). The one who diagnosed and cured it? The least-experienced member (errr, me). The solution? A new exhaust-flange gasket – one with a hole through it.

[noel shelleyParticipant @noelshelley55608]

ONE FOR Howard ! Having watched well over £60,000 spent on 2 pairs of similar diesel engines, I pair were fine the other smoked. The book said, and the experts did, with all the special tools. More money was spent on derated pumps, and varieing injector spray angles Etc. I found how it was set or worked hard to fathom. In fact I couldn't see how it worked. The good pair were set in one way, the other, slightly newer pair were set in a fancy way with fancy tools.

It turned out that whilst the book was right 40 odd years ago with a tolerance of 1 thou, but with bits of wear in the injection train Etc it didn't work now. The experts gave up and told me to do as I liked. I did ! Reverting to basic principles, ignoring the book and the timing marks I cured the smoke with a small torque wrench. Noel.

[Howard LewisParticipant @howardlewis46836]

Like Noel, I have often found that reverting to simple basic principles will lead to a solution.

Working on the basis that if you supply a Diesel engine with the right fuel in the right quantity, the right form and at the right time; it will run! You can't always know exactly what the designer had in mind, swirl ratios, surface / volume ratios, wall pressures, internal charge heating etc, so you just have to go back to basics to start things rolling before the fine tuning and attention to detail begin to take effect.

I suspect that a lot of the features and proportions found in full scale loco boilers were the arrived at empirically rather than theoretically.

I suspect that Sam Ell did most of the draughting work on "the Home Trainer", with a slide rule as follow up check, or even to explain what was found..

If you want to start scaling bubble sizes, settle the surface finish of the tubes and firebox first, to keep at least at least one of the variables at a constant value. But how do you measure the effects of localised scaling, or the hardness of the water?

In a reciprocating engine, conditions are continually changing, so will be the conditions under which the boiler is expected to supply steam, from second to second.

What works on one Club track may not work on another Clubs!

Howard

[Bob WorsleyParticipant @bobworsley31976]

I am not interested in diesel engines.

Re-read my original post, asked lots of questions, where are the replies.

Anyone do experimental work?

Found a heat book that used photography to measure the nucleate boiling bubbles, 1 to 6mm diameter.

LBSC must be spinning in his grave, I would like to communicate with him.

Conclusion, forget trying anything different.

[duncan webster 1Participant @duncanwebster1]

Bob, if you are so convinced you are right, do your own experiments, don't expect others to investigate your theories.

[Author]

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