Design of boilers

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

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

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.

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

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.

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

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.

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.

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

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.

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

It is not the knowledge of any form of engine, but the application of logical thought that solves problems.

It is far too easy to pursue some obstruse feature when thought about basic principles will lead to a solution.

You might well design and construct a boiler which is theoretically correct, but soon declines in performance because of scaling. on the outside of the tubes, and soot on the inside.

The boiler on 71000 was a poor steam raiser until the ashpan was opened up to improve primary air flow.

Even a good boiler can give mediocre performance if managed incorrectly, fed fuel for which it was not developed, or supplying cylinders with less than optimal steam passages and valve events..

The Holcroft / Gresley 2:1 valve gear wasgood, but deteriorated rapidly once wear in the linkage occurred.

And don't mention Bulleid's chain driven valve gear and it's behaviour, or his steam reverser!

Witness the locomotive interchange trials carried out by various UK railways. A boiler developed to work efficiently with Yorkshire coal and over the home metals would not produce equal results when fed with Welsh coal and run over a different roadbed. And if the firing and driving techniques are different, so will be the results!

The K Class "River" tank locomotives were dangerous on their own metals , (The Sevenoaks accident ) but were considered to be safe when tested over LNER metals; because the roadbed was was better.

Holcroft found that a lot of his tests on locomotives ( Locomotive Adventure vols 1 and 2 if you want to see more detail ) yielded inconsistent results because of oil being carried over from the cylinders and being carbonised in the blast nozzle, as well as thermal expansion changing the relationship between nozzle and petticoat pipe.

There is a huge shoal of red herrings just itching to start swimming and confusing the results of any test.

Howard.

My thoughts are simply that to run a boiler in public in the UK it either needs to be an existing design or where (unspecified) calculations are accepted by a boiler inspector and the whole thing accepted as sound design. If outside their experience they are perfectly entitled to refuse to test it, not because it is necessarily unsafe, but quite rightly because they are not familiar with any similar designs.

If I want to build a model locomotive then my options for experimentation are limited to non-existent if I want to run it in public and how many of us have private tracks that match up to those at many clubs?

As such this topic, while intellectually stimulating maybe, has little practical relevance in my case.

I’ve been following this thread with great interest. Thanks for the mention Nigel but I don’t consider myself an engineer… still have a ways to go.

My two cents worth… Thermal radiation depends on the assumptions, with most of my simulations based on grey body radiation, the formulation for the emissivity is adjusted using first principles to account for specific conditions like dust, and build up on the thermal pickup surface. I’ve found with the boiler simulations that using straight black or grey body assumptions results in the heat transfer to the water being over-predicted. The view factor is calculated in the FEA, a little computationally intensive but you only need to do it once for a specific model. This is relatively new, a few years back this was a hand calculation typically using a solver in excel (not difficult just takes a while to setup).

There are many experiments for onset on nucleate boiling with some of these results shown in textbooks; worthwhile to have a look at. The non-dimensional analysis used in HE design is based on these early experiments with the calculations and formulations really just curve fitting. Perfect for water/steam which is what most of the experiments were done on.

I agree with Noel, just build it and try not to overthink! For the boilers and locos I design I do the initial calculation and simulations to decrease the build time and to make sure I’ll be happy with track performance, but I make sure it doesn’t keep me out of the workshop for too long! I have built a Martin Evans loco with the boiler doing the job on the track, but the designs were I’ve put a little effort in heat transfer and flow dynamics is orders of magnitude better (I can’t take credit for this, the computer did the work!).

The attached picture is from a slide show I used in a presentation, I wanted to upload the whole slide show but I’ll spare you all the HT FEA jargon.

Oh, you deserve credit for the physics and maths!

The computer does not know how to analyse the physics of a miniature locomotive boiler - you need tell it how. You still need know the sums and how to tell the computer to do them for you.

Taking a GCSE Mathematics evening-class course over 20 years ago as a refresher for work reasons, I was baffled by the one topic new to school maths, hence to me: Matrices - taught as a pure abstraction linked to nothing else. I sought help from one of the scientists at work. Surprisingly, despite her PhD in Very Hard Sums, she could not explain the basic information I needed, so I was still baffled. She did though tell her work involved gigantic simultaneous-equation blocks soluble only by matrices; so I now knew their use - Finite Element Analysis (of vibrations, in her field). I realised the computer just made the arithmetic feasible. Though using a bought-in programme, she still needed to understand FE Analysis and extremely advanced mathematics.

As for the Matrix... It know the sort that is the clay in what used to be called "Boulder Clay"!

Nigel - Although I have never needed to use matrices since leaving University many years ago I do remember on lecturer (or maybe he was an instructor in a maths workshop?) explaining the need for them as follows -

You always had squared exercise books for maths in school and calculations were less confusing and easier to follow correctly if you kept to the squares. Well a matrix is the two or three dimensional equivalent for more complex calculations. Thank goodness I never met matrices with more than three dimensions then!

Worked for me for as long as I needed it to.

The OP asks many questions and raises many interesting points ! When steam engines were used as transport, they seldom went cold, even quite major boiler repairs would be done as soon as it was possible for the boiler man and his boy to enter. Tornado had problems with it's boiler not withstanding all the technology and calculation used in it's design, I was dismayed to be told that a 9F in steam returning to the shed would not be steamed for 2 weeks. No matter how slowly you build up the fire from cold there will be differential expansion and stresses that would not normally have occured in it's working life - for which it was designed !

With all the volumes on the steam boiler and it's associated engine All the information is available to build the perfect boiler ! It should be possible using modern electronic control to burn the fuel and get the best possible efficiency for any given set of circumstances.

A light sensor in the fire box and another in the funnel to control fuel delivery and combustion air, integrated into the requirement for power. Constant monitoring and control of water level fed from a feed water heater, no sudden rush of cold water. Pressure monitoring and control of fuel to make safety valve lifting and power wasting a thing of the past. A sensor in the steam pipe to monitor priming and increase super heating. This and much more fed into the ECU. ATC !

Or one could employ a good driver and fire man who knew the road !

If one wants to build a boiler does one build to prototype or use more modern design ? Does one build it to work well or please the rivet counter ? Do you build something the youngsters would appreciate the application of modern technology to or something that transports us back to our childhood ?

With copper and silver at near all time highs I cannot afford to build an experimental boiler ! I would invite the original poster to do so and give us the benefits of his investigations and design. Noel

I'd never even heard of Matrices until that course, and I assumed they are a new invention. So I was quite surprised to learn of their very deep ancestry; but thought it amusingly appropriate that one of their 19C developers, a certain Prof Charles Dodgson, also wrote two other fantasies - the Alice novels.

I don't understand them because though only boxes of simple arithmetic "sums" their completely unintuitive, fancy terms are never explained. Any likeness to any real mathematics alive or dead is purely co-incidental, let alone to physical things. So I could find nothing explicable, analogous or exemplifying, for them.

I'm afraid your lecturer's analogy is even worse than the introduction in the maths text-book. Having shown a simple table supposedly representing a shop's weekly sales figures, it stated that you could omit the headings naming the goods as long as you keep their order consistent.... then it launched straight into the mysticism!

Why couldn't my scientist friend not help me understand the basics of what she used at professional level?

She gave me what the teacher and books gave: the basic instructions. Not what matrices are and do to each other; not what "determinants" , "identity matrices" , etc. are. Nor why, when and where anyone might want to bundle little sums into three boxes, instead of writing them separately, and so on.

'

When it comes to designing the boiler for my steam-wagon, from almost nothing but a few photocopies of contemporary photos of the extinct vehicles, I used the standard formulae in the model-engineering books. That was for structure, not steam-raising power - I knew that would be beyond me even if I could find the methods.

However these do not give the strength-formulae I particularly needed, for the vertical, cylindrical inner firebox; a copper drum 5" diameter and about 12" tall, with most of the tubes lying along projected chords. I found something close in a professional designer's book written by an American chap called Roark, as well as discovering his tortuous formulae were governed by further "If A < B but >C where C > D" style equation chains.

Doubts set in when I saw at a major exhibition, a sectioned locomotive boiler failed before it had even seen its 7-1/4 " g frames. It wasn't my noticing the tube-plate flange bowed in by a desperate attempt to "planish" the shell down to meet it, but my I realising the builder had been unable to put enough heat, and rapidly enough, into the metal for the flux and silver-solder to work properly. Big cauliflowers of partially-decomposed alloy...

The clincher came at one of the Taunton MES exhibitions. 'Western Steam' trade-stand displayed a brace of freshly-made Shay loco boilers almost identical in pattern to my steam-lurry's patented form. Upshot of the conversation with Helen Verrall was my posting the dimensioned outline drawing to her, and duly collecting the finished boiler at the following year's exhibition. CE-marked, the lot.

Salient here was my asking for 4 superheater flues to give me the non-prototypical option (the engine is a compound but my replica is limited to 90pis boiler pressure), but otherwise I stated leaving all structural and fire-tube details to WS' methods and practice.

I still have my original boiler shell parts, machined, stored away in the loft in case I am tempted to try to finish it. The difficulty might be that of not frightening the society Admirers of Boilers with something not as conventional as a 6"-scale traction-engine or 0-gauge loco.

Noel I’m with you on the cost of copper and silver solder, it’s ridiculous! But this is where modern manufacturing techniques, modern design and materials can bring the hobby back to where it was in LBSC’s time, suitable for the young and people with limited resources. I personally think experimentation on boiler design should be more towards materials and manufacturing techniques, with efficiency and steaming benefits a second thought (from my simulations and personal experimentation I think there is little to gain by digressing too much from the published ME ratios, for copper anyway). To put things into perspective not one of my last three locos cost me more to build than the price of a single set of Tich castings and they were all larger gauges.

Personally I’m too pedantic and safety conscience to have my boilers made by anyone else, professional or otherwise.

Matrices are much used in 3D computer graphics - a single 4x4 matrix can contain all the information necessary to Translate, Rotate and Scale an object, itself defined by it's transform matrix - how they do this is very non-intuitive, but computers seem to like it.