Design of boilers

I'm no expert on fluid dynamics but I think you will find that it's the other way around. The smaller the diameter of the tube, the less likely that the flow will be turbulent as the Reynolds number decreases.I would guess that you would need a very high gas velocity through the small tube to get any turbulence.

Across the pond, they run a lot of propane and oil fired locomotives and they have found that putting 'turbulators' (strips of twisted stainless steel) in the fire tubes results in a significant increase in the steam raising ability of the boiler, due to them breaking up the stagnant boundary layer of gas on the inside of the tubes by introducing turbulence.

Jim Ewins did some tests years ago on a 5" gauge boiler that showed that useful heat transfer only occurred in the first few inches of the fire tubes.I would reason that that is because of laminar flow in the tube, rather than turbulent.

John

My favourite film clip this is what you get with too much draft, he's obviously opened the exhaust blast jet a 'bit'.

Starts cool, then it starts raining fire halfway down the track.

Oh no, that's two 'F' grades I've picked up today. As I'm sure you and Andrew are right I must get the textbooks out again. Good job I didn't design boilers for a living!

Dave

Can I once again encourage people to read Martin Johnson's article in ME instead of all this speculation. Flow in small tubes is just as easy/difficult to model as in big ones, but the same fundamental laws apply. Cooking recipes as found in some sources probably don't. The L/D^2 rule quoted is many is completely without foundation.

The really difficult bit is what is going on in the firebox, models are very different to full size.

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

What is Martin Johnson's article? Why not put in a link so someone without 120 years of ME can read it?

All this flow in tubes is down in the noise, most heating is done through the firebox and with a 1/4" of water space my point was that that was not all it could be. Just look at the size of the bubbles when a kettle boils. Another gotcha is the design of crown stays, how any water is supposed to flow around them is not at all clear. Look at the ones specified for Minnie as a poor example. No cross holes, only 5/8" apart.

It seems that the large firebox stays used in the Allchin are a good thing, in that they conduct heat to the water. So making the stays even larger could be an even better thing.

The point is that with an incandescent fire, temperature about 700C or so, then there is a lot of heat there. And extracting the maximum possible from radiated heat could be more effective than using tiny flue tubes. Gases have almost no specific heat capacity, why they don't heat up tubes. The suggestion about using solid copper heat sinks from the front of the firebox into the boiler might be more effective.

I would respectfully point out that an incandescent fire would be more like 1200*c. Noel.

+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."

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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

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..

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

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

Andrew

Dave

and another

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!

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.

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?

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

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

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