Design of boilers

Don't know if Helen will have much of that deposit sitting in the bank, expect she buys copper by the full length so has a fair amount of that sitting on the shelf which the deposits go towards but I suppose the rate metal prices seem to keep going up it could be worthwhile.

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but she has to buy the copper up front, so the £600 might just about cover that

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The dismal response was aimed pretty well at everyone, the near complete lack of any analysis and thoughtful comment on what took me several weeks to work out.

One person says that the smoke box gets red hot, another says that the boiler is 70% efficient, can't both be correct. Reading old books and they get efficiency readings of about 5%, which I would believe. This is why I introduced the Stefan-Boltzmann law, the boiler radiation losses are probably more than 50% of the heat that goes into the boiler.

I have also just spent several long evening trying to calculate form first principles the theory of staying, and it is now beyond my maths ability, see if I can find another book. I have used the Schaum's outline books but now find them incomprehensible. What is the difference between shear and bending moment stresses? I can understand the shear applied to a rivet, but in a beam?

I see that copper prices have risen over the last 6 months or so by $1000 tonne to $5,500, so probable can't afford to make them now.

Depends how you interpret that, the text I quoted said larger tubed boilers make the smokebox hot due to more heat passing straight through the tubes, so likely less than the 70% mentioned. Also TE boilers don't have the benifit of a nice flow of air to help the fire and exhaust so likely a bit less efficient than a loco. You are suggesting that a couple of big tubes will be better. I just offered another opinion from someone who has made a good number of boilers and run engines.

I'm sure that some of those that take part in things like IMLEC have tweaked their boiler designs and they don't differ greatly from the norm.

Well looks like you may only be able to afford 3 boilers and not 4 if the copper price is high.

If I was looking to improve boilers I would be looking at the materials more than the design nowadays

3D printing in particular could increase the internal surface area

The NZ rocket people print their own engines so there are tough alternative material systems about

Just use a mig type wire for the simplest systems a star trek laser beams for the fancy systems

Edited By Ady1 on 22/01/2021 10:57:27

Not true in my case.

Making parts is the easy bit, what interests me is the design and how it works and whether I can put numbers on the theory. Given some of the derogatory comments in the past I no longer discuss theory and design here.

Andrew

My reaction was only dismal in the sense it recognises the difficulty of what Bob is suggesting. There are too many variables in play at the same time!

Existing model boilers are based on full-size practice reduced to meet the constructional needs of scale models. Whilst scaled down boilers aren't the result of a bottom up design, they've evolved over a century of practical construction. I suspect, despite imperfections and misunderstandings, they are a good practical compromise.

Reading about boiler design in the 19th century, it started with wild guesses and ended with boiler design based firmly on scientific understanding derived from experimental evidence. None of the experimental work was done on small boilers! When boilers are scaled down, the big picture disappears into the noise. I don't think the thermodynamics of small boilers are understood, because small changes of layout have disproportionate effects, and they have never been studied comprehensively. No-one knows!

Much easier to predict the behaviour of a 3" diameter 20 foot long fire tube than the same thing at 1/12 scale. This is why one builder of a small boiler reports 70% efficiency, whilst another has a red-hot smoke box. The difference might be explained by the fuel alone; welsh steam coal has a much stronger radiant effect in the firebox than wood or slack where still burning particles are often blown out of the chimney. Or the difference is caused by something else entirely.

Rather than building a boiler on Bob's principles and testing it as a unit, I suggest it would be better to test each idea one at a time. If an all at once unit behaves well, it's hard to determine why. Which of several features is working? Likewise a failure might be due to one big shortcoming overwhelming several small improvements elsewhere. But which bit is wrong?

Be good if Bob's ideas could be simulated with a computer model rather than expensively building and testing real boilers, but I doubt the mathematical properties of scaled down boiler components are sufficiently understood; they would have to be determined experimentally. Again, this would have to be done one at a time, a lot of work.

Bob mentioned the Stefan-Bolzmann law. Strictly speaking the law applies to Black Body radiation, which isn't exactly a model loco's firebox. It describes what happens when an incandescent object behaving as a 'black body' is viewed. The law is more appropriate to radiation escaping through an open firebox door than what happens inside. I mention it not because Bob is wrong to consider it, but because I don't know how to apply it in a simulation, or even if it's valid to try in these circumstances.

May be worth Bob modelling his boiler ideas with a 3D CAD package - Fusion 360 can do FEM, and I think it does thermal flows as well. However, getting the model right, applying the analysis, and interpreting the results are all skilled work. Not easy. Worse, I'm not convinced Fusion would model a miniature boiler correctly because its database of physical properties are all full-size. So it might be accurate for 60532 Blue Peter and unreliable for LBSC's Tich. Or maybe Fusion or another CAD modeller is smarter than I think!

More power to Bob's elbow though - this is unexplored territory.

Dave

Edited By SillyOldDuffer on 22/01/2021 11:24:25

Sorry, but that T Deg to the fourth power is THE most important change that takes place inside the locomotive firebox to increase steam raising when it is needed most..

Yes of course, but perhaps this extract explains better what I was trying to say. First the 4th power rule, but at the end 'This assumes that the heat produced by the burning coal in the firebox is radiated under ideal conditions.'

I'm only suggesting the firebox of a scale locomotive isn't 'ideal conditions' and no-one understands the differences sufficient to model them mathematically. Could be wrong!

Dave

Having strongly disagreed with Bob in the past on various posts related to boilers I think there is small merit in his questions. I am sure with small changes such as flue diameter or even spacing there could be improvements to be made but those improvements unlikely to be in the measure of 10's of percent and probably limited to decimals of percent. Gaining better understanding of exactly what happens at our small scales can only be beneficial and others have trod the road before and not concluded there are significant gains to be made. Maybe that is because efforts have been concentrated on improving the standard fire tube, locomotive style boiler on which there is much full size information to draw from over many years rather than moving to a completely different style of boiler - which is difficult if you want to build a model of something that looks like it's full size counterpart! Maybe he should add brick arches to his list.

i agree that the calculations from first principles for staying of flat surfaces is indeed complicated. There are empirical rules of thumb and simplified formulae available and indeed tabulated 'standards'. The figures for fos quoted In the linked spreadsheet on a similar recent thread for the stayed surfaces of up to 70 indicate (if the underlying calculations are correct) just how conservative the solutions adopted in miniatures for stayed surfaces are when compared to quoted fos of around 3 on the circular barrel. The key for stay pitch is the resistance to deformation of what is effectively a constrained plate where due to the pressure one side is in tension and the other in compression and where the stress in the plate is sufficient to generate plastic deformation. SOD's reference to FEA in CAD would certainly save a lot of manual number crunching!

Paul.

Bob seems determined to misunderstand what others are saying, so I'll keep my dismal thoughts to myself until he produces experimental evidence or properly reasoned calculations

I'm not sure that there is all that much available to be gained, and there are other factors to be taken into consideration. A major one being that usually people want their model traction engines and locos to look something like the full size ones. This limits both the total size, and the outline of both types of boiler. Now, we know that for heat exchangers generally, the bigger the better, and ideally they should work on the counterflow principle. I suspect this is not really possible for a scale looking boiler.

Another thing to bear in mind is that when people talk about boiler efficiency, they are talking in terms of heat out/heat in, in BTUs or Joules. But even if you get all the heat out of the fuel into the steam, the overall thermal efficiency of the plant is going to be limited by the temperature at which the steam comes out of the boiler. With model boilers on saturated steam, this is usually going to be limited to 100 psi (see steam tables for the equivalent temperature.), and since the exhaust is not condensed into a vacuum, the effective exhaust temperature is not going to be much different. This is why superheating can make such a big difference, the higher temperature means the engine can be more efficient, so it requires less weight of steam, so the boiler has a better chance of keeping up. But then with reciprocating plant, the maximum temperature tends to be limited by what the lubrication can stand.

Quite a lot of full size boiler design has actually been aimed towards getting the best possible results out of the available cheap fuel. This can mean sacrificing a bit of efficiency to get better combustion, for instance allowing more space for complete combustion might be more important than optimising the tubes for best possible heat transfer.

John

Found something that might be of interest to a wider audience.

Spon's Dictionary of Engineering, mine is dated 1874.

There is a whole long section on boilers, but at page 441 it starts talking about heating ability of tubes and fireboxes. One test done was to divide a boiler, 5' long, into six sections. First was the firebox wall and 1" of tubes, the second 11" of tubes, the remaining four each 12" of tubes. After three hours the evaporated amount for the sections were 2ib 12oz, 2ib 13oz, 1ib 14oz, 1ib 6oz, 1ib 2oz and 1ib 1oz.

Many other thoughtful comments and tests.

Also a diagram showing heat distribution across a flue when divided into a core and two circular rings. Temperature in 700deg, centre out 700deg, middle ring 700deg, outer ring 200deg. Linear flow is very poor at passing heat to the tube, as you would expect. But why not put some of that stainless steel shavings sold as pan scourers in the tube? Even increase the diameter of the tube?

Tube size affects the ability of the fire to draw correctly and cool gas travels more slowly than hot gas.

The whole system needs to considered 'as one' from the damper through to the chimney bore and height.

Tubes need to be swept due to a build up of ash and soot. How would coils of scourer pad help transfer heat into the tube walls?

Possibly to cause turbulent flow ?

Turbulence is one reason that big fire tubes behave differently to small ones. Most of the flow in a 3" diameter tube is laminar (ie smooth), and predictable. The flow in a model sized fire tube is turbulent because the surface is big compared to volume. Although the difference could be modelled mathematically, I don't think anyone knows what the rules are. Big tubes are understood, small one's aren't.

In practice, I doubt there's any advantage in deliberately making a small fire tube more turbulent because it already is. The advantage might well lie in reducing turbulence to improve flow because it be good for the fire and keeping tubes clear.

Dave Halford makes an excellent point about the need to consider the system as a whole. It's because finding the sweet spot - if it exists at all in a small boiler - involves balancing several factors; increasing turbulence delivers one set of benefits at the cost of others. Not clear to me how the optimum heat transfer can be found, particularly as it depends on the fuel too.

Dave

That's marked as an F.

Read up on Reynolds number. For flow in tubes Reynolds number does depend on internal tube diameter, but also on flow rate. Ideally the tubes would be sized to ensure turbulent flow as this gives better heat transfer and also a lower pressure drop for a given flow rate. An ideal range would be 100000<Re<1000000.

There is confusion regarding the Stefan-Boltzmann law. It describes the total power radiated from a black body at a given thermodynamic temperature. It does not involve the temperature of the surrounding environment.

Andrew

What, another F, AGAIN? It's just like being back at school...