(model) Yarrow boiler info

[johan]

The Yarrow boiler doesn't seem to be popular. Can't see why as it is largely made of tube which is easy to get and apart from the main body you don't need stays. So I'm going to try and use one in my locomotive. Largely because the original experimental boiler is not a viable model construction as it contained a lot of screwed joints, square seamless tube and so. I have read Harris and seen his design n°11 but there is nothing on the hearth.

But now that the boiler itself is more or less drawn I find that I have no info on how the fire is build in such boilers? I would like to know the following details:
- does the grate sits below the lower drums?
- do you let the (burning) coal come in contact with the lower drums?
- do you let the coal come into contact with the bottom of the water pipes?
- how thick is a typical layer of coal in a model 5" loc? The grate would be around 60mm x 130 mm.

Thanks in advance for any input!

[uuu]

In a locomotive boiler, the coal will touch the firebox all round. So it should not matter. But you might be concerned about the robustness of the tubes as you rake the fire, and from the shovel as you throw coal down a quite deep space.

Wilf

[johan]

May 5, 2024 15:46:28 GMT uuu said:

In a locomotive boiler, the coal will touch the firebox all round. So it should not matter. But you might be concerned about the robustness of the tubes as you rake the fire, and from the shovel as you throw coal down a quite deep space.

Wilf

Yes, but the issue is that in a locomotive boiler, the only way out for the heat is through the tubes. In a Yarrow it is between the tubes. So if I block those passages with coal, what will happen? Maybe because of the hot coal touvhing the tubes it will be better, maybe because of the coal, the heating surface will be less because no hot gasses pass those parts of the tubes.

It is a bit of a gamble this boiler. But it is the only way I see to even have a boiler in this loc. There is place to put the fire below the tubes. But is this the right way to do it? If I could have a good indication of how deep a fire is in a normal locomotive boiler of about the same size then I'd have at least something to go on.

At the moment I'm guessing 2-3 cm. But it could be more. Honestly I do not even have an idea how large the coal size is! There has been written a lot about model boilers but I have so far never seen a single word about this part of the whole.

[uuu]

Aha - I see where you're at.

Every loco is different - I quite like those that need a deep fire, because I find it easier to avoid cold spots. I admire the people that can almost see the grate through the glow. But 2-3cm sounds like a good target. Coal comes in a variety of sizes - small is 0.5-1cm lumps, medium 1-2cm lumps.

In your Harris book you'll see some loco designs - and on some you couldn't have a 5cm fire, as it would come right up to the tubes.

Just musing on equivalence of your boiler to a standard one: The firebox in a loco generates a big proportion of the boiler heat, by absorption of radiant heat from the fire and the gases swirling above it. Hot gases radiate heat! A Simplex boiler has a grate area of 16.2 sq inches, I believe = 105 sq cm. Yours will have 78 sq cm. And the Simplex boiler has a square box above the fire with a lot of gas capacity and surface area. Your sloping sides may have less room. So I'm wondering whether your boiler would make less steam than a Simplex one.


As noted in the Harris book - this boiler is sensitive to water level. You'll need keep the tubes full of water, so the top joints don't overheat.

This could be a fun loco - what does it look like?

Wilf

[johan]

Ok, if I read that then 3 cm seems to be the better choice. I have been sketching a bit in the mean time and it looks like I could make it that way that I could change the level of the grate later without too much trouble.

I have no idea if this will be a fun loc when it is ever finished. But right now it is rather frustrating. I'm trying to make a model of the loc that is on the cover of the Kerr, Stuart Water-tube boiler geared drive steam locomotives" by Allen Baker rail-books.co.uk/products/kerr-stuart-water-tube-geared-drive-steam-locomotives-9781912995059 (no affiliation, the first link that turned up). I have a thing for the more experimental engines Problem (well one of the problems) is that a lot of information and plans are for the 4-wheel version that was build. While I would like to make the 6-wheel version that is on the cover but was never build. So the engine at the front, not at the back as with the 6-wheel ones that were build. Advantage of the 6-wheel version is that there is good info on the frame in the book on the diesel locs as those used the same frame and there is even one in restoration.

I really love those quirky industrial engines. Certainly experimental ones like this one that was never build (in that form). It means nobody can point out missing rivets or something being too large or too small. Choosing a narrow gauge version also makes for a rather chunky loc and the use of larger fasteners. Also keeping in mind that our track isn't very long, reasonably level and when we haul passengers you stand longer still then you run. So even a rather asthmatic loc isn't a big issue then.

But the boiler was an experimental a prismatic one consisting of square and round seamless steel pipe, screwed connection and a lot of welded parts. So not realistic to get that past the boiler inspector at the club. And also not a very succesfull one as the later prototypes had a different boiler. The prismatic form makes that getting a Yarrow boiler in that casing was possible. I do get a bit more steam reserve that way as well. And it is the closest to the original boiler in that it is also a water-tube boiler with a reservoir at the top end no tubes at the front or back. The lower drums of the Yarrow make that the grate is narrower however.

There is still a lot work to do. As the chassis/frame is a riveted construction throughout this needs to be drawn before I can even start to cut a single piece of metal. Likewise I want the boiler to be done and the plans for it checked and approved. I already have the confirmation that a Yarrow boiler of that size and general design is ok.

Once the boiler is off the list then I need to turn my attention to the engine. It is a complete separate engine and lucky for me sits in a closed box out of view. (one of the reasons I choose this loc) After that there is the detail of the leaf springs... absolutely no idea how to make those. But before I can even think about them the design needs to be far enough to have a good idea about the weight. So I'll be drawing for at least the rest of the year.

[uuu]

I was at the club today, and was observing a Sweet Pea loco in steam. At the end of the run I measured the grate. There is 3cm available between the grate and the firehole door, so you can't go deeper than this at the back of the grate.

Wilf

[johan]

Thanks for the measurement! I mailed another club member with a 5" loc and he will get back to me at the weekend. So 3-4cm seems to be the dimension to aim at. I also found that I have a book (1915 vintage) about ships boilers that states that the hearth should not be in contact with the lower drums of the Yarrow boiler and they should be shielded from it. Now I won't put up a firebrick wall but a stainless steel plate (and some insulation) might work.

But there is time... Going from some A5 sized copies of copies of copies of plans of locs that are only half related to the original to a (maybe) working model is proving harder then I thought. Yesterday I changed most of the boiler support frame because I came to the conclusion that while it looked good and might have worked it would be almost impossible to put together around the boiler. Everything is possible in 3D on a pc. But reality will always find you out.