Grate area for a small traction engine

[murray]

There is no such thing as "two many" projects!   I'm building a 2" traction engine.
Its a quicky, more toy than model, chain and sprocket drive, based very loosely on Savage's Little Samson because the 9" wheels will fit my lathe and 4" copper tube for the boiler is readily available.
There - justified.

That 4" boiler tube indicates a grate area around 10-15 sq. inches (from the ~4" width of the firebox). That would appear to be a bit of a problem for a single 1" x 1.25" cylinder. There won't be enough draft for a coal fire. And in keeping with the 'quicky' and 'toy' aspect I want to avoid a conventional wet firebox.

I've found a few creditable sources of boiler design information. Jim Ewans and his Ee factor, Prof. Bill Hall's boiler simulator, John Baguley's notes on same.
Jim Ewan's Ee factor ties the grate area to driving wheel size (therefore engine rpm) and cylinder swept volume. By removing the wheel from his equation I get a suggested grate area of around 2.2 sq inches for my single cylinder. Lets assume a small traction engine cylinder will go twice as fast as a locomotive's the grate area is still only around 4-5 sq inches. That seems very small to me, especially hung on the back of a 4" dia boiler.

By manipulating numbers in Prof. Hall's simulator it would seem the heat transferred into the water is very roughly 50-50 between firebox and tubes. However John Baguley notes that Jim Ewan's experiments show that only the first 1/3 of the tube does any work and that the majority of the steam production came from the firebox.

The current plan is for a Briggs style boiler in copper. The rear tube plate will be at the firebox front but the copper 4" tube shell will continue to the back, split at the bottom and opened to provide firebox sides. So all the sides and top of the firebox will be dry in 1.6mm or 2mm copper directly connected to the wet part of the boiler. I'm hoping copper's high heat conductivity will carry much (some?) of the radiant heat to the water. It won't be efficient but thats OK (its a toy!).  It will be easy to construct and with relatively little expensive copper.

I have a loco also with a 4" boiler but cylinders that sweep nearly 6 times the volume. Its 12.25 sq inch grate easily keeps up. So providing I can get the draft strong enough to keep the fire bright I'm hoping I can get 1/6 of the radiant heat into the water. And the conventional (albeit short) tubes can do the rest.

Very happy to hear any thoughts on this.
Thanks
Murray

[builder01]

If you need more draft, could you just add some sort of blower in the smoke box? Sounds like you will have plenty of steam to do the job.

David

[John Baguley]

Hi Murray,

I've just had a look at my Allchin drawings. The grate area for that is only about 10sq. ins and that feeds a 1.5" stroke by 1.25" bore cylinder so your boiler should have ample capacity for your smaller cylinder.

Don't forget that the width of the inner firebox will be a lot narrower than the diameter of the boiler - probably around 3.125 inches. The Allchin firebox is about 2.875" wide with a 3.75" diameter barrel.

Should have read your post more carefully! I realise now that you are thinking of a Briggs type boiler.

John

[durhambuilder]

I would have thought the firebox sides would melt or at least distort without the water space to keep them cool? I can get the fire on my 2"TE glowing almost white when pulling hard.

[murray]

I helped a friend of mine silver solder a 3/4 water pipe that was already buried in the ground about 6" away. Even with my largest propane torch it wouldn't get hot enough until we dug it out for a couple of feet. I suspect (hope!) the firebox sides would struggle to get to red heat.
Of course they'll be insulated on the outside. The horn plates are 1/4 inch away at the sides which I'll fill with ceramic wool. I know that's not much, I could move them further out without too much trouble.

I've seen steel Briggs boilers working in low light and hadn't noticed the backheads glowing but they were larger scale so I presume had room for a lot of insulation. With one little cylinder I think my draft will be weak at best so I'm not expecting to see white fire. How big is your cylinder DurhamBuilder? And your grate?

Perhaps I should at least put bushes in to later fit whatever you call the water walls made of pipe in a Briggs boiler just in case?

Prior art! I found this small drawing about 2/3rds down the page: link

Its the boiler for 'Eric' a 16mm garden gauge loco.   I'm not even including the firebox crown making it even simpler.  Its almost a pot boiler just with a few tubes.

Thanks
Murray

[murray]

Further development:



I've added an arch to reduce the direct radiant heat to the top of the firebox.  The crankshaft is only a short distance above there and I'm worried about the heat level.  Not sure what to make that from, perhaps steel.  I suspect the 1/4 - 1/2" of ceramic wool will prevent the steel outer casing (not shown here) from glowing red but it'll still be hot.

I've also brought out the top row of tubes into the firebox to gather some radiant heat.  Not sure if thats a good idea or not yet!  The ring in the foreground is the firehole so I can see if I'm going to shovel coal into the fire or into the tubes.

The bush along side the top tubes is one of two to fit water walls should that be found necessary later.
The grate is 4 x 2.5 for 10 sq inches of fire.  The 13 1/2" tubes gives me 19% free gas flow.

Murray

[murray]

Even further further (virtual) progress!   This is sitting over the GA for Little Samson.


The grate looks very small when you see it like this. I'm kind of tempted to ignore all that credible expert advice and make the grate about the same size as my locomotive with 6 times the swept volume...   hmm, now that I type that...

I've got even keener on the idea of bringing the tubes out into the firebox, to the extent I have two rows.  The worse the can happen? The ends of the tube can burn/melt and I'll cut them back to conventional lengths again.  The backhead and rear firebox wall will be removable so I can do that.   Try that on your wet back boiler! 

The engine is sitting on a 3mm plate (the black bit just above the boiler) so I can build and test it as a separate part.  The main bearings are mounted to this base too, separate from the horn plates.   It removes the hassle of mounting and lining up the cylinder and other parts on the curved top surface of the boiler.   Kudos to George of the PYRTE  design for that idea.

Murray

[murray]

So I thought an experiment may be in order.

I took some scrap copper about 30 thou thick and made two strips about 1" wide and suspended them side by side, one in water, one in air.  The distance from the support to the water is about 1.5"
Heres the setup ( Warning! Rigorous scientific methodology will not be in evidence here ):



Then I applied a generous serving of heat as evenly as possible to both strips at the top. I tried to keep the lower edge of the visible flame above the lower edge of the ceramic rod (it was wobbling around a bit here as I took the photo):


Its not really clear from the photo, and broad daylight doesn't help, but the right strip is glowing red all the way down except for maybe the last 1/4".  Thats a rolled edge (it was an old copper light shade).  Although it looks like it is glowing too, the left strip is only showing it's copper colour and then only near the top.    The water is warm but isn't actually boiling - the surface is just being disturbed by the movement of the copper strip.

I kept up the heat until the right strip showed signs of melting.   Here it is after I removed the heat:

Its pretty obvious the difference in temperature between the two strips.  The right dry strip was bright red all over at the time it started to melt.  It melted where the ceramic rod was glowing behind it but the colour was reasonably consistent all over.
In stark contrast the left strip was barely glowing red if at all (I should try this in the evening) and most of it shows no sign of having been hot at all.  A good inch above the water its still at its normal copper colour and is only discoloured where it was in direct contact with the ceramic rod (which was glowing red).

So I think one can conclude that A, copper is a good conductor of heat and B, water makes a good heat sink.   I've heard people I respect note that  it isn't always the end of the world if the crown sheet becomes exposed in a small copper boiler - not that I would suggest doing that! - this experiment would support that, provided there was still water in the vicinity.

My proposed boiler will have copper twice as thick but also twice the distance as above.  But also the water contact area will be much greater, as will the heat source.  Not sure how to reconcile all that however I'm confident enough that I think its worth a try.  But I might abandon the idea of extending the tubes out into the firebox.

I'm also going smaller, 3" instead of 4" because it matches the small grate area better, it gives me 1/2" all around to insulate it well including the cleading all the way to the smokebox and I happen to have some (the *real* reason!)

Murray

[jma1009]

Hi Murray,

This is just a personal observation.

Miniature traction engines have relatively small boilers when compared to a miniature loco of similar length. They usually have only one cylinder geared down. They do not usually incorporate superheating. The valve gear is usually far from optimal.

I would suggest that to put a Briggs boiler on 2" scale traction engine is a retrograde step and only justified by expediency in manner of construction and cost. You are losing at a stroke the benefits of the firebox radiant heat transfering to water legs on the sides and back of the boiler.

Jim Ewins showed that in miniature this is where most if not virtually all the heat is absorbed/transfered to the water in a conventional boiler. Very little heat is transfered through the tubes to the water in the barrel of the boiler.

You might just as well fire it with meths and fit Smithies water tubes into the firebox, and go back to model engineering pre 1923 and 'The Battle Of The Boilers' LBSC -v- Bassett Lowke.

The 5"g Boxhill boiler adequately provides enough steam for 2 x 1" bore cylinders with direct drive to 4 1/4" wheels. My own version has what I consider to be an improved boiler plus has better smokebox draughting and vastly superior Don Ashton valvegear.

Boiler diameter is 3 3/4".

It never ceases to amaze me how rudimentary and poor some miniature traction engine designs are, and how their owners and builders are so easily pleased.

And I speak as someone who has helped restore a number of fullsize traction engines.

One of these days I might build a miniature traction engine incorporating all the very best miniature loco principles.

Sadly, I dont see a Briggs boiler as the way to go with your project, and a proper boiler will be a better steamer in every respect.

Cheers,
Julian

[Jim]

I would tend to agree with Julian on this Murray.
True I have a Briggs boiler in my 3" SCC Burrell 'Devonshire' engine but that is because the boiler is also the the chassis for the engine and I lacked the specified thickness of copper and heating equipment to construct a compliant boiler able to cope with the all the stresses a working road engine is subject to.
It would be no harder to build a proper boiler than the one you're proposing, in fact you are already 9/10ths of the way there as it is.

Jim

[mutley]

Build a proper boiler and be done with it. The saving in time is small in comparison to the overall project.

[murray]

Thanks for your comments. I obviously did convey my intent very clearly, sorry.
I want to see how much I can simplify a steam engine and still keep it 'fun'. Sure, it won't be efficient, won't look very prototypical but it may still work well enough. I get a great deal of satisfaction from a hard working small loco, on the edge of adhesion, trying to stop the fire ending up in the smokebox, forming clinker, blocking tubes. While I admire locomotives well matched to their loads, quietly and efficiently going about their work, thats not for me. Each to their own.
I seem to have digressed..

>...only justified by expediency in manner of construction and cost. You are losing at a stroke the benefits of the firebox radiant heat transfering to water legs on the sides and back of the boiler.

Yes, I'm looking for expediency in building and reducing cost. But I'm not convinced I'm loosing all my radiant heat, that was the point of my little experiment above. The wet copper strip received as much heat as the dry one and it went somewhere. I presume into the water.

> It would be no harder to build a proper boiler than the one you're proposing, in fact you are already 9/10ths of the way there as it is.

Oh I think a wet leg boiler is harder. Much. It needs staying, it needs a mud ring, it needs firebox plates flanged on formers. An inner wrapper. Throat plate. It requires more heat, copper and time. It has the potential for a lot more leaks (from personal experience!). My proposed boiler has two tube plates, round ones at that. Turn the single former needed on the lathe.

I'm quite prepared for this to to fail. I guess failure would be poor steaming or perhaps burning, melting of the firebox plates. In that case I can build a more conventional boiler to replace it. But I suspect it may work OK. Only one way to find out...

Murray

[mutley]

The boiler on a traction engine is also the chassis. I'm all for experimenting but in this case I can't help feel your living dangerously especially if you want to make it from copper.

[Jim]

I agree totally with Mutley's comments. I must say, given your determination to press ahead with your plans regardless I'm left wondering why you're asking for advice in the first place.

Jim

[murray]

The boiler 'cleading' is actually steel pipe that takes all the stress. The boiler slides in from the rear.
I'm 'pressing on ahead regardless' because I want to experiment, try new ideas, not just repeat what has gone before.
I'm asking for advice and sharing ideas because thats what I thought these forums were for.

Murray