Wood cladding of boilers.

[Gas_mantle]

Hi

I'm looking for a bit of advice on how to go about wood cladding a small boiler, I recently bought a secondhand vertical copper boiler 3" in dia that I'm happy to spend a few quid improving.

I've bought some brass banding and some insulating lagging and intending to buy some mahogany planks to finish it all off but now that I'm commited to the idea I'm a bit unsure how to go about it and hope others may be able to help.

Are the planks and insulation usually glued in place before fitting the banding?  I'd have thought that might help in getting a neat alignment and generally make things easier to work with, but am I asking for trouble gluing something exposed to heat ?

As for the banding, how are the ends usually joined ? As it's a vertical boiler with 360^o cladding I'd like a neat join but with thin planking I think screwing into the wood isn't really an option.

Whilst I'm doing this largely for cosmetic reasons am I likely to see any significant improvement in efficiency ?

Thanks
Peter

[steam guy willy]

i always use elastic bands to hold them in place when I'm fitting them before attaching the brass bands, and i get the mahogany from the local toy shop that stocks lots of planks in different sizes....the ends of the brass bands are copies of the engine i am making.

[sbwhart]

Hi Peter

For the cylinder cladding I glue the wood to  piece of cotton material first with copydex then cut the cladding down to size before fixing them to the cylinder with bands, sticking them to the material first makes it easy to hold them in place whilst you fix the bands, can't see why this method wouldn't work with a boiler, even though the boiler will be hotter.

Hopes this helps

Stew

[Jasonb]

If you are going to have lagging below the wood then you will need something firm under the area where the straps go. I have done it by glueing up 3 layers of thin timber strips around a former and when set they become a rigid "plywood" ring.

Put a ring where ever you want a strap and fill between with the insulation, you can tack the rings in place with a drop or two of superglue as it only needs to hold during fitting.

I also use Stew's method of glueing to cotton (old hanky) but use PVA wood glue, make sure you don't get any on the sides of the boards. Cut a card template to get things right then transfer that to your sheet of planks. Apply the finish to the planks before sticking and finish both sides.

Banding I usually solder a small block of brass to each end, one tapped say 10BA (M1.6) and teh other a loose clearance fit, or you can do both clearance and use a nut.

[MJM460]

I purchased a marine type boiler from Miniature steam in the hope that it will be more readily accepted for occasional club running than my own designs.  It was supplied the mahogany planks.  I held them in place with rubber bands until the brass bands were in place and while I trimmed them to fit around the various fittings.

It was supplied with thin brass strips for permanent fixing.  The ends were just bent at 90 deg and drilled for screws.  Ok when placed at the bottom of a marine boiler in a boat, but for a vertical I prefer the idea of silver soldered blocks which a can be square or round as suggested by steam guy willy and Jason.

Boiler efficiency was hugely improved.  My first steaming, before the cladding, the gas ran out with plenty of water still covering the fire tube.  Now with cladding, I have to turn off the burner before the water disappears out of the sight glass.  This indicates significantly reduced heat loss resulting in more steam generation from a fuel measure.  Similar but harder to measure improvement would occur with coal firing.

I now have to improvise a load for the engine to see if steam consumption is slowed (due to lower rpm) or increased due to higher pressure required, to learn if I will then have the safety of fuel running out before water.  Otherwise I will need an engine driven feed pump, but running out of fuel is my preferred solution.

I realised afterwards that I should have put a layer of fibre insulation or possibly cork or felt under the timber for even better results, but of course that would mean sourcing replacements for the strips with cut outs for fittings as these will now be in the wrong place, and bands will be too short.  I am not sure if cork would smoke at the shell temperature, has anyone tried it?  No worse than cotton?  Fibre will of course need packers under the bands as Jason described.  Interesting to see that some glues are useable.

My own design boilers have metal casing so heat transfer is to the outside of the shell.  I am still deciding how to add insulation and a cover cladding to these, but I have enough measurements to put numbers on the difference when I get it done.

MJM460

[Gas_mantle]

Thanks guys, I feel a it more confident about tackling the job now and have ordered some small mahogany planks.

The intention is to try and use it to power my nearly completed Grasshopper engine, I've tried it turning the engine over moving the valve by hand (I haven't made the eccentric yet) and it seems to have a reasonable amount of torque at the flywheel but whether it will generate enough steam for continuous running remains to be seen.

It would be great to get better efficiency after lagging but I'm in virgin territory here so it's a lot of guess work.

When I've finished the engine I'll update my build thread and (hopefully) make a video of it running on steam 

Peter

[kvom]

Any insulation/lagging on such small boilers is likely to be insignificant.  It will be for looks.

Does your engine have cylinder cocks? Likely needed for operation on steam, as well as a lubricator.

[Gas_mantle]

I'm surprised that you think lagging on a small boiler is likely to be insignificant, I'd have thought there is more to be gained from lagging on a small diameter than there is on a larger one. Surely a small boiler has proportionally more surface area exposed in relation to its volume than a larger one has.

As a matter of interest to satisfy my own curiosity I'll do a few simple tests before and after lagging and see what difference there is. I don't have the means to carry out any truly accurate tests but ought to be able to do something rudimentary.

As for cylinder cocks my engine is a fairly basic model so I haven't added any, it's really intended to be an ornament and to run on steam very occasionally for short periods.

Peter.

[kvom]

I was going by what I've been told in the model steam locomotive hobby, where insulating these boilers doesn't do much good.  Air conducts heat less well than wood, but the lagging will make the boiler safer to touch.

The issue with cylinder cocks is that when the cylinder is cold, the initial steam introduced can condense into water, and the cylinder can potentially push it where it does damage.  On a piston valve engine it's more serious, but on a slide valve engine  the water can more easily be pushed back into the steam chest.

[MJM460]

Hi gas mantle,

The boiler performance is easily measured.  Replace the plug with a thermowell drilled to accept a thermocouple as is supplied with almost any digital voltmeter these days.  They seem quite accurate but you can check them with ice water and boiling water.  Especially useful to have one in the boiler space and one after a superheater if you have one. The result will surprise you.  I will post a sketch of a thermowell of you need it.

I weigh the water I put in the boiler, and weigh again the amount I remove (with a tube on a syringe) after the boiler has cooled.  A good household scale will weigh to +\_ 1 gm.  Similarly weigh your fuel.  When you get really keen on accuracy, postal scales which will weigh to +\_ 0.1 gm. But there are inaccuracies which probably make this unnecessary apart from reducing one source of error.  I am assuming you are using gas or Meths, coal is a bit more problematic.  If you are keen and have the fuel heating value, you can calculate an efficiency, and you can calculate the heat used in heating the whole lot from room temp. Separately from the heat to evaporate the water to steam,   but a simple ratio of gm of water to gm of fuel will give an adequate performance parameter to compare before and after insulation.  Ideally do several runs at each condition to give an idea of the variation.  And start from cold each run.

Theory predicts that a very thin layer of insulation is worse than none and this always surprises those who first read about it.  However I definitely had a significant improvement with one layer of mahogany strips so the problem occurs at somewhat less thickness, say a single layer of paper.  I would expect an additional layer of insulation under the strips would be very worthwhile.

The problem for both my marine boiler and your vertical is that the heat transfer area to the atmosphere from outside the shell is quite large compared with the area of the fire tube heating the boiler, so even after allowing for the different temperatures involved, the heat loss from the boiler shell is quite significant on a small boiler.

For the locomotive example, it would not surprise me if a thin layer of air trapped by the normal cladding was as effective as a thin layer if timber strips.

MJM460

[Gas_mantle]

Hi MJM

Thanks for the info, I was only intending on doing very basic tests to compare the performance before and after cladding - something like seeing how long it takes to boil and what pressure it can maintain with the outlet valve open etc.

What thickness of planking and insulation did you use ? I've ordered 1.5mm thick mahogany and I also have 1mm thick boiler lagging material (I have so 3mm thick material also but that looks too thick).

If the wood arrives this week I hopefully get it done over the weekend 

[Robert Hornby]

Peter,
If you check out "COLUMBINE'S BOILER" in the Boiler section on page 7 you will see how I did it. it worked Very well for me.
Robert

[MJM460]

My strips appear to be about 1.88 mm. Not easy to measure in place and I can't put my hand on one of the off cuts at the moment.

Not enough difference to matter.   I did not put insulation under, unfortunately did not think of it until I had cut around all the fittings and cut the bands to length.  But another time I will give it a try. 

In principal, more is better, but diminishing returns.  And after all for a model, appearance is important.  Which ever you use will make a difference and I would stay with what looks right.  The money you save on fuel will not make up for ugly!

MJM460

[Gas_mantle]

Thanks guys, interesting to read and see how other people tackled the job.

I have considered adding a purpose built gas tank like these :-

http://www.forest-classics.co.uk/ceramic_burners.htm

Can anyone shed any light on how much these kind of things hold ? I'm using a 2 ¹/₈^" ceramic burner and have no idea what sort of size I'd need to avoid constant refilling.

Thanks
Peter.

[MJM460]

Hi Peter,

My burner is only 1 1/8" dia ceramic type so not really comparable with yours.  It burns approx 30 gm of fuel in 36 minutes which heats up and evaporates approximately 250 gm of water, a little of which is carried over with first steam.  I generally buy the 230 gm disposable container of butane-propane mix gas for hikers stoves, but there is a larger container available if I start running more frequently.

I have the manufacturers gas tank and valve which I fill from the disposable container using the manufacturers filling device.  My fuel tank holds about 34 Gm and is quite a good match for my 3" centre flue boiler.  The issue is not frequent filling, it is not hard when using the manufacturers adaptor, but rather the desirable safety factor of running out of fuel before running out of water.

If you have a feed pump or injector you can of course keep topping up the water.  For a stationary boiler or a ride on loco or traction engine, even a hand pump will do, but on a model boat, once it is out of reach, fouling of the propellor with weed and other hazards mean it is harder to ensure the water is kept up, and the safer course is to use the smaller tank.

I would suggest you first try the adaptor which connects the burner to the disposable container, and keep careful note of your bench runs where you can keep a good eye on the level glass until you are confident of how much you burn during each run, then buy the little fuel tank in the size which suits you.  I am sure that your supplier will be able to tell you the effective capacity of each tank in his range.

MJM460