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Building A Model Boiler

A Layman’s Guide To Construction Materials

 Model steam boilers tend to fall into two main conventional categories (with a further one added at the bottom) despite there being many variations of these. They are basically constructed as either vertical or horizontal boilers.

From the point of view of efficiency and economy, the horizontal is better in that the heat is retained longer around the boiler, that way passing more heat to the water inside the boiler barrel.

Horizontal boilers are generally used in small model steam production plants requiring minimal height, much as are used in model boats or loco's, using either spirit or gas firing, whereas with the upright boiler there tends to be more draft created because of the design and this type is more suited to coal fired boilers.

As an alternative, when a large chimney is incorporated in the horizontal design coal can be used as a fuel as the chimney produces the necessary draft to aid the fire’s combustion.

In both types, generally speaking, the water is heated in a sealed container so that as the water heats up, it tries to expand – and this is before it becomes steam, causing the pressure to build.

With this extra pressure, the water needs more heat to turn it into steam, and as more heat is applied, the pressure becomes greater.

What happens with the greater pressure is that instead of the water boiling at 100C, the water temperature needs to be higher to actually boil the water and achieve steam production and a temperature of 120/130C is not uncommon in model steam boilers.

From this it can be seen that the container, the pressure vessel, needs to be sturdy to withhold that pressure as the temperature goes higher.

What is happening is that the heat input (work) is being stored within the steam and water so that it can be released and put to some practical use.

If left unattended, the heat raises the water/steam pressure and temperature so high that the pressure vessel ruptures, and as the water/steam exits the broken vessel, it reaches a much reduced pressure (the atmosphere) and flashes into scalding hot steam with such force it can be lethal to anyone in close proximity.

It has been likened to a bomb exploding with iron boilers in days of yore, and no doubt you have heard of this type of happening before now.

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What happened was that in the early years of boiler building and usage, many casualties were reported from explosions caused through lack of control by the boiler-man, usually leaving his boiler unattended to get a brew or maybe even to dally with any other distraction that walked his way, along with poor construction and maintenance methods.

And this is where the present day construction materials play a big part and we shall be considering steel, copper and brass for this purpose.

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In the present day the construction side of things is now much more controlled with large steel boilers where builders now need to use what is called ‘coded material’, being steel of a certain chemical standard, and welded to a similar standard, so that the vessel is able to withstand many times the proposed working pressure.

The main benefit of building a boiler out of steel for the amateur is the cost is less, but with the physical effort required for its production along with the need of various testing methods being used to check the wall thickness and joint structures, plus the fact that steel boilers rust on the inside means they can be far more costly in the long run.

Many steel models require the boiler to be re-fabricated after twenty-plus years of regular service because of this inherent rusting problem, along with the fact that steel corrodes inside the firebox, fire tubes and smokebox due to the sulphur content of the smoke and burnt coal itself.

From this it can be seen that the thickness of the metal is reduced over time and as this happens the metalwork is weakened. If this weakening continues the metal will reach a point of collapse owing to the pressure within. This situation develops from pin-hole leakages into a sudden rupture as it usually occurs under boiler pressure.

Another consideration is that when the boiler is at working temperature and is then allowed to cool, any pressure on the inside is turned into a vacuum or partial vacuum meaning that outside air pressure is trying to crush the boiler. This can be overcome by having a vacuum relief valve installed on your boiler, but this is not generally the case on model steam boilers.

It is to cover these possible catastrophes that regular tests need to be done on boilers, especially steel boilers.

Primarily this testing was aimed at early steam boilers but is now considered a test for all boilers working over a certain pressure, that way proving their worthiness to hold the necessary pressure.

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The best construction material for making your own model steam boiler is copper as boilers produced from copper are able to go on for decades as they tend not to have any adverse chemical reaction or thinning of the boiler material. Some does occur, although copper does tend to become work-hardened with use and boilers are constructed well over specification to combat this small problem.

However, they can be purchased ready made and able to be used with many live steam model engines in boats.

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Brass on the other hand, another modern choice used by many amateur, small-scale boiler builders, has problems that come to light after a few years use. It is by far the easiest and best looking metal to work with, but the problems become apparent once water has been used for some time.

This all stems down to brass containing zinc, which tends to seep into the water from the metal as an electrolyte. With this transfer and the heating and cooling, the zinc can become eroded into the water and deposited elsewhere, plus it becomes very brittle over a few years and can fracture. This is a reason to avoid old brass boilers like the plague, and especially so if they are coal fired.

Modern model brass boilers tend to be made of a brass that contains less zinc, and are therefore more durable, but even so, the manufacturers of these small boilers keep the pressure down below two atmospheres (less than 30lbs/sq. in.) so that should there be any failure of the brass or the fittings, the escaping steam will be minimal owing to the lack of pressure and the miniscule size of the boiler.

On top of this they use spirit, tablets or gas firing to reduce the risk of chemical reactions with the zinc in use.

So, all-in-all, the best option for the long run, although the most expensive initially, is to use copper as your main building material. Obviously this will be governed by the scale of the boiler you wish to build when cost or strength may be a consideration, but on the smaller scale, say up to 4 pints/2 litres total capacity, then copper is the one.

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A further alternative to having a boiler barrel (a steam and water container) for heating purposes, is to have what is called a mono-tube boiler or a flash steam boiler.

This essentially is a single tube containing a small amount of water which is heated to the extreme, that way expanding the steam into dry steam where the pressure can be up to 2,000 lbs/sq.in or more owing to the heat input. It is virtually like running an engine on compressed air.

Obviously a pressure gauge is of little use here and the throttle control is mainly governed by the heat input, but also by the addition of water droplets, so this boiler type is used where no consideration for the fuel efficiency of your boiler is a problem.

These boilers tend to be used with very high speed/short stroke or turbine engines where pressure is not the issue, but the volume of driving gas is paramount.

The main concern with this type of boiler is to have no soldered joints in the tube as the very high temperature of the tube is likely to be above the solder melting point (assuming silver solder (brazing) has been used).

Normally for this type of extreme heating a stainless steel tube is used, but by keeping the heat input down a little, copper tube can serve the same purpose.

George



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