Building a coal fired boiler

Hi

forgive me as I'm a beginner.

I want to build a coal fired steam engine boiler of less than 5" in diameter. I've calculated the thickness of the walls to hold in the pressure. I'd prefer to use steel as I'm good at welding, but steel rusts and if its too thick the heat won't get across.

A lot of people suggest copper, but copper is astronomically expensive for a tube of suitable diameter and thickness to make a coal fired boiler.

What other materials would be suitable, can brass be used? I guess that would be cheaper?

What is the cheapest way of building a workable coal fired loco boiler that's long lasting and capable of holding pressure? 

Also where is the best place for materials, I've tried Macc models but they don't give much of a description on the thickness of tubes etc.

thanks

Edited By Simon Robinson 4 on 19/11/2015 15:32:09

Brass is not safe for all but 'toy' boilers as it is vulnerable to corrosion and becoming brittle. There are good designs for steel boilers out there, but the thickness depends as much on allowances for corrosion as for sheer strength.

If you want to use your boiler other than at home, you will need it to be certified and insured. that means a recognised design or getting the design approved by a boiler inspector. In any case the inspector should want to inspect the built at two or there stages during the work. The cheapest way to do this (and access a lot of helpful expertise) is to join a model engineering club.

It certainly can be done, because Fizzy of this parish has done it: www.model-engineer.co.uk/forums/postings.asp?th=65907&p=1

You will probably find Australian discussion of Briggs Boilers useful as well, see here for an example of building a steel briggs boiler:

www.narrowgauge.iform.com.au/boiler.html

Neil

Brass is not suitable for the boiler nor any fittings which are in contact with it as brass degrades. You are unlikely to get a certificate for a boiler made of brass. The same applies to stainless steel. Although you may think a relatively thin steel sheet is strong enough a huge wasting allowance is required so you can't use it. Copper will of course give fewer problems with ageing. A 5" dia. steel boiler would probably be 1/4 " thick or more and must be made by a coded welder using certified boiler plate and inspected throughout construction including test pieces. Copper is best and probably easier to insure. Colin

Macc give the thickness of all their C106 copper tube here, this is the type to use for a boiler, don't know what your calculations have come up with but 10g (3.25) would be the usual thickness for a 5" dia barrel.

Whilst I wouldn't advocate steel for small boilers, the idea that it's thickness and low conductivilty means less heat transfer is a falacy. The thermal resistance is almost all at the gas/metal interface, the resitance of the metal is very small

Though its additional thickness can mean less waterspace around the firebox or if you keep that the same then the grate area has to be reduced mybe by 25% on a 5" dia boiler. Also you can't get as many tubes into the boiler barrel so again loose heating area, expanded copper tubes into a steel boiler is a compromise.

Edited By JasonB on 19/11/2015 18:25:37

Thanks

The 2" dia 10g (3.25) is kind of what I'm looking for I'd be looking at 8" in length for the boiler. Would that be a suitable boiler tube for coal firing?

thanks

Simon, you said you wanted 5" in your opening post now 2"

If you want a smaller 2" boiler then the thickness can be reduced, depending on your working pressure then 16g (1.6mm) thickness would be a likely candidate. But at those small sizes coal firing can be very difficult, nice for the smell and effect but for practical running gas firing will be easier to control and probably give more steam output. As a half way measure charcoal is an option.

Thanks

So now its a marine boiler not a loco style

Can you give us an idea of what you actualy want to use the boiler for, what pressure you want it to work at, how much steam you need to produce and post your calculations. Without this sort of info we will only be guessing at a suitable thickness (though an educate dguess) and can better say if what you propose will work.

The only certainty is that silver solder will be the best solder for the job.