Building a coal fired boiler advice?

[Simon Robinson 4Participant @simonrobinson4]

Hi

Im a beginner. I'd really like to build a coal fired steam locomotive. I'm presently at the design stage. I will be working with steel and will be stick welding. I have a few queries.

1) What is the smallest diameter of boiler that can be coal fired? Is 45mm dia too small?

2) With such a small boiler is it still crucial that the fire box is totally surrounded with water as in larger locomotives and that stays are used even if the steel is of substantial thickness?

3) Is 3mm sufficient thickness for the walls of such a small boiler?

My initial designs involve cutting a third of the way into the bottom rear of the steel boiler tube and welding in a square fire box which will extend at least 30mm up into the boiler tube and be 35mm wide and 70mm long. The rest of the box will extend below the boiler but at that point it won't be in contact with water. This bit will include the grate and ash pan.

[Simon Robinson 4Participant @simonrobinson4]

[JasonBModerator @jasonb]

Posted by Simon Robinson 4 on 27/10/2015 14:35:38:

Hi

 

1) What is the smallest diameter of boiler that can be coal fired? Is 45mm dia too small?

For practicallity and efficiency I would use copper for anything below 125mm

2) With such a small boiler is it still crucial that the fire box is totally surrounded with water as in larger locomotives and that stays are used even if the steel is of substantial thickness?

It would not be practical to have a water space around the firebox at those sizes

3) Is 3mm sufficient thickness for the walls of such a small boiler?

You need to do some calculations but I would say not as there would be no allowance for wasting (rust) metal will rust at the same rate be it 3mm thick or 10mm thick

My initial designs involve cutting a third of the way into the bottom rear of the steel boiler tube and welding in a square fire box which will extend at least 30mm up into the boiler tube and be 35mm wide and 70mm long. The rest of the box will extend below the boiler but at that point it won't be in contact with water. This bit will include the grate and ash pan.

You will have such a small volume of water and such a large proportion of steel that you will be unlikely to get any useful steam out of a coal fire, won't be a lot better with gas firing

Edited By JasonB on 27/10/2015 14:46:25

[Simon Robinson 4Participant @simonrobinson4]

Here is a rough sketch diagram of the fire box cross section.

[JasonBModerator @jasonb]

Simon, have a look at one of the two ways you can include photos in your posts here

[JasonBModerator @jasonb]

Just saw this book for sale, something like the boiler shown would be a far better bet, about the same size at around 50mm dia.

Edited By JasonB on 27/10/2015 15:48:51

[colin hawesParticipant @colinhawes85982]

Steel is a no-no for such a small boiler; it is likely to lead to problems in steaming and in testing and as already pointed out more thickness is required to allow for corrosion. Colin

[nigel jones 5Participant @nigeljones5]

thats one small boiler! I much prefer to make mine from steel but I have to agree with all of the above. 3mm will withstand any pressure you are likely to generate but unless you are a very good welder it will, and yes i am confident on this, leak.

[Neil WyattModerator @neilwyatt]

You may want to Google Briggs Boiler for ideas on making a 'dryback' coal-fired boiler.

The boiler of Tick is 63 mm diameter, I am sure smaller one have been made for 2 1/2" locos.

It certainly is a proven design that proves a ~45mm wide grate will work for a coal fired boiler.

 

Edited By Neil Wyatt on 27/10/2015 19:31:18

[JasonBModerator @jasonb]

Neil if that Tick were made from 3mm thick 50mm dia steel it would have a grate of 25x 50 and the amount of radiant heat from the fire if you could keep it going would be so small. The one I mentioned earlier is this.

[Neil WyattModerator @neilwyatt]

I wasn't suggesting scaling down Tich, just that its grate was similar in size to one for a 45mm diameter boiler.

The one in your drawing is effectively a Briggs.

N.

[Paul LousickParticipant @paullousick59116]

A typical Briggs boiler

[Neil WyattModerator @neilwyatt]

Worth noting two things: the Briggs can be improved with circulation tubes in the firebox and that the firebox needs to be insulated well (including the backhead), which adds significantly to its external dimensions.

Neil

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