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Coal fired Cornish Boiler

How a Stuart S50 build led to an appropriate boiler

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Driver 9221218/11/2014 19:56:46
7 forum posts
7 photos

Having returned to model engineering after a 33 year break I decided the best way to ease myself back in was to use my newly acquired lathe and mill to build an S50.

The build had its issues, but thanks to Tubal Cain's "Sally" serial in ME we got there.

A boiler was required, but most (all) horizontal boilers off the shelf seemed to be gas or spirit fired and designed for marine applications. I could not find a coal fired model horizontal boiler - either design or finished article - for love nor money.

So having read K.N. Harris's excellent book, I took inspiration from one of his G.A. drawings of a Cornishesque boiler design.

My boiler is now built and tested (by a professional inspector, who approved my design). I would like to share the journey.

It's 6" dia and 13 inches long. I will expand on the detail in my next post...with images.

Bazyle19/11/2014 22:15:14
5210 forum posts
201 photos

Looking forward to it. Inspiring first post.

Driver 9221220/11/2014 20:02:11
7 forum posts
7 photos


Please forgive the quality of the images, but hopefully will give you an idea.

Making a working scale replica of a Cornish boiler would, given the original design of a centre flue containing the furnace and galloway cross tubes with two exterior side brick return flues and a single exterior flue taking gases under the base of the boiler to a tall chimney, result in a pretty big model - by my calculations it would have been huge. I obviously decided this was completely impractical, so took inspiration from the 'Sweet Pea' style marine boiler and designed it so it could appear as a Cornish boiler and stay within model proportions, suitable for a 1' x 2' bore and stroke engine. My intention is to build the Stuart Victoria next.

The back head is pretty key to giving a sense of authenticity so I did try to model this on a real boiler. (See pictures) You will see that my solution was to put a 3.75 inch diameter firebox inside a 6 inch barrel. The firebox is around 5 inches long and is secured into the back head with silver solder and a riveted staying ring, which accommodates the back head and grate assembly. The back head was machined from a 4" diameter x 1" thick piece of cast iron solid.

There are 16 smoke tubes and the whole boiler is 15 inches long which allows for a 2.5 inch deep smoke box which accommodates a 'steam dryer'. There is a steam blower built into the chimney base, fed from the stem dome.

From the design MSWP is 80psi, so the initial hydraulic test was 160psi for 30 minutes. The boiler is silver soldered throughout - by me. All bushes and the domes are bronze. All materials were sourced from Blackgates, and the cheat was I used pre flanged 5" Springbok smokebox tube plates and the firebox tube plate was supplied flanged. My design and initial drawings were run through a computer model by my boiler inspector. I also own a 4" Scale TE.

The grate is Blackgates 5" gauge stainless section, which I formed to create a basket 3.5" x 4.75" x 1" deep. I have test steamed it on anthracite grains and it steams reasonably well. Well very well once I got the hang of it. If you look at the nearly finished image, you will see the ash from today's steaming.

I bought a 6" Southworth steam pump off a chap in Stamford and made the Stuart hand pump from castings to supply feed water.

Anyway, I can expand in more detail if anyone is interested.



Edited By Driver 92212 on 20/11/2014 20:08:50

Edited By Driver 92212 on 20/11/2014 20:11:47

JasonB20/11/2014 20:12:29
18094 forum posts
1992 photos
1 articles

I've added them for you. The way to do it is when typing your reply there is a small black camera icon along the top of the reply box, click that and then you will get a new window where you can select your album and the photos in it.


PS nice boiler


EDIT looks like you worked it out while I was doing them

Edited By JasonB on 20/11/2014 20:13:52

Driver 9221230/11/2014 17:13:51
7 forum posts
7 photos

Steamed the plant today...

Happy to answer any questions smiley

Edited By Driver 92212 on 30/11/2014 17:19:55

fizzy30/11/2014 21:24:51
1710 forum posts
115 photos

Very nice! Why don't you blow the used steam up the chimney? When I have produced similar items I have found that the incoming cold water cools too much and it wont continue to steam until hot again, adding blast exhaust soon fixed it.

Michael Gilligan01/12/2014 07:45:19
15688 forum posts
683 photos
Posted by Driver 92212 on 30/11/2014 17:13:51:

Steamed the plant today...


Lovely job.


Driver 9221201/12/2014 09:44:43
7 forum posts
7 photos
Posted by fizzy on 30/11/2014 21:24:51:

Very nice! Why don't you blow the used steam up the chimney? When I have produced similar items I have found that the incoming cold water cools too much and it wont continue to steam until hot again, adding blast exhaust soon fixed it.

Many thanks for everyone's compliments.

You are quite right in what you say about the steaming rate and feed water issue. I did consider exhausting the steam up the chimney - locomotive style - to create draft, but with a plant like this I saw several issues so my thinking was...

1. There are long steam supply and exhaust pipe runs involved. That equals a lot of condensate initially and if the plant has been shut down for a bit. I decided to follow the model boat route with the inclusion of a condenser on the exhaust from both engine and pump to trap condensate and used steam oil. Unfortunately this means the exhaust steam pressure from this is considerably reduced from boiler pressure.

2. The direct exhaust 'blast' from an S50 wouldn't actually do much on a boiler of this size. The feed pump's is stronger, but doesn't run all the time. It could be a lot of work for little gain.

3. A really strong steam blower would be the best option. Before turning on the pump (or closing the bypass) to add feed water a good firing and plenty of blower would keep things in balance.

4. While the boiler isn't 100% authentic, a chuf chuf at the chimney of this type of boiler isn't either.

So far I will admit I have struggled using the steam pump and it knocks the pressure back to about 40psi. But I think I need more time to practice firing technique and setting the pump rate and bypass opening. It's early days currently. Using the hand pump nice and steadily doesn't affect the boiler pressure unduly providing the fire is nice and bright.

I rule nothing out - after all this is a prototype so if you can expand on your set-up/experience I'd welcome it.

Driver 9221206/12/2014 10:31:37
7 forum posts
7 photos

Having thought about Fizzy's post about feed water I have just ordered materials and fittings from Blackgates to make a feed water heater. The plan is to use the exhaust steam from the duplex boiler feed pump to pre-heat its water output. This should raise the feed water temp to around 80 degrees C once it is up to temp.

It will be a coil indirect type which will double as an oil trap - I will incorporate a drain for the condensate. I might then look at routing the exhaust steam from this up the chimney. One step at a time.

Driver 9221205/01/2015 14:04:27
7 forum posts
7 photos

water heater.jpgBoiler feed water heater made - but not tested yet - over Christmas.

Can anyone advise from the images etc. whether routing the exhaust steam from it up the boiler chimney will draw the fire to any significant extent?

Max working pressure is 80psi. Average is around 60psi. The Duplex boiler feed pump steam cylinders are 5/8" bore x 1 1/4" stroke. They exhaust into the feed water heater drum which is 2" dia x 2.75". The steam heats 12" of coiled 3/16 20g copper feed water delivery pipe. The steam then exhausts through a 3/16 x 20g pipe.


Johan van der Werf07/04/2017 20:58:02
1 forum posts

Looks very good. I started my hobby again by building the Stuart H10 and I want to build a coal fired horizontal boiler too. Same as you, I want to build a bigger engine later. Do you share your drawinhs for this boiler including calculations?


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