Correct construction of firepit Scotchboiler

[Stoomsnor]

Hi you guys!

I have a question concerning the appropriate construction of a Scotchboiler firepit if that is the right word (Mind you I am Dutch ...)

The picture below is taken from the inside of the boiler, believe me you don't want to be there...
As you can see the frame of the steel bars at the end does not level with the bottompart of the firedoorframe. The entire frame can be removed and hinges on the back of the firedoorframe. In my opinion the frame should level because only in this way it is possible to draw the coal out in a proper way in case of overheating. At the steel plate in the leftcorner you can see a weld. It seems that the frame used to be wider but was narrowed down.



The next picture is taken from the outside front of the boiler.
As you can see the framepart where the bars are hinged in is positioned quite low in the opening designated for primary airfeed. It blocks part of the passage for the incoming air and leaves only very few room for an ashscraper / rake. As the ashes build up it will probably clough up.



The next picture shows us more detail of the primary airfeeddoor.
As you can see there is a weld on the right of the hingeplate for the bars, which proves that the frame indeed was wider before, cut through and welded together again. One can see even more clearly now that the part is blocking access. When I bought the boiler a "spare" bar came with it, now I am thinking it was a part of the original setup! In my opinion levelling the frame at this point would improve the airflow as wel as the accesability for an ashscraper / rake. However by levelling it more space would occur at the sides causing the air leak through so that would have to be filled up with tapered pieces of steel.



The next picture shows us the other end of the frame holding the bars. You can clearly see that the midsection has been cut out and welded together again.
It looks like the intention was to lower the intire frame for about 3cm (1,25"), but for what good reason?



The next picture shows us where the frame-end now nicely fits against a steel coverplate where it originally would be for about 3cm (1,25") higher.



The final picture shows us the space the coverplate covers between the two headers of the waterwall. The boilerinspector adviced me to fill that space with the stone-like isolationstuff.



The only reasons I can think of why such a modification was made is to be able to put larger blocks of solid fuel on the bars or to heat a larger part of the surrounding steel cilinder. Another reason as far as the rear end of the frame concerns could be that the top of the little wall would be to high up. I red there should be a minimum distance of 21cm (8,25") between the top of it and the topside of the steel cilinder.
I studied a lot of books and drawings and noticed that most of the bars are not horizontal, they tend to drop a bit to the end. For my understanding that is done to prevent coal rolling out of the firedoor and for giving a better aerodynamics for the primary air.
All together I tend to lift the front-end of the frame to level with the doorframe, leave the back-end as it is and fill up the gaps occuring on the left and right then.

Then there is also the absence of a little wall on the top of the rear steelplate. In my opinion there should be a steel construction to form such a wall or made in such a way that it could hold a fireproof stone to prevent coal from falling in the section, act like a accumulator and cause the air to rise and mix with the gasses emerging from the coal. What size should this wall be? I can't find anything about sizes. On scaled drawing it appears that the top of the littlewall does not exceed half the diameter of the cilinder.
All together I tend to weld two thick 2" high steel bars on top of the back steelplate with enough room to fit a fireproof brickwall not exceeding in height compared to the center of the cilinder.

Then there is my concern for the weldings. How high will the maximumtemperature get only using manualfed coal of 7250Kcal and ventilated with air in a natural way?

Well I suppose that was more then one question in the end...
I am sorry for not being able to use the right words for all the stuff
On the other hand you guys try and put together one line in Dutch, I am looking forward to it !!!

[Mike Rometer]

Couple of things occur to me after reading that (it wasn't difficult the English is way better than my Dutch).

First is, if it has been so modified, was it because there has been a problem with this (or another identical) boiler and these are the perceived cures, or had it been wrongly built. The sensible way to check that is by recourse to the original drawings, if available.

The quickest way to dump any fire is to pull up the fire-bars with the rake so it all drops through and can be raked out of the ash-pan.

Marine or Scotch type boilers nearly all suffer from a lack of height in the ash-pan requiring frequent clearing. Problems will occur when the ash builds up and stops the air flow, causing the fire-bars to overheat and in severe cases fail.

Large lumps of fuel (coal) are not normally advised as they take too long to burn through and can leave holes in the fire. For full sized locomotives the pieces, according to one sage at least, should be no bigger than a mans fist.

Edit: - Another thought occurs to me has there been a change of mind over the fuel to be used? Changed either from coal to wood, or vice versa.

[Stoomsnor]

Hi Mike! Thanks for the fast reply!

In the picture below the boilersdrawings are visible. Every piece of the boiler has been built exactly according these drawings. The drawings have been officially approved, so I think technicaly it will be OK! Further on the boiler carries an official certificationplate, so that's not an issue too.
The downside of the story is that the frame and its bars are not on the drawings...



I am familiar with the concept you described with the bars being able to fall down. I think it's a great way to get a quick heatloss, sadly my boiler can not be equiped with a contraption assuch. The positive side in my view however is that more heat will be contained in the firepit.

Due to the 10mm (0,4") space between the bars I guess the right coalsize would be 30 (1,2") to 60 (3.4") mm Antraciet.
It just ocurred to me that a thick steelplate came with the boiler that could be used to cover half the space of the bars. It had been put there to make the boiler work only hard enough to drive the steamengine and in doing so being able to make adjustments to its valves.
From what I heard from the son of the previous owner the boiler had always been planned to burn coal. Now he was not into his dads hobby sadly enough, so I'm not to sure about that.
It really does make a lot of sense an other type of fuel was going to be used . That would explain the low level because oil would burn with less air needed and the little wall was not needed anymore. Perhaps an other type of burner will take a certain amount of space .
Also it might have been that the really big 25HP-engine could not be sufficiently fed so he had no choice but to switch to an other type of burner.
Mmmm, I am not into gas or oilburners yet, but I will be!

Thanks a lot Mike for pointing me in this possible direction! I am going to figure this out

[gondolier88]

Frank,

The grate should never be level with the firedoor- for the same reason it makes it easy to rake out, it also makes it easy for the fire to fall out should you be in a heavy swell or ground in shallow water- the last thing you want is your fire bed on the engine room deck! Looking at the grate in its current form I would say it looks correct, but you are correct in saying that you'll have to keep an eye on ash levels- not a problem when the ash is easily raked out on a boiler like yours.

I would not recommend anthracite for a Scotch boiler; it burns very hot and in a boiler with a dry ash space such as yours you will soon burn through the grate. You could get away with burning a mix of steam coal and house coal- bunker of each either side of the boiler would be easiest way. As I mentioned earlier, there are tricks of the trade to getting the best from a long thin grate- it's easy to make steam and have a fire, but also easy to have clouds of smoke billowing from the funnel and not make much steam. As for size, what's called 'doubles' here in the UK would suit best- anything around tennis ball size to grapefruit size. Anything smaller with grate you have will either cause clinker all the time as it fuses between the frame or you'll go through far too much coal as it falls through the grate half-burnt.

Greg

[S. Weaver]

I'm with Greg on a little lip at the door.

Your inspector is correct about a small, firebrick wall at the combustion chamber end of the grates. This little deflection wall can be part of your brick/insulation package in the bottom of the chamber. It seems to me that a wall built from standard (North American) bricks would suffice: 9" x 4" x 2", with the bricks laid lengthwise on edge for a 4" height. Use good mortar and do not give the brick any reason to miss-adjust itself. It will do so at the most inopportune time.

As for fuel, as Greg said, you will have to experiment. There is plenty of room for slicing out ash, and it seems you have ample room to wrangle with a clinker should one develop. For the ash, I would make a small thin hoe, with the bottom curved to fit the ash pan radius, and keep the pan free and clear as much as possible. These grates could also fire seasoned hardwood.

Greg's caution about anthracite is also good. I fire the home with anthracite seven months out of the year with an Italian stove (Russo), but our Pennsylvania anthracite is a different animal than what I understand to be available there.

As our auto salesmen say back here, "Your mileage may vary ..."

Best of luck!

[steamboatjack]

Hello,
I am in agreement with all of the above, it looks OK as is but needs a fire brick wall at the back to deflect the hot gases and stop any fuel getting past the grate, This design of boiler is very similar to the type once built by Cochran's of Annan.
Regards
Jack

[Stoomsnor]

Hi you guys!

Thank you very much for reflecting your answers to my questions!
The picture-image is getting clearer and clearer already
However, many questions will follow, you have been warned...

Thanks again to all of you!!!
Frank

[DetroiTug]

Quote: "The grate should never be level with the firedoor-"

My boiler suffers from this and even worse, the bottom of the firedoor is below the grate, and yes hot coals falling out is a problem. So much so that the wood floorboard in front of it is pockmarked with burns. If it was a wood or fiberglass hull it would be a much larger problem. My plan is to put an aluminum diamond plate floor in eventually and maybe rework the door or build a steel bulkhead on the grate. On the other side of this, burning wood requires a large fire door for stoking and manipulating large and irregular pieces.

-Ron

[Stoomsnor]

Hallo Ron!

Thank you for your reply, it's a really good suggestion!
I think I'll go for a steel plate as well!
With regards
Frank