Firebox Arch?

[Soot n' Cinders]

I've been thinking about putting an arch in my shay's firebox to increase fuel efficiency, especially when burning wood. To me it seems like it eats way more wood than it should compared to coal, especially considering my wood is good cabinetry oak and maple scraps and my coal is some dirty alabama junk. And if the arch would help keep the flues from getting as sooted up from coal, that would be a bonus with my dirty coal.
Unfortunately I havent been able to find a whole lot of info on setting up an arch so I sketched up an idea on a screenshot of the CAD model of my boiler and would like some feedback, both on my sketch and whether or not I should even fool with an arch.

[K. Browers]

Hello Tristan,

Your shay will be really nice when you complete it considering the work you have already completed.

An arch should give you some gains in fuel economy and I expect it would reduce cinder and ash accumulation in your tubes. Installing an arch would be a valid experiment. Male it removable, possibly attaching it to your ash pan so it could be easily removable.

I think you might be overlooking the different calorie content of your fuels though. Dry hardwood has about 7000 BTU per pound and coal has about 12000 BTU per pound so you would need to burn 1,75 lb of wood to have the same heat output of one pound of coal. Coal is also more dense than wood so it could seem that you are burning a big pile of wood compared to the same heat value of coal.

Cheers
Karel

[Asteamhead]

Hello Tristan,
Good idea to add in a firebox arch to improve combustion!
Position looks right but please shorten the arch to about 5" (horizontal) versus 4" (vertical) distance from the firebox back- versus crown sheet.
That will improve access to the tubes and make cleaning somewhat easier!
To construct brackets on the ashtray may be fine in case you can't add supports to the side sheets. Please think about dividing the arch in two parts in favor getting these out through the fire-opening of your boiler.
Stainless steel 316 Ti (1.4571) about 1/4 " thick will do fine as material for both the arch and brackets
In case you TIG-weld two short 3/4 open pipes fitting (loose) into the lower tubes you don't need those brackets at all .
Please look to a similar solution for my A (photo).
By the way you clean your tubes by firing wood with a good draft!
Good luck!
Asteamhead

[pat1027]

I have an arch in a 10" boiler burning coal. It makes a noticeable difference in how the boiler steams and minimizing derbies that get pulled into the lower tubes. The arch is a simple stainless plate that fits into a slotted bracket mounted just above the grate on the rear tube sheet. Tubes are brushed from the smoke box end. The arch is removed once a year for the annual boiler inspection. I'd have to put a tape on it but the clearance around the end of the arch looks similar to what you have shown.

[Bill Shields]

Irrespective of the good suggestions regarding arch position, the cross section area for gas flow (top of arch to sides / top of the firebox which forms a rectangle), should be about the same as the total cross section area of the flues.

Keep in mind that you need to get wood into the firebox (looks like you have), but generally speaking, having the end of the arch closer to the firebox door will help make the arch work better (keeping the area guide described above in mind).

[Marty_Knox]

Arch.pdf

(18.61 KiB) Downloaded 257 times

The Master Mechanic's Locomotive Front End Arrangement, 1936 revision, calls for the area over the arch to be 110% to 120% of the minimum net tube and flue area.

[Soot n' Cinders]

Ok, lots of things to take in this morning! So a few responses... I dont need to worry about getting the arch in and out through the door nor do I need worry about punching the tubes from the rear. Being a Shay, the bottom of the firebox has no obstructions so things can easily go through the bottom. And with the water tank attached to the frame, I have to punch through the smokebox anyway.
Now onto the adjustments to the design. So it sounds like the arch should only be about half as long as the firebox, with the front against the tube sheet and the rear ending about at the middle column of stays. The other thing, with 16 3/4 flues and my firebox about 5-7/8" wide, I need to raise the the rear of the arch to about 1.375" from the crown sheet.

Looking, Im questioning how much effect the arch is going to have. But I can easily make a test arch out of some scrap sheet metal I have laying around. Wont hold up very long, but should be long enough to get a feel for it.

[Fred_V]

looking at your drawing i wonder if your flue brush will come all the way clear of the lower flues before hitting the arch. if not you will get a stuck brush; they don't like to reverse within the tube.

[Short Line Tom]

Some more good informations on the topic were published in:
LIVE STEAM Mag (September / October 2011)
"Install a Firebox Arch" by Jeff Frost

Cheers!
Tom

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