From napkins to fabrication VFT boiler design

[Spanky]

Hi all,

Let me introduce myself. My name is Josh, I'm from St. Criox Falls, Wisconsin, USA, and steam engines have fascinated me ever since I got to ride on a Rumley Oil Pull tractor at River Falls Days. My dream and goal is to build a 14-16ft steam launch and power it using Ray HasBrouck's #10 engine (I might scale his drawings to give me a bigger engine) and a VFT boiler that my dad and I will hopefully be able to design and fabricate ourselves.

Over the past few day's I've been putting all my boiler ideas onto paper (my dad says i have too much time on my hands, as the kitchen counter is starting to overflow.) I'd like to pick your brains and have you weigh in on my design as it progresses.

Design Goals- Design and build a durable and affordable coal fired water leg VFT to operate at 125 psi and have a service life of at least 10 years.

Design Overview- Dimensions 16" OD x 30" tall, 13.25" x 14" tall firebox, fire grate will be 4" above the boiler base plate, 61 3/4" tubes 14" long (I calculated about 16 square feet of heating area)

Just wondering what size piping should be used for the various parts, I'll be posting a few CAD models when I get them done either this weekend or Monday that I'm most certainly need refinement on.

Thank you all in advance for whatever help, tips, do's and don'ts, and time you provide to help me with this project.

Josh

[fredrosse]

From Another Topic:

Fred I'm wondering how are the fluetubes oriented? As your comment about switching to a 12in furnace shell has me stumped on how to fit 61 3/4" tubes to the top of it... right now I have them perfectly vertical for ease in construction and because it just makes sense to have them that way. Should I decrease the distance between the waterside walls of the tubes? In my drawings it's 1/2" tube wall to tube wall.

Tubes should rise vertically from the furnace tube sheet to the outlet tube sheet, as you propose in your design. Two options you might consider:

For a 12 inch pipe (12.75 Outside Diameter, OD) x .375 wall furnace, you could put 61 tubes, 0.75 OD, on a 1-1/4 inch equilateral pitch. The outside of the tube holes would be at least 1/2 inch away from the Inside Diameter (ID) of the furnace pipe. This allows enough room to weld the tubesheet to the furnace pipe. For 61 fire tubes 0.75 inch OD, 0.083 wall, 1 foot long, this gives 9.3 square feet Heat Transfer Surface Area (HTSA) I would not recommend this, as the tubes have not much flow area for hot gasses, 16.33 square inches.

For the same size furnace, you could put 43 tubes, one inch OD, on a 1-7/16 inch equilateral pitch. The outside of the tube holes would be at least 1/2 inch away from the Inside Diameter (ID) of the furnace pipe. This allows enough room to weld the tubesheet to the furnace pipe. For 43 fire tubes 1.00 inch OD, 0.095 wall, 1 foot long, this gives 9.1 square feet Heat Transfer Surface Area (HTSA). I would recommend this, as the tubes have better flow area for hot gasses, 22.2 square inches. The 0.095 wall tubes will last longer, and the open gap between tubes is reduced from 1/2 inch to 7/16, still acceptable.

For these small tubes, I would recommend about 12 - 15 inch total length for each tube. Longer tubes will not pick up much more heat. You might try to make an EXCEL spreadsheet to explore options like these.

[Spanky]

Thanks for the input I will switch to the 43 of the larger diameter, How tall should the furnace area be if i plan on firing coal/wood? Also a question about the picture what are the two "leg" looking pipes at the bottom front, one on either side? Mudholes?

[fredrosse]

If you make the firetubes 12 in long, then you could make the firebox about 12 inches minimum, 16-18 would be my recommendation, resulting in a boiler shell 24 to 30 inches high.

The boiler in the picture can be mounted horizontally (with the top row of tubes not used) or vertically. The large connections are for blowdown - cleanout access. Built in 1979, 12 inch pipe, schedule 20 (1/4 in wall) shell, but since it only has 19 tubes the surface area (HTSA) is small. Coal fired, used 24-7 for 4 years. Hydro test at 550 PSI a few years ago. I was thinking of using it in the horizontal position, with propane burners into each firetube, may still do that some day.

[Spanky]

Not a half bad idea. Would look pretty classy with some lagging and brass. In the horizontal wouldn't you need to build a smoke box? Speaking of smoke boxes, on a VFT is there a gap between the "smoke box" (not sure if that's the right term) and the boiler shell? I saw a picture of one where there was a hoop of metal, presumably to hold up the "smoke box," attached to the top tube plate by six welded on tabs spaced equally around the circumference. This made a gap of about 1/2" between the boiler shell and the hoop. Is this for draft purposes? I thought it was strange as I'd never seen it on any other VFT pictures I've looked at... I'll post the picture when I find it

Josh

Edit: found the picture it's in the "Boiler Modification" topic... *face desk*

[fredrosse]

"wouldn't you need to build a smoke box" Yes, somilar to the smokebox on a typical steam locomotive.

"Speaking of smoke boxes, on a VFT is there a gap between the "smoke box" (not sure if that's the right term) and the boiler shell? I saw a picture of one where there was a hoop of metal, presumably to hold up the "smoke box," attached to the top tube plate by six welded on tabs spaced equally around the circumference. This made a gap of about 1/2" between the boiler shell and the hoop. Is this for draft purposes? I thought it was strange as I'd never seen it on any other VFT pictures I've looked at.." I think that ring is a spacer for the insulation and lagging that would be applied to the boiler later. You generally do not want to let cold air in the smokebox, that will "kill" the natural draft produced by the hot chimney gasses.

For the "Margaret S." boiler, I use a stainless salad bowl (16 inch OD, 7 inch flat on bottom) as the smoke hood. Cut out the bottom to fit the chimney pipe. You can't find a cheaper smoke hood, use commercial kitchen equipment in stainless steel where you can. The stainless can take the heat, but is expensive unless you can adapt some piece of kitchen equipment.

[Spanky]

Ha! I love the mixing bowl smoke hood. Are there two, one inside the other, to insulate it and keep the exterior one from heat discoloration? And I got Pro-E installed... now to just remember how to use it... and figure out how to post pictures into these replies.

Also changed the design again, swapped out the central flue tube for a fusible plug. So down to 42 tubes of 1" OD, Thank you Fred for the idea of fewer tubes of a greater OD.

Josh

[fredrosse]

Just one salad bowl. This boiler uses turbulators, so the flue gas temperature is quite low, no heat colors on the stainless. The photo was taken after several full fire outings. I do however have fiberglass insulation and a metal cover over the entire boiler when its all together.

[farmerden]

Not meaning to change the subject but I noticed, Fred, that your u-joints are not aligned with each other .Was that to eliminate a "flat spot" in the drive line? Den

[fredrosse]

Den, I do not understand your question. The keyways on the crankshaft are both cut in the same orientation, and I guess the universal keyways are shifted?

I selected the large universal joints as I wanted to be able to take some flexing of the hull, but the shafts are basically very close to inline condition, so the universals don't do much, if anything, with respect to mis-alignment of non-parallel shafts.

One design feature was definitely required, I needed a coupling that would allow virtually no torsional flexibility, as the paddlewheels are the only "flywheels" for the engine. In a single cylinder engine, there is forward torque thru the power stroke, then the flywheel has to feed some power back to the engine at top and bottom dead center. If there was something like a Lovejoy flexible coupling between the engine and its flywheel, the torque reversals on every engine stroke would be troublesome.

I wanted to use bellows type couplings, they provide the torsional rigidity that is required, and would be ideal in this application where angular mis-alignment is very small. They would also be safer, as there is just a smooth outside surface, nothing to possibly catch clothing, etc. However, the bellows couplings cost several hundred $$ each (I lurked on e-bay for years, looking for a good deal on some, they didn't show up) so I bought the truck universals, $25 each new.