Boiler/Feedwater on the John Fitch

[Brenton Baker]

Wanting to keep threads intelligible without taking up too many threads, I have decided to make a separate thread for the boiler and feedwater system.

Attached is the boiler diagram for reference, as well as a P&ID of the feedwater system (safety valve, blowdown valves, boiler pressure gauge, and whistle omitted from the boiler for clarity).

The three-way valves on the auxiliary hand pump allow the pump to either pump water from the hotwell to the boiler, or from the makeup water tank to the hotwell (care will have to be taken to avoid pumping water from the makeup water tank directly into the boiler).

[Brenton Baker]

After attending the Great Delta Steamboat Meet and reading Audel's Engineers and Mechanics Guide, I am making some changes that I hope will make the engine and boiler a bit more realistic.

I was originally planning on diesel burning because of the lower cost and potential for biodiesel, but I am beginning to consider propane, and so designed a boiler for that, taking into consideration the reported lack of radiant heat from a propane fire (most of the heat transfer surface is in the tubes instead of the water leg). The tubes are also much smaller due to soot becoming a nonissue with propane.

Hand holds and fittings omitted in this drawing. Boiler shown ready for assembly (tube rolling and welding, shell assembly and welding).

[87gn@tahoe]

Brenton,

I just saw this post. I think you may want to re-calculate your heating surface figures. I got just over 12sq.ft. using just the OD of the 91 1/2" tubes 13" long.

[Brenton Baker]

Oh, goodness, I forgot to update this. I'm not looking at Propane for fuel on the John Fitch anymore; looking at the BTU/HR, I'd be looking at several 20# tanks to get a few hours in. I don't like the idea of five or six propane tanks sitting on a boat, and neither does my father (though we will probably use it for the first boat).

So I am not looking at the 91-tube boiler anymore.
Thank you for double-checking those numbers, though.

[87gn@tahoe]

What fuel are you looking to utilize?

[Brenton Baker]

Right now the plan is for diesel, propane atomizing.
These drawings are not complete. I am going to leave them as-is until I get more experience (read: have a clue what I am doing).

The original (19-tube) boiler is the choice so far.

[fredrosse]

" (care will have to be taken to avoid pumping water from the makeup water tank directly into the boiler)." I do this all the time, not ideal, but OK to do.

Looking at your boiler drawing, the "2 inch mud ring" appears to be cut out of two inch thick steel plate? That plate actually can be no thicker than the rest of the flat plates within the boiler.

The boiler you have drawn has the upper tubesheet flooded with boiler water. This type Vertical Firetube Boiler (VFT) is commonly called a "submerged tube plate boiler". The design was more popular about 100 years ago, but is rarely seen today. Most VFT Boilers just use a single tubesheet at the top, and the last few inches of tube not submerged in liquid water can give a slight superheat to the output steam. This configuration is satisfactory, and far less expensive to build. I have some concerns of potential "priming" for the layout you have drawn, there may not be enough steam release surface available.

Note when calculating boiler heat transfer surface area, all surfaces, including the boiler tube surface exposed to the hot combustion gasses is counted, not the surface in contact with the boiling water. For firetube boilers this means that the tube inside diameter is relevant.

[Brenton Baker]

Fred - thank you for looking over and critiquing my plans I got as far with them as is shown, before realizing that:

1. I don't know enough about these things to be making any final decisions and
2. I ought to build a boat first (ANY boat, even if less-than-ideal, so long as it is SAFE and relatively inexpensive to build) before I try to build the John Fitch, so that I can make mistakes with it rather than with the more expensive John Fitch (whose Indian Paddle mechanism will give me a run for my money by itself).

That being said:

Would the first boiler I posted in my thread "Engine for the John Fitch" be closer to a realistic design (minus the errors in calculation and the omission of every part but the tubes, sheets, shell, and door)?

Also, the 2" dimension was probably a mixup on my part (2" thick vs. 2" water leg width) but of curiosity, why must the mud ring be no thicker than other flat surfaces?

[fredrosse]

" Would the first boiler I posted in my thread "Engine for the John Fitch" be closer to a realistic design? ANS Yes that is more typical of a water leg VFT design.

"why must the mud ring be no thicker than other flat surfaces?" ANS it does not require more thickness because the 1/2 inch thickness you have for the tubesheets is strong enough for that bottom ring. It could be made thicker, but that would just make things heavier and more expensive. That weld should also be full penetration on both the outer shell and the furnace tube, not an easy task with very thick plates.