Here is a pic of 2 of the heat exchangers. The trombone coil is something I picked up from EBay. It is roughly 75' of 3/4" stainless steel tubing. The larger tube is 2.25" stainless steel, the U bends are something I had left over from an old project I worked on years ago. I had some straight tubing cut and one end swedged so it would slip over the ends of the U bend. It makes the connection stronger and keeps it in alignment easier. There is roughly 285 square inches of surface area inside the 3/4" tubing and 960 square inches in the 2.25" tube.
From another angle. The 2x4's are temporary spacers. Steel support brackets will be installed once it goes together for the last time.
This picture shows the last 2 heat exchangers. One is a 2" diameter SS tube with 848 square inches of surface area, the other is a 1.75" SS tube with 174 square inches.
From another angle.
There is no particular reason for the different size tubing, just using up left over materials. If I did not have those SS bends, I would have gone with a standard size of 2" and had each heat exchanger tube formed all in one piece to avoid the seams and additional welding.
Here is a shot of the mock up to where I am now in the construction. The flu pipe is also SS, it is .062" wall thickness sheered and rolled into an 8" diameter pipe. I'll TIG weld the seam before final assembly. You can weld stainless steel to mild steel with a 308 SS welding rod or wire. You use 309 SS rod for welding SS to SS.
Last night I made a cardboard mock up of the front plate for the boiler. It will look just like the rear in size and will have the openings cut for both the fire box and ash tray doors. The support structure for the tubular heat exchangers will weld onto the front and rear plates. The heat exchangers themselves will not weld to any of the steel structure or supports. They will float on the supports so the materials can expand and contract with the different temperatures and not create any stress fractures due to differing rates of thermal expansion of the materials used.
Next week I will have my grating materials finished and get those tacked in place. Once that is located, I can drill the holes in the back wall for the primary combustion air tubes to go through.
I will also have my fire brick and can mock up how it fits in the boiler and then measure for any support or retention brackets needed to keep the brick in place. The upper retention bracket for the brick will also support the 1/4" SS burner top plate and the 2 secondary combustion air pipes.
I am going to use the fire brick on both sides and the far end of the fire box. This should prevent the steel pipe from ever burning out. Or at a minimum give me 20-30 years of use.
I will have a piece of SS plate that fits against the back end of the fire box that will cover the brick, this will hopefully prevent the brick from being damaged when logs are thrown into the boiler.
On top of this structure will sit a SS water storage tank. Right now I am planning on a 24"wide x 36" long x 30" tall tank. There will be a 9" diameter tube rolled and welded into one end that the flu pipe will run up through. The wall thickness will be .062". A tank this size would give me a net storage capacity of 104 gallons after you allow for the loss due to the flu pipe. The size of this tank is still being thought out, we will have at least 2 x 150gal hot water storage tanks in the house, one for the in floor heat and the other for sink and shower needs.
The total square inch surface area of this tank will be around 4000. That combined with the 2200+ inches in the heat exchangers should give me enough surface area to absorb all the radiant heat needed from the fire box. This whole unit will be housed in an insulated steel or concrete block structure.
I'll post more pics in a week or so as the work progresses.
On a side note, an idea I am throwing around is how to utilize all of the extra heat available inside this building that houses the boiler. Moving hot air from place to place out of doors is not efficient, but I have an idea to take an old electric oven and remove the outer casing and mount it in the side of the block wall. There should be more than enough hot air available to heat the inside of the oven. That way we can use it for baking bread or roasting meat. Not sure how to control the temp inside the oven yet, possibly a series of vent holes to let in outside air to cool down the oven to the desired cooking temperature.
I've seen cob or clay ovens where they build a fire inside, once the clay is hot, they put bread inside and close off the entrance. I am planning on an outdoor kitchen for summer cooking to keep the cooling load on the house to a minimum. The boiler would be a winter only use, but it would save on some of the propane need we would have for cooking. If things get bad, everything will count. Just a thought at this point.