|As noted, the basic design of this boiler
follows the Lune Valley design, having multiple small coils emanating from
a central drum. In the original Lune Valley design, the tubes
were steel, expanded into a forged flanged drum. This is rather
impractical for home manufacture, so I did some adapting, using copper
coils rather than steel, silver soldering the coils into steel nipples
welded into the drum and welding the central pressure vessel together from
standard pipe rather than using flanges. Note that the 4" and 7" caps
and the 7" to 4" reducer are commercial forgings - reasonably inexpensive.
The weld-o-lets I used for all pipe connections are also small forgings.
The root passes on all the welds are 6010 DC and the cover passes
are 7018. After the coils were soldered in the entire assembly was
pressure tested to 400 psi. One of the welds securing a weld-o-let
leaked a drop or two; it stopped almost immediately.
Some key design considerations if you're contemplating your own moderate pressure water tube boiler design for hobby steamboat use:
||Here I am fitting the coils into
the nipples for a test before soldering. In order to bend the coils, the
copper tubes were filled with sand and wound on my 1942 10" Atlas by hand
using a form made from maple. I turned the jackshaft pulley by hand
with the lathe in the lowest gear, guiding the carriage with my other hand.
In general, winding the coils was not difficult, but was rather tedious.
After winding. the coils had to be annealed in order to splay them to fit
the boiler (see previous picture). This was done with a large
weed burner. After annealing, I used steel wool to clean the
ends of the boilers.
The support for the boiler was a temporary fixture on casters; this allowed the boiler to be readily spun about its vertical axis and made soldering, etc much easier. If I were to do this again I'd make it higher - my back aches just looking at this picture.
|Here's the casing for the boiler in a
photo that makes the frame pretty visible. The rings top, center
and bottom are made from 1" x 1/4" solid rolled on the roller I made
to build the Otter's canopy (XXX insert link to future canopy page
here). The inner jacket is 22 gage 304 stainless; I chose this
because I got it surplus at $2.50/lb. If I had my druthers
a more heat resistant grade would be better, but so far this appears to
be working quite well.
This structure was difficult to get aligned correctly; the rings weren't exactly round and they didn't want to stay aligned vertically. I think it might be easier to just avoid the vertical members and just use the outer shell to hold it all together.
|The view of the emoty casing from above.
The bottom is 12 gage or so steel; the supports seen as a X in the bottom
are 1" x 1/8" on edge as is the inner ring. On top of this is placed
1" Fiberfrax insulation, a disk of 16 gage stainless and a layer of lightweight
insulating firebrick. I originally lined the entire combustion
chamber with this firebrick, but it proved too weak to handle the stress
of trailering and thermal shock without being mortered into place.
The combustion chamber is now lined with 2" of Fiberfrax and an innermost
surface of 321 stainless. The later is warping slightly after several
years of use.
One can see here the fire door (the lower hole) and the burner entry (the upper tangenially cut hole). If I redo this ever, I'm going to fit a larger fire door and some sort of grate or ash pan so I can raise initial steam on solid fuel. Right now I use propane, but this is a pain to have in the boat. Once steam is raised, of course, I use steam at 30 psi to spray diesel in a tangential swirling flame. Since the flame wraps neatly around the firebox, we have little trouble with the fire going out due to a gust of wind or the like.
|At this point the pressure vessel has
been bolted in place and I've installed the 1" FiberFrax that forms the
insulation for the outer jacket (18 ga 304 stainless). A fellow
steamboater arranged to have the outer jacket rolled for me. Everything
here was tacked in place while clamped into position, as it's very difficult
to hold tolerances tight enough to do anything else.
Wear a respirator when using FiberFrax - it's nasty stuff.
|The plumbing has been added here.
From upper left clockwise the nipples are:
|Plumbing in place with economizer tubing
(1/2" copper) added. You can also clearly see the upper steam connection
for the safety valve, stack draft and whistles. Unlike the lower
main and auxilary steam connections, this one doesn't have an internal
pipe leading up into the steam dome. This was a mistake. I may
yet replumb the sight gage to use this as the upper connection as the boiler
appears to steam better with a high water level, and raising the level
of the connection would allow the glass to show these higher
|The Otter's boiler ready for hydro-testing. The orange hose blew off from the pump during hydro, shooting 400 psi water (there was some air left somewhere) all over the place.|
|The Otter under test with her new boiler in 1996, just in time for the meet in Victoria BC.|