2008 - 2009.

John R. Bentley 2009.

Model Watertube Boiler

-  A miniature industrial D-type boiler  -

Stack Construction - the full disclosure

Marking out the octagonal stack base
made from more of that same steel plate!
The stack bottom flange will be made smaller and gusset pieces will later be brazed between it and the stack.  The upper strengthening hoop will be replaced with a simple flange version of the same.
The tabs are cut - almost ready for bending
I carved deep bending grooves with a Dremel cut-off wheel
Well... that was simple
As mentioned, the round bottom flange will be made smaller
Joints silver brazed and the base temporarily copper plated
It's plenty strong!
Trimming the base in the lathe
The pressure disk is just a piece of scrap
Later the base's corner seams will be filled with a lower-temperature
silver solder and it will be finished to resemble stone or concrete
The stack top flange
Cutting out strengthening hoops
Drilling the bottom flange for the stack mounting bolts or studs
Cutting a large rectangular breech opening near the base of a full size steel stack can allow it to buckle under its own weight.  To offset this, two strengthening pieces are attached on each side of such an opening.

I have even seen this in a large brick chimney.  To accomplish this easily in the model, I simply attached the pieces to two of the hoops.  They will later be silver brazed to the stack.
Another view  -  digital film is cheap :-)
You will see something similar to this on every steel stack with side breeching
(the size exaggerated by the camera)
The copper-plated steel stack after silver brazing
I added a drain for stack condensation (& rainwater!)
Inside view
(instead of the usual full-diameter funnel-shaped bottom, I will be using
an angled bottom plate to direct any condensed water running down the stack walls to the drain outlet)
(note the welded seam - further down the page you will see a notch
in the middle brass ring of the economiser to accommodate this seam)
The angled plate was silver-brazed in positionsoldered
Size reference
The economizer
The tubes cut for the economizer
This upper ring in this shot fits the inside diameter of the stack
After brazing 12 tubes to the ring
Four more tubes in the centre will surround a single central brass downcomer
The said inner four tubes
(the fifth central tube passes completely through the bottom header/tank)
Bottom view of all the tubes brazed to the lower (cold) header/tank
Overall view from below before the other tank is added to the top
Curving a stainless steel sheet (to become a support for the pipe connections)
Tank bottom cover attached
(bracket and inlet fixture in foreground)
Note above that the central outlet tube from the upper (hot) tank is installed
(this tube does not actually touch the cold tank as it passes coaxially through a short hollow stay)
The finished bottom assembly
(to avoid heat transfer I used stainless steel for the threaded hot outlet and the bracket)
Here's an inverted close-up of the warm outlet tube passing through the lower cold tank
The top tank's cone-shaped cover being turned
The small round baffle surrounding the downcomer might help equalize the flow through the other tubes
The completed apparatus with top hoisting hook
(Well....almost complete - there will be a sheet wrapped around the tubes
covering the lower half of the upper section above the middle ring)
Outdoor view proving our grass did turn green in May of that year
Stainless steel nipples support the entire economizer unit inside the stack
Trial fitting
Hoisting 'er down
Eventually insulating washers will be used on the hot outlet (lower) nipple
Here's what the boiler flue gasses see as they enter the stack breech from the boiler
I think the full-size version ("Economite") is a monotube arrangement - one continuous run of pipe, folded many times to fit into a reasonable section of stack.
In my unit, the flue gases enter from the side and, due to the middle ring closing off the stack around its circumference, the gases are forced to stay within the central area of the nest of tubes until they pass the top of the encompassing copper shroud.   The top conical tank is smaller in diameter than the stack by a respectable amount - allowing the gases to exit around its periphery.
(the shroud is crudely sketched into this photo, as I haven't attached an actual copper sheet yet)
Note the tiny bit of space around the bottom of the shroud - it will allow condensation to run down to the bottom of the stack and out the drain.
I gave the boiler plant its first overall coat of paint.  This was just to eliminate visual distractions so I can prepare the surfaces for final finishing.  I'm not going nuts over the degree of finish quality, I just want it to be uniform - this is intended to be a workhorse boiler.
Clad in high temperature aluminum undercoat
I will probably make imitation manhole assemblies and push them in the holes where the drum heads are exposed
Obviously most of the boiler front will be obscured by the burner and the fittings
I'm still not sure about final colours ....
There are lots of ugly spots, but from the beginning this thing was never expected to look good - only an old workshop boiler to be ready for testing engines.
At this point (mid-June 2009) seven months of construction time have been put into this boiler.