John R. Bentley 2008.
Firebox and Naphtha Gas Burner
From the Naphtha Engine's Flash Boiler - constructed from the 1887 patent drawings of Frank W. Ofeldt
The firebox is made from cast iron since it will be subjected to considerable temperature swings
while providing the required mechanical strength to support the heavy boiler casing and stack.
This lower view shows the location between the valve cover and the copper casing
It started out as a common oil tank filler cap for a domestic heating system
The cap is made from galvanized cast iron
The cap is quite thick and I felt I could cut wastage by "finding" my firebox within its walls
Turning off the outer surface
Depth of proposed firebox marked with a grease pencil
Outer machining completed, and the two mounting holes drilled
Marking out the secondary air holes on the bottom of the firebox
( Taig 4-jaw chuck, Grizzly Rotary Table & Taig Micro Mill all put to good use )
I left the galvanized coating on the inside bottom (possibly better heat reflection?)
(obviously, the ring burner will hover over the spaces between the hole rows)
A view of the secondary air vent holes and the clearance from underneath
(the large hole in the firebox front is the touchhole for lighting the burner)
Note in this trial setup, the firebox large internal thread used a lot of space...
It doesn't show but the black space around the lower coils is nearly consumed by the threads
Milling off those offending internal threads
( Rotary tables and mills are amazing things! )
This stainless steel fitting couples the vaporizer coils to the top of the engine
The length of this fitting provides the correct amount off fire space below the coil assembly
There is more clearance showing around the coils now, since the threads were milled away
The outer casing slips on over the firebox
Part 2: Making the ring burner
Center drilling, off center!
Eccentric turning
After swinging end-for-end in the lathe the eccentric hub is completed
( note that the end-drilling has been started and will be completed later )
Drilling the junction
Marking out the burner holes with a centre drill on the mill
Drilling the entire burner tube from end-to-end
(I had to reverse the piece and go in from the other end)
The finished straight burner waiting to be curled
The holes are not yet all drilled all the way through
Wrapping 'er up!
It is quite blackened from annealing
It is starting to look a bit like an FM transmitter antenna!
Trimming the tips
A plug for the ends
( the holes are still not drilled through )
Re-trimming after silver-brazing the plugs into the ends
Main burner ring
An auxiliary starting burner will curve around (and slightly below) the outside of the right arm
Main burner in position in the firebox
Two holes at each end will remain closed as I don't want fire in this area
(due to a brazed joint in the boiler tubes at this location)
The mixing tube attaches to the burner here
The very first fire test (using propane)
I will be adjusting the orifice size to get exactly what is needed, but even this puts out plenty of heat to boil naphtha satisfactorily.
So while these adjustments are yet to be made, this is darn promising for the first try!
Part 3: The Injector Valve
The injector mixes naphtha vapor tapped from the coils with induced air to supply the ring burner
The injector sits on top of the gas mixer tube
( the supply tube from inside the boiler to the valve is not yet installed )
Mixing tube
( I cut this from solid brass bar )
( the hollow globe sits atop this tube - together they form the venturi shape )
Starting construction of the mixing globe
After polishing
( you can see the Canon camera and trigger finger in the reflections in these shots )
Here the mixing tube and globe are screwed together for drilling
Air intake for the mixer globe
The setup prior to adding the valve on top
The valve body under construction
The vapor supply pipe and packing nut beside a standard Eagle pencil
Milling the hex flats with a thick saw on the Taig mill
Construction of the bronze jet under the microscope on the Taig lathe
Threading the stainless steel valve stem
( that's a 1-72 tpi thread - the green drop at the bottom of the die is A-9 cutting oil )
Checking the valve stem/wheel centering in the 4-jaw chuck on the rotary table
Milling the grips in the periphery and adding cooling holes to the handwheel
Parting off
Valve and mixer unit complete (less the mixer tube)
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