A Monoflue Gas Fired Radiant Burner Boiler
steam engines and locomotives.
3D CAD visualization.
Basic concept boiler drawing (click on above image
for 2000 x 1400 pixel version).
Note: The design has been updated during construction, with a more tightly
wrapped spiral strengthening on the flue as per the 3D CAD drawing plus
A 4" test boiler has been made with John Elliot based on the
above design. The boiler has all the major structural elements but was without
the small accessory fittings when testing since higher than normal pressures
A live test bed is now being build to test the "in use"
operation of the boiler.
1. Completed boiler - after 860 psi static pressure test (S.P.).
Click on image for large 2200 pixel wide image (964 kB).
2. Completed boiler - showing stays and bushes (S.P.).
3. Staying diagram for previous picture (S.P.).
4. Parts that go into making the boiler (J.E.).
5. Test assembly (J.E.).
6. Test assembly (J.E.).
7. Partly constructed boiler (J.E.).
8. Brazing setup - final assembly used 130 kW burner (J.E.).
9. First pressure test to 240 psi - no leaks (J.E.).
10. The second pressure test - 860 psi (J.E.).
Images 1-10: views of the
test boiler, components, trial fitting and testing (photos - S. Parker and J.
I have designed the boiler with the aim to meet the requirements
of the Australian copper boiler code which requires bushes on the longitudinal
stays. This is not the case here in the UK but one of the objects of this design
is to define a basic boiler which can be made and used worldwide.
The steam pipe has also been de-rated because of the steam
collection slots and the main support for this section of the front and back
heads is from the adjacent two stays. Note the splash guard which is fitted onto
the top two stays.
The only large piercing of the outer boiler shell is the primary
safety valve which is of a special design which will live under the sand dome
and exhausts through two pipes directed downwards under the boiler and between
the frames. It has a sufficient aperture to exhaust steam at a minimum of four
times the maximum evaporation rate.
Secondary "safety" valves are connected to a manifold on the
steam collection pipe and these are set to give normal boiler "regulation" and
pressure indication to the driver.
In normal operation the burner will be regulated by the steam
The spirally wound flue support can be seen in the pictures,
which adds to the evaporative surface area available.
Drawing Design Notes
The drawing is a schematic
rather then a fully engineered drawing and although dimensioned the measurements
are indicative rather then hard and fast (the major length of the boiler and
size of smokebox are more or less accurate to fit the loco).
The burner prototype is a
radiant stainless steel mesh over a slotted tube fed from a modified 3.1KW
blow-lamp burner. The active part is some 10" in length (note the slots vary in
size from one end to the other).
The stainless steel mesh
will need replacing from time to time, so the burner assembly should be
accessible for maintenance. Higher temperature materials could be used if
The mono-flue as shown is 2"
O/D 10 SWG (0.128" - 3.25 mm) tube . It is important to note that the collapse
pressure for a tube is NOT the same as its burst pressure. I have included a
close pitch support spiral (that emulates conventional support rings) to ensure
that the flue retains its round shape, having been advised that this are
necessary to strengthen the flue for it's 8x margin.
N.B. The 3D section shows
spiral on a 1/2"
pitch which is designed to give a substantial increase to the surface area in
contact with the water. Easy to draw in CAD, not quite so simple to fit and
Flue Tube Collapse Pressure:
There seems to be a certain
amount of ambiguity surrounding collapse pressures of copper flue tubes. While
the burst pressure, density of stays, number of rivets, etc. all seem to have
formulae, flue tubes are specified by a table.
Tables are fine, but don't
describe the reasoning for the values they contain. As it is important to design
in safety as the primary component of any boiler I have been undertaking some
research and I am working on developing a formula which fits experimental data
derived from testing by copper tube manufacturers.
The graph below is from the
Copper Tube Handbook - Figure 3.
Fig 3: Cu Tube Collapse Pressures
The formula will be for
collapse pressures of annealed copper flue tube of various thicknesses and
diameters, where length is six or more times the diameter. Note that the
existing commercial formula as used for steel pipe in bore holes does not work
for the small diameters in copper as used in our boilers. Also this does not
cover shorter pieces where the length is similar to the diameter - such as is
found in marine style boilers as used in the Sweet Pea design.
N.B. These figures assume the
tube is actually round (less than 1% eccentricity) and undented!
I always assume fully annealed
tube. This may not be the case in practice with standard copper boiler design as
the tubes are brazed at the ends only and the centre sections may retain a
degree of hardness. BUT without destructive testing - which rather defeats the
point - one cannot determine the amount this might be, so one should always use
the fully annealed values.
Note (2005): This is clearly
seen in the high pressure test as the profile of the boiler shell shows the
effects of fully annealed sections (see
large 2200 pixel image).
Fig 4 shows the affects of
heating on the yield strength of copper. NOTE: This is not the operating
temperature, but the maximum temperature the material has been raised to during
fabrication or operation AT ANY TIME.
Fig 4: Copper Yield
Strength v Heated Temperature
This graph demonstrates why it
is so important to have an adequate safety margin in hand for safe operation.
|Metric Conversion 1000 psi = ksi x 6.894 = MPa
1 MPa = 145.03 psi
The steam collection slots are
indicative and would be made finer and in a greater number than that shown,
allowing for scale build up.
The superheaters also shadow
the top of the flue and may provide some protection should the water level be
low and slosh (shouldn't happen normally, but...). There are two superheater
tubes, the left hand one feeds the left side piston and the right hand one the
right side piston, i.e. they connect directly and thus reduce the complexity of
Note (2005): Subsequent to
my original design I am now looking at the benefits of having a separate
superheater module with it's own dedicated radiant burner. This would produce
superheat on demand, and allow close regulation of the final dry steam
temperature (of particular benefit for use with PTFE cylinder components).
The siphon baffles are a major
feature and should help promote circulation in the boiler. Although in
implementation they are rather different from that which was in full size use
they will operate in a similar manner. The baffles would also be appropriate for
a G1 boiler, although perhaps rather more fiddly to fit.
Conclusion and Disclaimer
The basic premise is simplicity
and strength, including minimising the number of incursions into the basic
pressure vessel structure.
Although I have gone through this
design fully with calculations any new boiler construction should be rechecked
against the actual materials being used. Even small changes in gauge thickness,
diameter or length can have a big effect on safety margins.
In my final implementation (see
pictures) I following the Australian regs closely (for example more stays than
those originally detailed in my drawings were used). I also used bushes for the
stays which is not always common practise.
Any boiler design should be
thoroughly checked using accepted practise before being built. Showing a boiler
inspector the calculations and having them to hand for insurance purposes
demonstrates "Due Diligence"!
Recommended maximum internal
working pressure for a copper tube with brazed joints with saturated steam is
120 psi. See
Copper Tube Handbook for further Information.
This design originated as I needed a boiler for my own use which
I might have some chance to be able to construct myself, or at least have made
at not too great an expense.
I decided that gas or liquid
fuel firing was most appropriate for my requirements, as it can greatly simplify
the overall design.
Fig 1: Single through five flue options for a 4"
Taking into consideration the
size of burner(s) required to generate sufficient heat to produce efficient
evaporation I chose the mono-flue "Cornish" option as this seemed most
appropriate for a 4" OD boiler (and the simplest in construction).
Larger boilers for bigger
locomotives would use two or more flues of up to 2" OD which is the maximum
usable diameter for 10 gauge tube.
Whilst in appearance this
concept looks like a big sister to a Gauge 1 (c. 1/32nd scale) boiler the design
was done from first principles (my original inspiration when I first started
looking into boiler design options was the Lentz boilers of over 100 years ago).
Form follows function however...
The Radiant Burner concept has
been successfully used in Gauge 1 scales, with documented improvement in
operation compared to conventional gas burners.
Image 4: Radiant Burner by Kevin O'Connor
See the Southern Steam Trains
Notes - Radiant Poker Burners for more information.
Variations on a Theme:
of Helsinki, Finland has built a three flue 51/2"
boiler, for his 4-4-0 American locomotive No:
3003. On August 26th 2002 he steamed his locomotive for the first time. The
boiler went from "I wonder...?" to "It works!" in two months!
Engineering & Live Steam Locomotive webpage for further details.
Compare Kevin O'Conner's nifty little Gauge1
burner to Jan-Eric's 6 KW burner - and there are three in his boiler.
... and then built a two flue boiler for his
0-6-0 tank loco "666".