Solid Fueled Flash Boiler Experiments
by Steve Redmond
I recently read with interest your discussion of flash boiler
drive. It was a subject I was interested in last year just before I
stopped work on my experimental engine.
Last year I actually built an experimental flash boiler for solid
fuel. It worked, though not well enough, and I thought I'd share the
information I have on it with you and others here.
I've seen several old articles on flash steam generation in
gasoline torch fired boilers. These basically assume the form of
copper or steel tubing wrapped in lengthwise flattened coils with an
additional wrap of coil radially -- corkscrew style. The coils were
housed in perforated sheet metal tubes and employ superheat. They
were used to power early model aircraft and boats.
They were fed by a force pump run off of the steam engine. The engines were radial piston types with rotary valves.
As you know I cast my own metal parts, both for the disk turbine,
and the lathe and milling attachments I used to build the engine.
The foundry unit I use for melting metal is a Dave Gingery style
charcoal fired bucket type with a homemade fireclay and sand-mix
lining. This easily melts aluminum, and reportedly can melt brass if
you are patient enough and provide enough fuel. That means it can
produce temps near 3000 F, and 1000 degrees F is a piece of cake...
It occurred to me that the foundry might, with some re-design, make
a good heat source for a flash boiler. I'm very interested in
producing a solid fueled engine, rather than a fluid fueled one --
that's why the foundry furnace was of interest, as opposed to the
gas torch designs of the early models. Solid fuels other than coal
are renewable and common and work with the total carbon atmospheric
balance we already have, rather than introducing bound carbon into
So I basically built a new foundry, only this time, the chamber
was sized to fit a coil of 1/4" copper tubing around a 2"
central pipe which extended above the top of the foundry as an
exhaust stack. I decided to fill the central pipe with charcoal
briquettes. A tuyere of about an inch diameter ID pipe entered the
bottom of the combustion chamber/pipe stack for forced air-in via a
small high speed fan.
The coils were protected from the direct flame of the combustion
chamber but surrounded by 4" of fireclay/sand/vermiculite
insulating lining mix.
As an experiment without a force pump, I
hooked the coil up to a needle valve in line with house water
pressure (about 40 lbs.). I thought I might be able to possibly get
40 lbs. of steam pressure before the feed water was blocked and since my turbine had spun on 30 lbs. of compressed air, I might
see some rotation. This was an admittedly crude setup, but I wanted
to see whether it would work at a minimum level -- to decide whether
it was worthwhile to pursue further.
I fired the rig up, and after a few minutes did see steam issuing
from the turbine exhaust. The rotor turned rather sheepishly and a
trickle of water also exhausted the port. I couldn't adjust the feed
water to get consistent steam. It was either running dry or dousing
the rotor. That was about it. It was disappointing, but not discouraging, and as in any failure,
provides insights into the practical needs of a successful boiler.
These are no doubt conclusions others might have pointed out in
advance, but you never know until you do it yourself.
1.) accurate feed pressure and volume adjustment is essential to
the running of a flash boiler. Much more critical than a
conventional boiler feed. Feed metering is THE problem for flash
boilers: feed must follow steam requirements of the turbine..
2.) wet steam of marginal heat content will condense inside the
turbine preventing spin-up
3.) steam should exhaust without condensing.
4.) a massive conductive turbine housing like mine (thick cast
aluminum) will act as a condenser. Conventional steam engines used
wood lagging (insulation) around steam cylinders to retard
5.) I needed a larger combustion chamber. The small amount of
fuel was burnt too quickly and a column of ash retarded heat
6.) I needed better heat transfer to the coils. The protective
pipe housing was too protective -- too thick. Oxide discoloration
indicated that the heat was highly localized, rather than supplying
the full length of the tube and coils.
7.) I needed higher pressure in feed and steam output. Enough
surplus to prevent condensation in the engine. This would lead to
some inefficiency, which would depend on the ratio of the minimum
required exhaust pressure/temp to the total drop from input.
As I said I'm not discouraged by the results, and I certainly
believe that such a blast furnace/solid fueled flash boiler
configuration could work on a demonstration engine -- if the feed
design and combustion chamber proportions were properly worked out.
On the positive side, the furnace's forced air input may yield
better combustion efficiencies than older style atmospheric flash
boilers, and may reduce emissions because of the extremely high
heat, and controlled excess air.
I also believe that there are inherent advantages in this
configuration because of the massive insulation and small boiler
coil volume -- in this case I hoped that if it occurred, a 1/4"
diameter tube blow-out would be minor, contained in the furnace, and
directed, if anywhere, up the stack.
The biggest problem of steam adoption is the inherent danger of
contained pressure in a large vessel. If we can get past this, then
small solid fueled generators may be viable. The final advantage is
that this is a very simple design to build and experiment with by
comparison with other boilers, and the fuel is considerably safer
than gasoline or propane (the usual model engine choices) and easily
extinguished with water.