Solid Fueled Flash Boiler Experiments
 by Steve Redmond

March 2004

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 it.

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.

They are:

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 condensation

5.) I needed a larger combustion chamber. The small amount of fuel was burnt too quickly and a column of ash retarded heat development

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.

Best Regards,

Steve Redmond

Website: www.sredmond.com