Building a Steam Balloon Flight Boiler

Although it is not immediately needed for the first Steam Balloon flight which will be a "dribble" type flight, for the sake of research, we are starting to make a flight boiler. To arrive at the maximum possible steam production capacity (in kilos per hour), using the minimum total weight of boiler, water being boiled, and burner, is not an easy problem - especially for a novice in the art like myself. There are subtle trade-offs between boiler efficiency and boiler weight; like for example, is it worth getting another 2% efficiency in fuel utilization, if the boiler is 4 kilos heavier? The answers always tend to depend upon the projected mission profile....

The first question that arose was: What basic type of boiler?

In theory water-tube boilers are the lightest type, but they are hard to design and build. Also the complication of pumping the feedwater arises. The requirement for a Steam Balloon flight boiler is to be able to produce a large quantity of steam per hour, but only at atmospheric pressure. (A Steam Airship boiler would be quite another story, and more complicated). With a water-tube boiler, this requirement entails multiple parallel circuits, and there would be a great possibility of these getting out of balance, with a few of them working flat out and quite a number not doing very much. Once such a boiler is built it is very hard to find out just what is going on inside it!

Therefore in the interests of simplicity we have decided to make a "Stanley Steamer" pattern fire-tube boiler, but rather different from anything that has ever been built before, as far as we know. Anyway this simple design cannot get out of balance, requires no feed pump, and is comparatively easy to build (still not easy, though).

We have started by making the end plates, which may well be considered the hardest part. In the interests of keeping down the total weight, we need to minimize the volume of water in the boiler, which means getting the tubes very close together. The smaller the diameter of the tubes, the more heat transfer area we can pack into a given volume of boiler. However drilling a large number of small holes near one another in the end plates with sufficient accuracy is a very challenging task. Therefore we have purchased some pre-punched stainless steel plate, with holes arranged in a hexagonal pattern. The most appropriate size available was 1.5 mm thick, with 5 mm holes at a pitch of 8 mm. We intend to drill the holes out to 1/4" (6.35 mm), which will leave a 1.65 mm bridge between each hole and the next - about the minimum for structural strength, we think. The advantage of using the pre-punched plate, is that the positions of the centers of the holes are absolutely accurate. Enlarging them to 1/4" will be extremely easy because the drill will be self-locating, and no problem of mis-placement can arise.

Thus we have cut out two circular plates, and now we are proceeding to weld outer rings around their edges. The next job will be to fill in the remaining half-holes at the borders between the rings and the main plate portions. This is quite fiddly! The resulting end plates will have a total of 2227 holes each. The photos show the rings roughly bolted into place, to locate them in preparation for welding.....

What we have not yet decided upon, is how long to make the boiler tubes. With such small diameter tubes, it is fairly clear that the firebox will have to be blown; it is only a question of what pressure to use. Experiment may be the best teacher; I can't find any literature on any comparable boiler with such small tubes. Anyway, if for example the tubes were 45 cm long, then the boiler would have about 20 square meters of heat transfer surface. I think that it would take quite a high blower pressure into the firebox, to use this heat transfer surface effectively. In principle, given a high enough blower pressure, it might be possible to produce about 600 kg/hour of steam with this boiler.... Even getting so much steam out of the boiler will require some fairly sophisticated manifold structure! There really won't be much in the way of liquid water in the boiler - it'll be more like a froth or foam....



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