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ON LIGHTWEIGHT BOILERS

Here we describe the possibilities for very lightweight boilers.....This page is rather tentative; it is a rehashing of voluminous information given to me by a specialized steam consultant, unhappily not currently available....

There is a fundamental distinction between the two basic types of boilers: fire tube boilers and water tube boilers. Water tube boilers are more promising for weight minimization, but their construction is not so straightforward.

WATER-TUBE BOILERS FOR BALLOON USE

The estimated weight (not including the burner) for a state-of-the-art lightweight high efficiency water-tube boiler capable of boiling 500 kg of water per hour at atmospheric pressure is 40 kg. The next figure shows a projected such boiler in axial sectional view, suitable for gravity feed with minimum of 3 m head. (The labelling of the coils appears somewhat mysterious because their three-dimensional arrangement is very difficult to show clearly in any figure). Such a boiler would be ideal for the large multi-person balloon described in the flight modes page, flying in the jacketed reboiling mode; and it is much lighter than the boiler conservatively postulated there, which is assumed to be a fire-tube boiler of very lightweight but conventional construction.

LIGHTWEIGHT HIGH CAPACITY
WATER-TUBE BOILER
FOR BALLOON USE

This cylindrical boiler is 40 cm in diameter and 50 cm high and made of stainless steel. There are 30 parallel circuits each about 3 m long. The tube is 12 mm I.D., 22 swg wall. All the tubes are connected in parallel by manifolds, not shown, housed in the conical upper and lower cover assemblies. The inner bank consists of 4 coils, A-B-C-D (31/2 turns); the next bank consists of 5 coils, A-B-C-D-E (31/4 turns); the central bank consists of 6 coils A-B-C-D-E-F (21/2 turns); the next bank consists of 7 coils, A-B-C-D-E-F-G (21/4 turns); and the outer bank consists of 8 coils, A-B-C-D-E-F-G-H (11/2 turns). The coils are pitched at 40 mm centres. Odd and even banks are wound heterochirally. Vertical tie rods, not shown, are provided in the spaces between the coils.

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(perspective view of similar unit)

This boiler could be scaled down, and would result in a lower capacity boiler very light in weight, suitable for the one-man balloons described in the flight modes page, flying in the jacketed reboiling mode.

WATER-TUBE BOILERS FOR AIRSHIPS

For an airship which is to be steam propelled, a lightweight boiler of a rather different type is required for supplying steam at high pressure. A possibility is a boiler of similar design to the boiler illustrated above, but with all the coils connected in series as one long tube. The feed water is forced by an engine-driven feed pump into the outer coil and progresses inwards from coil to coil. It becomes entirely vapourized into steam by the end of the third coil, and is then superheated by the inner two coils. This boiler is capable of producing sufficient steam to drive a reciprocating engine developing 50 hp. It could readily be scaled up as required.

The 1926 study by Burgess, described in the prior art page and given in full here, is also relevant. This much larger boiler is of quite ingenious construction. My sources have told me that modern practice could improve on the steaming capacity / weight performance of this boiler substantially.

THE BURNER OR FURNACE

One of the features of the use of steam lift gas on the projected scale is that fuel oil could be used as the flight heating fuel, rather than LPG as with current hot-air balloons. It is not very clear that this would be advantageous for a free balloon. Fuel oil is much cheaper than LPG and has a higher heat content per kilogram, but a fuel oil burner would inevitably be heavier than an LPG burner. Moreover, the fuel oil would need somehow to be supplied to the burner. One possibility would be to use a vaporizer type burner (like a "Primus" stove), and in this case pressure feed and a hand pump would suffice. The burner would be started and warmed up using propane, and could then be switched over to burn fuel oil. As an alternative to fuel vaporization, if sufficient power were available, the fuel could be fed by a powered pump to a spray burner, and a blower fan might also be provided. However a gasoline-powered onboard generator would be required, since electricity is the only practical power option, and this would entail complication, weight, and expense. Of course onboard electrical power might be useful or even required for other applications.

In the airship case propane is not a viable alternative to fuel oil because of the high quantities of fuel that will be consumed, and accordingly the complication of a power-driven burner, pump, fan, etc. cannot be avoided.

An interesting possibility is that solid fuel such as high quality coal (anthracite) might be very effective....



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