Here we describe our first preliminary small-scale Steam Balloon experiments.
First, we sewed up a miniature test envelope of the classic ball-and-cone balloon shape. We used twelve gores, and the radius of the ball was 50 cm while the bottom cone angle was 60 degrees. Thus the area was 3.5 m2 and the volume was 0.6 m3. The lift therefore, when filled with steam, was 3.76 newtons - about 383 gm. The material used for this first small envelope was second-hand conventional hot-air balloon fabric: ripstop nylon with one side polyurethane coated. The bottom of the envelope was left open, and an umbilical tube was led into its side. We sealed the seams with latex, and later gave the entire envelope a latex coating because it was too porous and allowed leakage of steam.
We also built a loosely fitting insulation jacket for this envelope from a light flexible material which is marketed in the UK for putting behind central heating radiators to reflect the heat away from the wall back into the room. This material, called "Mangers Radiator Reflector", surprised us with its effectiveness. It consists of a 2 mm thick sheet of light plastic foam with layers of reflective "Mylar" type film on both its sides. We made this jacket with simple duct (duck?) tape.
Here are two pictures of this envelope with jacket fitted, test-inflated from my trusty old vacuum cleaner which blows as well as sucking:
We then set up a simple experimental test rig, of which this is a rather crude diagram:
The steam source was a 2 KW boiler from a do-it-yourself steam wallpaper stripper, which produced a little over 3 kg of steam per hour. The steam was fed into the side of the envelope through the umbilicus, and a clip was put on the bottom of the envelope to restrict the escape of the steam, so as to build up pressure to fill the envelope.
When the system reached steady-state with the envelope inflated, both steam and condensed water were being continuously expelled from the bottom of the envelope. We adjusted the clip until the envelope was fairly taut, and then kept the system running for some time, measuring the amount of water that dripped out. The rationale was that the extra steam passing through the envelope and blowing out, which was moderate in amount, made no difference to the amount of water condensed.
In fact, using a single 2 KW boiler, it proved impossible to inflate the envelope in its naked state without the jacket; the steam supply was quite inadequate. Therefore we were not (yet) able to establish a value for the naked condensation rate. However with the jacket fitted we succeeded in getting the envelope filled with steam and taut, and in keeping it that way in steady-state. (Unfortunately no photograph is extant). I think this was the second time in the history of the world that any flexible bag has ever been filled with steam and kept full.... the first appears to have been Mr. Brian Boland's inflation of a full-size balloon in Iceland with geothermal steam - see the prior art page....
The figures obtained were as follows:
First test: 2.2 kg of condensed water accumulated in 1 hour
Second test: 2.3 kg of condensed water accumulated in 1 hour
Furthermore, when the envelope was removed and the steam supply was allowed to blow freely to atmosphere, the amount of water droplets falling out from the steam (not condensed from it) was about 400 gm/hour. In fact these boilers are actually designed to produce very wet steam which carries as much water as possible, which is the most effective for wallpaper stripping. We realized that this was a defect in the experimental setup. Anyway, deducting this extra carried-over water (the accumulation of which on the inside of the envelope of course does not involve any heat transfer), we arrived at our conclusion:
Condensation with this jacket .... about 530 gm/m2.hour
In retrospect from the point of view of what we know now, this seems like a relatively high (and even impossibly high) effectiveness for this rather thin and light insulating jacket. We conjecture that the reason was as follows. The "Mangers" insulation material is quite stiff, rather like the material of an obi or a Noh type kimono (not so stiff as corrugated cardboard, but stiffer than paper). Therefore the weight of this small jacket was not enough to make it conform closely to the inflated envelope enclosed within it; in fact, in many places, there was an air gap of a couple of centimeters or more. This air gap would be expected to have a very substantial insulating effect on its own, additional to the insulation effect of the jacket material itself.
Unfortunately this air gap effect cannot be duplicated on a larger scale. With a larger test envelope - let alone a full-size balloon - the weight of the jacket is quite sufficient to pull it downward closely against the envelope. It's only on the small scale of this test that the stiffness of the jacket material can overcome the pulling-down effect of its weight and maintain an air gap.
Anyway, we weren't happy with the porous nature of this first test envelope, so we proceeded to the second series of small-scale experiments.
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