PREPARATION FOR THE MID-SCALE EXPERIMENTS

THE TEST ENVELOPES

First, we sewed up a couple of larger test envelopes, which you can see here inflated with air:

The material we used was the same as for the envelope of the second run of small-scale experiments - "Gelvenor" siliconized balloon fabric described here. For sealing the seams, we used the same method as before - we glued strips of the fabric itself over the seams, this time both on the inside and the outside of the envelope, using Dow Corning 785 silicone rubber bathroom sealant. The final result was excellent, and the seams were completely impervious and as strong as shoe leather.

In anticipation of the production of the insulating jackets, we wanted to simplify the pattern for cutting out these envelopes. One of the difficulties we had experienced with the setup for the small-scale experiments was that the insulating jackets were rather difficult to make. Sewing up the envelope took quite a time, but at least there was only one of it. However for these mid-scale tests, we anticipated, and still anticipate, producing quite a large number of insulating jackets from various different insulation materials, and performing a lot of tests. Making jackets from twelve gores each quickly became very tedious, basically because it was a question of joining a large number of seams in order to produce a three-dimensional shape from a two-dimensional sheet. It seemed that a better method would be to use a simpler pattern, and accordingly we decided to use two cones fitted to opposite ends of a cylinder. The exact details of the pattern can be seen here. We produced two such envelopes of slightly different dimensions. As you can see, they became virtually spherical when inflated to moderate pressure.


PARAMETERS OF THESE TEST ENVELOPES

When inflated, Envelope (A) had area of 8.85 m2 and volume of 2.28 m3, and Envelope (B) had area of 9.28 m2 and volume of 2.46 m3. A steam feed connector was connected at one end of each envelope and a condensed water drain at the other.

THE TEST RIG

We then constructed a more elaborate experimental test rig, of which this is a rather crude diagram:

The rationale for the new steam distribution arrangement was as follows.

First, we realized that passing excess steam through the envelope and blowing it out of the bottom along with the condensed water, as we had done with the small-scale experiments, was not really a good idea because it courted several types of inaccuracy. Therefore we arranged a S-bend type water trap at the bottom of the envelope which allowed the condensed water to drain out while preventing the escape of steam, while as a side branch from the supply we arranged a steam vent under about 3 cm of water, which allowed excess steam to be blown off at about 300 pascals.

Second, we realized that it was necessary for the steam to be superheated somewhat, in order to ensure that it was completely dry, because even a few percent of water in the steam would distort the results. Therefore we built and installed a superheater, described later.

This photo shows the steam distribution apparatus in place with one of the envelopes hanging upon it, inflated with air (and complete with security guard):

(Note: later we built a more durable version of this steam distribution apparatus from copper piping rather than plastic. It did exactly the same job.)


THE BOILER

It was evident that our steam requirements had increased beyond the scope of the previous boiler setup, so we built a custom boiler with a water capacity of about 80 kg, fitted with four electrical immersion heater elements, nominally each 3 KW. We fitted it in a custom case and insulated it with 3 cm of polystyrene. Here is a photo:


Thus the total steaming capacity with all elements running was a nominal 12KW, i.e. about 18 kg of steam per hour. We did not install more heating elements for additional steaming capacity, although later it might prove desirable at some stage, because in any case the electricity supply in the premises we were using would not stand more than 12KW drain.

THE SUPERHEATER

The superheater was designed so that it would be able to completely dry the steam flow, assuming that the boiler was running at its full capacity of 12 KW and its output steam was about 4% wet. Therefore (fairly obviously) this meant that the superheater should be able to transfer 500W of extra heat to the steam flow. We built the superheater by enclosing a standard ceramic type 500W bathroom heater inside a 1 meter length of aluminum tube with a polished interior, along with 5 meters of 22 mm copper pipe in a zigzag arrangement, painted black. Then we enclosed this radiative heating unit in a thick wrapping of rock wool insulation and a custom case. This shows the interior of the superheater with the unit in place (an extra redundant length of the black-painted copper pipe is shown beside it):

We connected a standard dimmer unit for regulating the heater power between 50W and 500W, and installed a temperature sensor to monitor the interior temperature. We also fitted temperature sensors upon the steam input pipe and the steam output pipe.


This shows the superheater in its finished form:


Finally, this photo shows the boiler and the superheater set up ready for operation:



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