As you know if you have been reading my Practical Airship Design series, the airship I am describing/designing uses low pressure steam as the lift gas. While not anywhere near as powerful a lift agent as Hydrogen or Helium it has several advantages that I have discussed in that series here and here.
I have decided to do a some quick experiments for myself to see how steam worked as a lift gas. Nothing fancy really just a quick check on how steam behaves at low pressure in an enclosed space like our airship's lift bags.
In order to test this I needed a few items:
1) A source of low pressure steam of fairly large volume.
2) An envelope to contain the steam.
For the Steam source I decided to use a small 1/2 liter electric kettle, one that has no automatic shutoff just a whistle to tell when it is boiling. It would keep boiling until it was dry if I let it.
I thought finding an envelope would be a bit tricky as it has to be light enough to show if there was any lift available, but not be too sensitive to the temperature of the steam (100c at sea level of course but about 98C at our elevation). I wasn't sure how most light weight materials would handle the heat.
The lightest materials I had on hand were Safeway shopping bags and green garbage bags. These are plastic of course so I was concerned that they would potentially be damaged by the steam. Easiest way to test was to put a chunk of each in boiling water and see what happened.
Making sure my wife was out of the kitchen I filled a saucepan with water and got it boiling furiously on the stove. Then I immersed the samples, and after a few minutes I found that surprisingly they were completely unaffected! They didn't stretch or show any signs of being softened by the boiling water. They also didn't seem to give off any smell as a result of being boiled (so I escaped the kitchen and my wife unscathed). I decided to use these as the test envelopes for my lift experiment.
The first "proof of concept" experiment was to simply fill a shopping bag with steam from the kettle and see what happens. Anybody who has blown air into a shopping bag with a fan knows that the bag will expand as the air is pushed into it. In my case I wanted to see if the steam was doing any lifting not just expanding the envelope by pressure. How to do that?
The trick is to leave the bottom of the bag open to the atmosphere. That insures that there can be no over pressure inside the bag to hold it up. If the bag stays "inflated" even when it is open at the base then the inflation must be the result of the lift from the steam.
I fired up the kettle in the back yard and when it was boiling I placed the shopping bag over the kettle. I collapsed the bag into a long package first to exclude as much air as possible. As the steam beagn to flow the bag began to fill and lift the surface until it was nearly inflated! The bottom was open and the steam was not coming out of the bottom yet, which showed that there was no pressure building up in the bag.
I expected to see a lot of condensation of the steam on the surface of the bag, but there was very little during the few minutes of my test.
Next I did the same thing with the green garbage bag. This showed the same behaviour, the bag filled and extended even with the bottom open to the air. The garbage bag is much bigger of course and it was able to hold itself up by the lift of the steam. While I was doing this a slight breeze came up which kept collapsing the bag so I was not able to fully inflate it.
Again there was very little condensation on the inner surface of the bag. I'm not sure why that is unless the bag, by heating up to nearly the temperature of the steam, prevents much condensation.
My next set of experiments should be interesting. I will try the garbage bag again inside and see if there is sufficient lift to actually lift the bag from the ground! I will also measure the temperatures of the bag surface and the steam inside. Allowing the bag to cool by removing it from the kettle will show how fast the steam condenses.
Stay tuned for part 2 coming soon.
Keep your sightglass full, your firebox trimmed and your water iced.