[balloon-makers] H2O inside a Balloon Envelope

Thomas Goodey thomas at flyingkettle.com
Mon Apr 30 08:01:09 CDT 2001


> I ain't much of a chemist, but I calculate that the amount of
> water vapor generated by a balloon burner is, by weight, about 1.45
> times the weight of the propane burned.  

I think a bit more, but as a ball-park figure, one and a half times.

> As a hot air balloon
> flight progresses, more and more water vapor is introduced by the
> water vapor generator we usually call the burner.  If the balloon
> is using, say, 15 gallons (63.75 pounds) per hour, then about 92
> pounds of water vapor is added to the envelope each hour.  This
> would be about 12 gallons! 

That 92 pounds of water vapor gives a lift of about 90 pounds, in 
principle. Which is a lot better than hot air (that's the principle of the 
Steam Balloon).

But it doesn't all remain in the envelope. Once a hot-air balloon 
envelope is inflated fully, the injection of more gas (like hot 
combustion products) means a loss out through the open bottom of 
the envelope of an equal volume of gas. Not only does this gas 
carry away heat, which is wasted, but it also carries away some of 
the water vapor, which was previously added into the envelope.

Of course the humidity in the envelope does build up gradually, but 
you can't rely upon ALL the water vapor staying inside.

> The good news is that this water vapor makes the air in the
> envelope less dense, or "lighter", than it would otherwise have
> been, giving us more lift than we would expect.  Neat, eh? 

That's certainly true. It's a bit like the "bulles d'orage" which the 
French make (described on my website).

> However, I have from early in my ballooning career noticed that
> envelope temperature always rises in the early part of a hot air
> balloon flight.  

You mean, temperature at the top, I suppose.

> This would indicate a loss of lift.  

I don't quite get this.....

> I can only
> hypothesize that the level flight temperature rises because a
> temperature differential develops early in the flight between the
> air near the bottom and the air higher up.  Usually a hot air
> balloon takes off with the air within well mixed due to all the
> heating that has just occurred to inflate the balloon and make it
> hot enough to fly.  As the flight continues, with its relatively
> infrequent heating, the air at the top of the balloon is probably
> hotter than the air near the bottom.  This would explain the
> apparent rise in "envelope temperature" during the early part of
> the flight. 

This seems to make sense.....

> One of the lessons in all this is to remember that we measure
> temperature at only one location in the balloon envelope, usually
> high up, in order to avoid damage to the fabric from overtemping. 

That has historically been the reason for temperature monitoring, 
and it is very valid.

> I think most of us tend to assume envelope temperature is always
> constant throughout. 

Of course it cannot be anything like uniform. There is both a vertical 
layering effect, with the air at the top of the balloon hotter than the 
air at the bottom, and also a core effect, with the air in the central 
parts of the balloon hotter than the air nearer the skin. How do 
these two interact? wellllll ...... nobody seems to know the details.....

> I've been thinking about building the instrumentation to suspend
> in a balloon envelope to allow reading temperatures at many
> different heights in order to try to gain a better understanding of
> all this. 

This would be very interesting, and would cast a lot of light on a 
very obscure subject which is of the greatest importance for hot-air 
ballooning. Surely someone, sometime, has already done this?

Thomas Goodey



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