H2O For A Lighter Than [Dry] Air Balloon

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Back when I was living in SE [95% r h humidity] swamps I got this idea:

Get some filmy material like dry cleaner bags, make a balloon, insert a few drops of water, set it in the sun to evaporate and fill the balloon.

A g-mole of H2O is 18 grams, dry air 29. In the desert SW the balloon would rise.

Well, I've been out here in the SW for a couple of years and I STILL haven't tried the H2O balloon idea.

Bret Cahill

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----- Original Message ----- From: "Bret Cahill" Newsgroups: sci.engr.mech Sent: Thursday, March 24, 2005 2:59 AM Subject: H2O For A Lighter Than [Dry] Air Balloon

Let's assume the air is at 30 C, the evaporation stops at 100 % humidity,

at 30 C you have 6% by volume water,

with the outside air dry and 100% humidity in the balloon gives lift per cubic metre of air in the balloon of

(( 29 - ( 29 x 0.94 + 18 x 0.06 ) ) / 29 ) x density of air

density will be about 1.3 Kg / m^3

lift is about 0.03 Kg / m^3.... 30 grams per cubic metre is not a lot of lift :-)

Jonathan

Barnes's theorem; for every foolproof device there is a fool greater than the proof.

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No dry cleaners round here either. Maybe you can get some mail order? Best of Luck - Mike

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Actually, looking at some tables with physical properties of dry air, we can see that the density of dry air at 30 degrees C is 1.165 kg d.a. /m3 (d.a. = dry air).

We also have 1.116 kg d.a. /m3 w.a. (w.a. = wet air = 100% humimdity) and 0.02733 kg H2O /kg d.a. (at 30 C)

Therefore, in one m3 of wet air we have 1.116 kg of dry air and

0.02733x1.116=0.0305 kg of water, which makes a total of 1.1465 kg w.a./m3

Therefore the lift will be approximately 0.0185 kg/m3 which is a little less than 20gr... The classic birthday-party-balloon should weigh about 10gr. If we could blow half a cubic meter of wet air in it (about 200 times more than it can take) and take it to a very very dry place, then you'll have your balloon!

But its a good thought anyway.

S.D.

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We're good for more than 30 C. In fact, we'll be doing better than FORTY (40) C in a few weeks.

Every time you breath here you are losing water.

The governor is screaming bloody murder because the forest service is overhauling all its fire fighting tanker planes while a lot of Johnson grass is drying out in the desert.

Know what happens to a Sahuaro full of water when a brush fire hits it?

Popcorn.

Bret Cahill

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Wouldn't a balloon full of air with water vapor be *heavier* than dry air? You're taking a balloon full of air and adding water...how does that make the contents lighter? The water may evaporate, but there is still conservation of mass.

Don Kansas City

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don't "fill" the balloon with air. As it gets hotter, the water vaporizes, increasing the volume of the balloon, but not the mass, and fills the balloon.

The OP's suggestion is that, once vaporized, a mole of water and a mole of air take up the same volume, but the water weighs less

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Soap bubbles might be worth a try but you can be certain they won't last long when the humidity is in single digits.

Bret Cahill

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Dear Bret Cahill:

The thing I didn't see addressed is that all the water in the balloon will not evaporate. It won't do this until ~ 100 deg C, when the partial pressure of water reaches 14.7 psia. Pretty hot for a hot air balloon, you'd need insulation, and altitude would only make the problem worse.

David A. Smith

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The balloon would be partially filled with excess dry air to evaporate the water.

The summer dash board temperature report is generally over 65 C. A well designed greenhouse balloon -- maybe a black bottom or black sheet inside -- might beat the dashboard temp.

Scaling up definitely helps. On a windless day it would be easy to roll a really big bag out on some asphalt to inflate.

Nevertheless the material will still need to be very thin to work.

Bret Cahill

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Water has to have something to evaporate *into*. The only way to make it turn into water vapor without any air present is to *boil* it, which requires temperatures over 212 F at normal atmospheric pressure. And this temperature would have to be maintained as long as the balloon is inflated. This is high-school physics, guys. Aren't there any real ME's in this group anymore?

Don Kansas City

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Looking at the 150 degree F line on a psychrometric chart:

0.043 lbs of of water / lb of air at 25% r. humidity and 0.0214 lbs/ lb at 12%, 0915 lbs of water per lb of dry air at 50% r. humidity.

This looks linear so:

100% humidity => ~ 0.18 lbs H2O/lb dry air @ 150 F.

The difference in mole weight ratio of water vapor to air is (29-18)/29 = 0.38.

Neglecting the water in the dry air -- we get into the single digits r.h. in May & June -- the difference in weight of humid air to dry air per lb of air is 0.18 X 0.38 = 0.068 lbs/ lb.

A cubic yard of dry air is ~ 1.7 lbs, so, assuming a cubical shape, six square yards of material would need to weigh less than 0.11 pound or

1.8 ounces total or less than 0.31 oz per sq. yard for a 3' X 3' X 3' balloon.

A dry cleaner bag is about 12 ft^2 or 1.25 yd^2 and weighs an ounce ~ .8 oz/yd^2 so the volume needs to be at least 17 cubic yards using dry cleaner bags.

Something with a > 7.5' side would work.

Bret Cahill

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Dear Bret Cahill:

You won't get the lift you get from adding a small amount of CO2 and H2O to an air-filled bag... namely a hot air balloon.

David A. Smith

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Black trash sacks lift quite nicely, when warmed in bright sunlight. Apparently

Brian W

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You let the excess air out when the water evaporated - moist air is in fact less dense than dry air.

Brian W

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Everything has a vapor pressure. Water at ambient temp has a vapor pressure that can displace some air. It's measured in RH. This is high-school physics, it's true.

?

Brian W

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How do you do that? Your balloon has a finite, fixed volume. That volume is initially full of dry air, which has a definite weight. You add a mass of water. The volume can't change. How can this ever result in less mass?

Don Kansas City

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The poster I was replying to was proposing that we put water alone into the bag. Without air there can be no evaporation. It will remain a liquid forever.

Don Kansas City

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. . . .

< Black trash sacks lift quite nicely, when < warmed in bright sunlight.

Sounds easy; I'll give it a try. I have the thin filmy dark bags that rip open with more than two sweaters and the sun here will break down asphalt in weeks. This will be easy to demonstrate, but from my calculations, the sun will need to warm the air inside more than 70 F higher than ambient for a 1 m^3 bag to fly.

But if you could only hope for a 45 F temp rise due to passive solar then you'ld need a hot dry day and 100% humidity inside for flight from a small ~ 1 m^3 bag.

Without the water the bag would need to be over 300 cubic feet because the difference in density due to a 25 C temperature difference decreases the density by about as much as the humidity and you get about the same numbers as above when only the humidity was considered.

Overlooking the temperature difference was a big oversight but it's in favor of the balloon: It reduces the size by an order of magnitude -- a garbage bag.

Some other minor errors:

1. the weight of the dry (5% r. h.) air at 40 C needs to be used. That's 1.9 lbs/yd^3 not 1.7

This reduces the volume of the balloon by 30%.

1. the weight of the dry cleaner bag is 0.7 oz, not 1 oz.

This reduces the size of the balloon 60%.

1. The air isn't completely dry in June. It's about .005 lbs water / lb air. This is a negligible increase in the size of the balloon
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. . . .

< Black trash sacks lift quite nicely, when < warmed in bright sunlight.

Sounds easy; I'll give it a try. I have the thin filmy dark bags that rip open with more than two sweaters and the sun here will break down asphalt in weeks. This will be easy to demonstrate, but from my calculations, the sun will need to warm the air inside more than 70 F higher than ambient for a 1 m^3 bag to fly.

But if you could only hope for a 45 F temp rise due to passive solar then you'ld need a hot dry day and 100% humidity inside for flight from a small ~ 1 m^3 bag.

Without the water the bag would need to be over 300 cubic feet because the difference in density due to a 25 C temperature difference decreases the density by about as much as the humidity and you get about the same numbers as above when only the humidity was considered.

Overlooking the temperature difference was a big oversight but it's in favor of the balloon: It reduces the size by an order of magnitude -- a garbage bag.

Some other minor errors:

1. the weight of the dry (5% r. h.) air at 40 C needs to be used. That's 1.9 lbs/yd^3 not 1.7

This reduces the volume of the balloon by 30%.

1. the weight of the dry cleaner bag is 0.7 oz, not 1 oz.

This reduces the size of the balloon 60%.

1. The air isn't completely dry in June. It's about .005 lbs water / lb air. This is a negligible increase in the size of the balloon

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