Air tank for filling bladder

[flyyboyy]

My home compressor should be about 125 psi, but it is older and the gauges are not reliable. I am sure I get 100psi but maybe 120. Some home compressors will go to 175. The air tank I use is rated to 150. I am sure you won't fill a big kite with a 5 gallon tank at 125 psi.

I see a 8 gal one on Kijiji. Maybe that would do it?

[600]

Using the Ideal Gas Law (pV=mRT), to find required air tank volume:

air tank pressure: 100psi = 689.48kPa
if temperature is 25C = 298 kelvin
very rough estimation of bladder volume: pump volume approx. 2L x 150 pumps for big kite = 300L
pumping LE up to 8psi = 55.158kPa

find mass of air in kite:
(55.158kPa)(.3m^3) = m(.287kJ/kg*k)(298k)
m = .19348kg

for air tank:
(689.48kPa)V = .19348(.287kJ/kg*k)(298k)
V = .024m^3 = 6.34 U.S.gal = 5.28 imperial gallons

Can't guarantee this is right, but using my skills so far (second year mechanical engineer) it's the only method I know.

Might have to take into account that when you're filling up the kite, pressure is decreasing in the tank, so temperature is changing (I've assumed it stays the same as outside temperature, which would only be true if it filled really slowly)

Edit:
Actually, I'm pretty sure this isn't right because I think I've assumed all the air from the tank transfers to the kite. It would actually have the same pressure as the kite at the end (would take awhile to get there as you would know if you ever pumped up a car tire that was at 20psi with an air tank at 25 psi - takes a while to get to equilibrium) so:

(55.158kPa)v = (.287J/kg*k)(298k)

v = 1.5506m^3/kg

mass transferred to kite = .19348kg so air tank initially needs m - .19348kg

hmm not sure where to go from here.. things aren't as easy in real life where you don't know flow rate and heat transfers etc.

Can anyone help?

[flyyboyy]

Interesting post 600 ! Thanks.

Bladder uses 5 PSI max for larger kite.

As you fill the kite the temperature of the filling air would drop, so after you filled it, you would probably be wise to wait a bit to allow it to increase in temperature so you could see the pressure rise. Otherwise it would increase in temperature/expand and potentially overpressure/damage or rupture the bladder.

It would be wise to try it at home to see how much the temperature/pressure relationship is.

Cheers
Al

[FBKC]

Hey 600, I think you're over thinking it.......just assume temperature effects are negligible (good assumption based on small volumes and low pressures). I am thinking a small kite is about 150 L and big kite is 300 L, maybe that is far off, anyone know the average volume of a kite?

Therefore P1*V1 = P2*V2, just remember you have to use Absolute pressure for this to work (i.e. P1 = 114.7 psig, and P2 = 19.7), therefore:

Here the final volume if the tank is depressurized to 5 psig (19.7 psig), and then final volume minus tank volume for filling a kite:

Tank Volume (V1)_______Final Volume at 5 psig (V2)_______Useful Volume (V2 - V1)
2.5 Gal (11 L)______________64 L ________________________ 53 L
5.0 Gal (22 L)______________128 L_______________________106 L
7.5 Gal (33 L) ______________192 L_______________________159 L
10 Gal (44 L)______________ 256 L_______________________212 L

And the same results if the tank is filled at 150 psig (164.7 psia)

Tank Volume (V1)_______Final Volume at 5 psig (V2)_______Useful Volume (V2 - V1)
2.5 Gal (11 L)______________ 92 L_________________________81 L
5.0 Gal (22 L)______________ 184 L_______________________ 162 L
7.5 Gal (33 L)______________ 276 L________________________243 L
10 Gal (44 L)______________ 368 L ________________________324 L

So it seems like you really need a 10 gallon tank filled to 150 psi to be able to fill a big kite. If anyone can speak from experience I would like to hear from them.......i am interested in having an air tank in my truck for the occasional quick pump up.

[Billy B.]

I have a 10 gal tank and can fill a big kite 16-17m and a 11m to pressure with it. I have used it on ozone kites that needed 8psi, I now fly north gear and it only takes 6psi so maybe a big kite 16, and medium kite 13??

[AndKite]

This is what u need. http://www.pumpkiteup.com/

they call it a "venturi valve", basically the CO2 in this case passes a nozzle in high speed, pulling more air with it thus being able to inflate the kite with more air than the tank can hold. Should not be to hard to build one of those nozzles, just a small diameter hole to get the airflow at high speed, then increase the diameter and allow surrounding air to enter through vents, thus pulling more air in.

Or, maybe im wrong. Bring out the math!

[Nowind]

That Volo thing says it takes 30 seconds to fill a kite. As I stated before, I use a ball valve on the ens of my hose (3/8" dia hose, 1/4 " ball valve) When I open that ball valve, the kite is almost instantly (3-5 seconds) inflated. This is the beauty of the system. I used to use a regular pneumatic type trigger blow valve but is was slow.

Billie, I can only get one big 14-17M kite out of my 10 M tank. Maybe I am not filling it as much as you? But it is all my little home compressor will do.

[Nowind]

Correction 10 gallon tank, not 10M

[FBKC]

Correction 10 gallon tank, not 10M

So that makes sense, you use a 10 gallon tank at about 100 psi, and that fills your kite about once, so your 14m is probably close to about 200L volume

I like the Volo pump idea, however if you already have a comprssor, this is a lot cheaper:
http://www.harborfreight.com/11-gallon- ... 65595.html

In the end though, isn't it just way simpler (and cheaper) to pump it up manually!!!

[Nowind]

FBCK,
There is no way that anything is easier, simpler or cheaper than my 10 gal. tank assuming that: 1.) I don't have to move the tank from the back of my pick-up truck and 2.) You already own a compressor and tank (as I did).

If I have to walk any distance to set up or travel with limited room for gear, a regular pump is the way to go. But for my everyday drive down to my local spot, inflate, go. The tank can't be beat. I have had other people stand around and ask if they can fill their kite and I have to laugh and say sure, but there is not enough air left to blow out a candle.