2-liter Coke Shower

[Inquisitor]

This will definitely get me a Space Cadet vote!

Givens:
Designing a 40’ Cat.
Some spaces are to be sealed up for unsinkable specifications.
The liquid foam weighs 2.0 lbs/ft^3.
I want to pressurize the fresh water system in the boat.
I’ve seen airplane modelers used 2-liter Coke bottles as pressure tanks on their planes.

Could I use 2-liter Coke bottle in these sealed spaces, foam around them, rig tubing to them and pressurize them. I would think they would…

Be of less total density than equivalent foam.
Be a whole lot less weight than a steel home pressure tank.
Take advantage of a space that can’t be used for anything else.
And make the faucets/showers run a lot better than the pid’ln electric pumps.

… and if so… does anyone have a clue about their bursting pressure?

I might just have to dig a whole in the back yard and start using the air compressor on a couple every week. Maybe the neighbors won’t complain too much about the cannon’s going off.

[bazza]

pressure tanks have a rubber bladder in them. the bladder has air in it. when the water enters the tank under pressure that is greater than the air in the bladder the water pressure shrinks the bladder the as the water pressure drops the bladder re expands keeping pressure on the water. simply filling coke bottles with air wont work as after a period of time the air dissolves into the water and you end up with a lot of small water tanks that you cant get your water out of.

[cyclops]

What amount of cubic feet of air, at what pressure, do you want to maintain, for how many gallons of water used?

[marshmat]

By all means, give it a try. Test them with the air compressor first of course, see what they can take before they leak or blow, then pick a good safety factor.

If you keep each bottle at about 1/2 air, 1/2 water you will get a system that can put out a nice water pressure for some time. (Too much water in each, and you'll run out of pressure all the time and have to run the air pump.) Grade 7 kids do this in science class all the time.

Given the info that Cyclops asked for (but in metric of course!;) to make things easier) it's pretty easy to figure out the parameters of your system.

[gonzo]

Last night I was watching in "Myth Busters" and they pressurized three liter soda bottles to 140 PSI to make rockets.

[trimarandan]

I'm actually doing the same thing for my boat. the burst pressure wont be much of an issue, thoes bottles hold alot but i'm trying to make my tank out of PVC, if i can find a pipe with a big enough diameter to make it economical I'll just cap both ends and i have a tank. the regular PVC pipe is 'schedule 80' i believe its rated at around 280 psi.

[Inquisitor]

More out of curiosity, did you burst any or get a burst pressure somewhere? Even if they don't work for pressurization, they'd have less density than foam.

[trimarandan]

actually i have presurized a similar unit made the same way with up to 200 psi even considering I used PVC cement to glue the caps on, it held it like a champ. Remember PVC is the same stuff you would use to make a spud gun so i have no doubts that it would hold more than enough water pressure for a shower and a couple of sinks. also you wont get any bulging like you would with a 2-liter bottle, even if the bottle wont explode it'll swell up on the sides. what size of of water pipe are you thinking of using to supply presurized water to the sinks? with a larger diameter pipe you would require less pressure, and by having the tank closer to the sink itself would also decrease the need for high pressure. My only concern with this tank is finding a way to make the tank big enough to accomadate your water consumption.

[Inquisitor]

I still would like to see about the coke bottles mainly because
· they're free
· they reduce the density in the “unsinkable” areas

I'd test them in the open to get burst strength for them independently. Your point about expansion is important. I’ll get a large cardboard box, putting several bottles in, then poring and curing foam and then pressurizing. Have to see if the foam would eat the bottles. See how expansion cycles affect that.

But I would only be using them for their air volume/pressure that would ride on top (figuratively, not literally) of the water in your “PVC” Tank. Thus the water tank(s) could be filled to near 100% of water and thus could be smaller.

[trimarandan]

by using more than one air tank (bottle) you could get more consistant pressure, do you know what pressure you'll be runing at?

[Inquisitor]

I think that depends on what I can reliably expect from the bottles. Pressure vessels ususally run with pretty high safety factors (like 10+). I hope to get to the low side of what runs in a house... whatever that is.

[trimarandan]

at best i dont think you'll be exceding 80 to 90 psi. if you keep the presurizes water lines big enough you wont even need that much.