Vacuum Pumpless

[SeditiousCanary]

OK, I'm getting closer to actually trying my hand at vacuum bagging. Here is the simple question.

Has anyone ever done it with water pressure?

Seems like a five gallon water cooler bottle with the bottom cut out and a few hoses would work just dandy. Anyone here ever tried this?

[Ollie]

A column of water about 33 feet tall has about the same pressure at the bottom as the atmosphere does at sea level. So, if you make a vacuum in the top of a closed chamber with a column of water, the strength of the vacuum will be in proportion to the depth of the water below the vacuum. If the column of water is three feet deep, the most vacuum it can produce is about 1/11 of an atmosphere or about 1.33 pounds per square inch. You would want a column of water about six feet deep for bagging wood skins over beaded styrofoam and a column of water over 10 feet deep for bagging fiberglass or carbon skins over closed cell styrofoam using 14 mill mylar molding surfaces.

[gnofliwr]

Interesting idea from outside the box. But a vacuum punp can create a remarkable amount of pressure over a large area. Let's examine how much water you need to equal a vacuum bagger.

We'll use white foam as an example (blue/pink/gray can take higer pressures). White foam is usually vacuum bagged at between 7 and 10 PSI (much higher and it deforms) - let's use 8 PSI as a target. 8 PSI is convient because a gallon of water weighs roughly 8 pounds ("a pint (16oz) is a pound the world around") so we need one gallon of water per cubic inch to get 8 PSI. A gallon is 231 cubic inches so we need a column about 19 feet high (by 1" x 1") to contain a gallon. That's kind of unwieldly.

That's not going to be too effective in vacuum bagging terms; however, water might be comparable to stacking solid weights. A 400 sq" container 1-2 feet deep loaded with water would contain 20-40 gallons and weigh 150-300 pounds. That might be more managable than loading dumb-bell weights, books, concrete blocks, etc.

[SeditiousCanary]

Hmm...

The source I found indicated that:

1 PSI = 2" of Hg = 28" of Water
-and-
Atmospheric pressure is about 14.7 PSI at sea level
-and-
Hg is 13 times as dense as water

I mean I have some Hg, but not that much. Geez...

My presumption would be that with a bag and a shut off valve inline with the "water pump", you could evacuate the bag with a cycle or two of "pumping" the bag. Evacuate a sealed air tank to serve as the revisor, then connect the bag to the revisor with a regulator. I know there isn't as much vacuum created with a water system, but you aren't trying to get 30" of Hg either.

Sound reasonable?

[SeditiousCanary]

Further info...We (and when I say We, I mean myself) are talking about a wing with a volume of about 22 cubic inches.

A gallon is 231 cubic inches.
Five gallons is 1155 cubic inches.
Therefor, a five gallon water bottle would be 52.5 times the volume of the desired wing.

The aforementioned density difference between Hg and water seems to largely be offset but the huge difference in volume (in this case, 1/4 scale wings are another story).

I had thought about totally submerging the thing (I have a friend with a 10 foot deep diving well), but I couldn't do anything once it was in the water if it started to leak, let along keep it underwater easily for 24 to 36 hours. It just seemed like using gravity to do the work and a shutoff would be tons easier

Also, this will be a HiLoad foam wing core.

[Ollie]

See:
http://www.newadvent.org/cathen/14784a.htm
http://www.charlesriverrc.org/articles/foam_vac/scobiepuchtler_simplebaggingsys.htm

[RSCherry]

Ollie is correct- you need a column of water approximately 33 feet high to produce 1 atmosphere of pressure. The VOLUME of water doesn't matter, as long as it's enough to get 33' high you will have 14.7 psi at the bottom of the column.

The real problem I see with this method is leakage- it's impossible to get a perfectly sealed system, especially with vacuum. Hence, we all use a vacuum pump and disregard minor leakage.

If cost is the issue, I have heard of many folks converting old refrigerator compressors into vacuum pumps.

[SeditiousCanary]

OK, just to clarify, I have NO INTENTION of submerging the bag. This is a case of using gravity to draw air out of the bag. Water will be in a beer keg tub and a five-gallon water cooler bottle will be submerged in the tub. It should look a bit like this:

http://members.cox.net/seditiouscanary/GravVac.jpg

Yellow is the beer tub.
Cyan is the five-gallon water bottle.
Blue is the Vacuum hose.
Red is the wing cores.
White is the bag.

Now, assume there is a valve on the top of the five gallon bottle so you can seal it, open the top, sink it back into the tub, close the top, open the valve, then relift the five gallon bottle to remove more air. I know there will be some in aqueous oxygen in the water, and if you pulled enough water, it would start to give off hydrogen and oxygen, but that's much closer to 30" of Hg then I would ever get to. I'm thinking 10" to 15" of Hg. I think it would be rather easy to do with this method. Seeing that I can "reload" the five-gallon bottle, I should be able to get the equivalent of 35 feet of water/15 inches of Hg/7.5 PSI.

[SeditiousCanary]

Further that by adding a tank to hold a vacuum with a regulator....

I didn't feel like drawing all that in AutoCad. Sorry for being lazy...

[Ollie]

I listed the wrong link in post #6. Jon Stone was kind enough to point out the wrong link in my post. I have now corrected it to:
http://www.newadvent.org/cathen/14784a.htm
This is a brief bio of Torricelli who invented the mercury barometer after investigating the reason that a suction pump could not lift a column of water more than about 32 feet.

When the pressure gets down to about 1/15th of an atmosphere, water boils at room temperature. That's why a water based barometer can't quite make it to one atmosphere but a mercury barometer can.

When tapping the partial vacuum produced at the top of a sealed column of water, the air remaining in the vacuum bag will "dilute" the vacuum. The bigger the evacuated chamber before it is tapped and the less air in the bag before tapping the vacuum, the less the vacuum will be reduced by tapping it.

It would take a lot of mercury to create a vacuum chamber large enough to be a practical source of vacuum for bagging. The best configuration would be a large spherical vacuum reservoir atop a slender vertical tube about 32 feet long terminating in a tub of warer. The bottom of the tube would be shut with a valve imersed in a tub of water. A valve in the top of the vacuum reservoir would admit water to fill the tube and reservoir. Then the valve at the top would be closed followed by the valve at the botom being opened. The water would drain out of the reservoir into the tub until most of the tube remained full of water, creating a partial vacuum in the reservoir. The air and other gasses dissolved in the water would come out of solution diluting the vacuum in the reservoir along with the air in the vacuum bag after it was conneded to the reservoir. There would be a lot of practical problems to overcome to get enough of a vacuum to successfully use the system for vacuum bagging. Any leaks in the vqcuum bag would have to be small enough and the vacuum reservoir large enough to maintain sufficient vacuum for sufficient time for the epoxy to fully cure otherwise the work would be ruined.

[SeditiousCanary]

This is what I meant about not needing to pull so much of a vacuum that the water starts to separate out into hydrogen and oxygen. My plan was to have a shutoff at the bag and draw the vacuum, then shutoff the bag. This would prevent the water from separating out into it's principal components. The other advantage is the quantity of air and the ease with which you could suck out of the bag. I'm not aiming for more then 20 inches here, just enough to get a set of wings out of it/ If you think I can't do it, that's OK too, but I don't think the bag will leak enough to cause major problems. I do plan on watching it rather closely for a full 24 hours. I think the longest it will go unattended is four hours. I would make sure the air tank and regulator were in good working order first to have extra insurance.

[Ollie]

When water evaporates it becomes a gas that still consists of water molecules. It does not dissociate into its component atoms of oxygen and hydrogen.

[gnofliwr]

I mis-understood the mechanism you were proposing. I think you solution has merit. I think the problem you're going to have to over come is leakage in the vacuum bag. Your system is going to have to hold a vacuum for several hours - my experience with a pump and one way valves and a one gallon tank is that the pump runs for 10-15 seconds every 5-10 minutes. Some of that leakage is in the valves, but even pinching all of the lines, the vacuum still drops and the pump needs to run.

- Roger

[Ollie]

If the vacuum reservoir had a volume of about 50 to 100 gallons it might work very nicely. However the cost of such a reservoir and the problem of putting the tub three stories below the reservoir calls into serious question the economics of such an arrangement and the problem of siting such a device.

I think it would be easier to put the time and effort into a second part time job to earn the money to buy a good continuous service vacuum pump. I suspect that they become available on E-bay for less than $150 from time to time. About six four hour shifts at some place like McDonalds would cover it and be a lot quicker and easier than building a contraption.

[chemsurfer]

Originally posted by Ollie
....I think it would be easier to put the time and effort into a second part time job to earn the money to buy a good continuous service vacuum pump. I suspect that they become available on E-bay for less than $150 from time to time. About six four hour shifts at some place like McDonalds would cover it and be a lot quicker and easier than building a contraption.

I found a cheap pump on e-Bay that works just fine... It's not continuous service, but I have a vacuum switch (cost about as much as the pump) to turn off the pump at the pressure I select.

My pump also will not re-start under that heavy a vacuum, but I managed to find a used electric valve that will release the vacuum to let the pump restart (there's a great used electronic junk store here). The bag is isolated from this by a check valve so that it doesn't lose it's vacuum.

I also built a vacuum reservoir out of Black ABS pipe.

My total cost for the system, about $70-80.

Brad

[Dano13]

I am not familiar with what SeditiousCanary is actually trying to do, but wouldn't a plain old mechanics hand vacuum pump work?
Unless you needed something like we use for A/C work to remove all moisture and contaminants within the system, then it must keep a constant vacuum for several hours...

In response to someone else's comment about using refrigerator compressor/pumps, they work excellent. I have built several over my 30 years of working on vehicles and never spent more than $30 to make one. A couple were actually free. The only problem with these "poor man's vacuum pump" is that you need (MUST) give it some type of lubrication at start-up and and after disconnect from whatever you are doing with it, then cap the fittings. These "pumps" are not designed to be open to the atmosphere in any way.

Dano13

p.s.
Feel free to PM me if you would more info on the "poor man's (refrigerator) vacuum pump". I'm not sure the discussion of it belongs in this forum or not.... :)

[Ollie]

Hand vacuum pumps are low cost, low volume and capable of high vacuum. It's the low volume that may give difficulty. Some people that can get consistently good bag seals use them. One of the by products will be the development of a really strong handshake. Tiny leaks require vigorous pumping every few minutes for as long as it takes for the epoxy to cure. See:
http://www.charlesriverrc.org/articles/foam_vac/scobiepuchtler_simplebaggingsys.htm
I tried it and desided that it wasn't for me.

[chlee]

Has anyone tried this setup? The reservoir could be on the second or third floor of a house, and the end of the hose could be at ground level. After a siphon action is set up, a valve at top of the hose arc is opened to the vacuum bag. Air from the bag will bubble into the water flow due to low pressure in the hose; as the bubbles in the flow get smaller, the pressure at the bag will approach (rho)(g)(h) below atmospheric pressure, where (rho)(g) is the specific weight of water and h is length of the hose below the water surface.

The water in the reservoir could be periodically refilled -- or refilled via an upstairs faucet.

Interestingly, I think that if the hose length is long enough, the pressure at the top of the hose arc will go below the vapor pressure of water and the water will "boil" (cavitate).

- Chung

[SeditiousCanary]

Originally posted by chlee
Has anyone tried this setup?

Great idea, but I live in the desert and a single story house. I don't have the height and my neighbors would rat me out to the water company for wasting water.

[Ollie]

Chung,
I think you need to imerse the discharge end of the pipe in a tub of water to prevent air bubbles from entering the pipe and floating up to the valve and entering the bag. Also, what keeps the water from entering the vacuum bag? Why not connect the vacuum bag directly to the vacuum in the reservoir?

[chlee]

My drawing above is just a variation on a chemistry lab technique to create a partial vacuum to pull liquids through filter paper. A T-tube is connected to a faucet nozzle in this orientation ||= with the bottom arm connected to a long hose and the side tube connected to whatever requires suction. The faucet is turned on, and air is drawn in through the side tube. The method above is the same thing, except you use siphon action from your own upper-story water bucket rather than the hydraulic head from the town water tower to start the flow.

An advantage of the illustrated method is that it doesn't require a huge reservoir. Since you're actually removing air from system rather than just stretching its volume, large amounts of water aren't required. You could also catch the outflowing water in a bucket at the bottom to avoid wasting water, and then periodically haul it upstairs to run it through again.

Ollie, I'm not quite sure why bubbles would come up from the end of the hose, but I'll think about this. The device is just a siphon tube with a small amount of air leaking in from the top. But since you'd want to collect the outflowing water anyway, immersing the hose end is no problem.
I also don't quite understand why water would enter the bag: the pressure at the T-connector should always be lower than the bag. But I suppose a one-way valve would be useful here to prevent accidental backflow and to keep water vapor from entering the bag.

Finally, after all my verbiage, I'm inclined to agree that this is not really worth the effort. I went to a local repair store and got an old vacuum pump and hooked it up with relay, switch, gauge and reservoir for around $60. It will hold around 20" Hg for several hours after a ten-second motor run.

- Chung

[Ollie]

Chung,

Thanks for the explaination. I think I understand a little better now.

[Ragged]

Have a look at our pump kits at www.roarockit.com there is a good review at http://www.rcgroups.com/forums/show...ght=vacuum+pump
on the Etap system.

[steve lewin]

If you're going to resurrect these old threads I guess they should now be in the Composites forum.

Steve

[RMD72377]

I used a pump from a refrigerator and a 11 gallon portable air tank. I had most of the fittings and the gauge. The most expensive thing was the switch which I got from ACP for 40 dollars

[Mighty Mik]

I have an old Sears air compressor that has a tank like that, and a Kryotek computer case, which has a refrigerator pump in it i'll repurpose. What are the 2 objects in the inlet line? One looks like a filter, the other white thing is...?

[RMD72377]

the thing that looks like a filter is a check valve and the white thing is a vaccum tube reducer. The tube thickness on the pump was much fatter than the tube size for the valve . Both are available at auto parts store. I got the check valve from ACP when I orderd the switch.