How Useful is a 7 Gallon Air Tank?

It were me, I'd use a surplus 20 pound propane bottle. Remove the valve, leaving a 3/4" NPT female hole, get a 3/4" NPT plug, drill & tap in two places 1/4" NPT, thread in an air fitting with check valve on one side for filling, and thread in your hose on the other side.

GWE

This is pretty much what I do. I've got a two-jug oil-less compressor that is like yours. "Noisy as hell" as you say.

I've got it plumbed into a bunch of oddball tanks in the crawl space, namely one of those seven gallon air pigs, a propane tank which was purchased *new* because it was on sale for a price I could not turn down, and some odd ww2 surplus oxygen bottles. They're in the crawl space so if they detonate nobody gets hurt. :^)

I've got an ASME hard-seat blow-off on the manifold for 150 psi, and a check valve type pressure relief set at 125 on the compressor. I put a redline at about 110 psi on the manifold gage. For random tire inflation and shop cleanup I have to run the compressor about every four or five days. It's important to valve off all air outlets hard with a system like this or you come back after a day or two and the gage reads zero.

I bought that compressor about 18 years ago with the idea in mind that it wouldn't last very long, but long enough to tide me over. The darn thing just won't quit.

Jim

On Sun, 03 Sep 2006 22:03:08 GMT, with neither quill nor qualm, snipped-for-privacy@alum.mit.edu (Doug White) quickly quoth:

If you regulate it down to 30 psi (OSHA says that's "safe" ;) it should outlast the duster by several months.

I operated with a pair of those smaller (5 gal) tanks and a 3/4 hp Ingersol Rand oilless for about a decade. (It's a noisy little bastard.) You can bump 'em up to 125 psi without fear since they're rated at twice that. I hook both up to my large compressor now if I need more capacity, like whole-shop, open-door, full-pressure dusting. I'll also use them for painting now that I bought an HVLP gun.

They're really handy to take where there is no air, too. Muddin' offroad, etc. Get one. They're cheap ($25ish) and DO come in handy.

Newp.

I would not bet on that! Ever see the damage an air tank can do when it ruptures? I saw pictures of damage caused by one failing a few years back, it took a 20x 20 foot section of roof off the building and a similar sized part of the concrete block wall! I do admit it was a larger tank, somewhere around 200 gallons. I would not want to be standing on the floor above the seven gallon pig if it blew! Greg

There's a water separator on the feed line, and I blow the tanks down regularly to remove water from the bottoms. There's an ASME hard seat valve and the tanks have been substantially de-rated. There's also a lower pressure blow-off to limit the delivery pressure from the compressor.

But they're stored in location that's pretty much bomb-proof. Brick on all sides except the front. Your concerns are valid but they could likewise be said about *any* air compressor tank. The ultimate solution is of course to have a brand new ASME certified tank delivered to your house each week. :)

New is always better.

Jim

No kidding. Fortunately you could not, there is no floor over it that you could stand on.

I'd be more worried about the similar one I keep in the garage for topping off tires. No barriers around it at all.

Jim

Thanks for the comments.

I have determined that the vapor pressure of the stuff in the dusters is around 83 psi at room temperature. That's considerably higher than I expected. Maybe that's overkill, but I have to wonder if regulating down to 30 PSI air is going to do nearly as good a job as the dusters do. I've been toying with analytical comparisons between the two approaches, as much out of curiosity as anything. If a duster has 10 oz. of 83 PSI stuff, how does that compare with 7 gallons of 100 PSI air? Is the fact that the Tetraflouoethane gas from a duster is 3.25 times heavier than air make a big difference in it's "dusting" effectiveness? The duster will put out 83 PSI until the liquid is all gone, which gives it a bit of advantage over compressed air.

For example, how much does 7 gallons of 100 PSI air weigh? According to one web site, 100 cf of air weighs 7.5 lb at 1 atmosphere. 7 gallons is just under a cubic foot (1 cf = 7.48 Gal), and 100 PSI is about 6 atmospheres, so the tank will be holding roughly 6 cubic feet of air. That comes to around 7 ounces, which is much more than I expected, and not that far off from 10 oz. in many dusters.

Looking at gas volume, 7 gallons of air at 100 PSI is about 43 gallons at

1 atmosphere. Duster gas is 0.235 m^3/kg, which translates to 0.067 m^3 for 10 oz., or only 17.6 gallons of gas at 1 atmosphere. This makes it seem like the air tank is a big win.

Time to buy a tank. The reason I picked 7 gallons is that there is an aluminum one that several places sell that is both light, and presumably rust proof. I think a regulator is a good idea, so I need to look into that next. I'm sure the Harbor Fright & Northern Pathetic catalogs have something cheap, but I'm more than a bit suspicious of their stuff.

Doug White

I think the formula for computing force on a sail (be it 100 square meters of mains, or 1/1000 square millimeter of dust) is not dependent upon the mass of the gas -- that the force will increase linearly with the mass, but at the square of the velocity.

so... (if I recalled that correctly), at the same velocity, the TFE would be

3.25 times more effective at moving a "sail". I don't think I know enough to know how much velocity you're going to get from the little can's straw vs. your air gun nozzle, OR how much 'practical cleaning' you can do with a .5mm i.d. straw vs. a 1/4" i.d. air gun.

LLoyd

Yes, if only so you can stretch your seven gallons out a bit. Without one, you tend to use more when the tank is full. Once it drops below 30 psi though you'll need to crank the regulator up a bit.

Jim

On Mon, 04 Sep 2006 17:20:10 GMT, with neither quill nor qualm, snipped-for-privacy@alum.mit.edu (Doug White) quickly quoth:

--snip of my comments--

I use a (non-OSHA) rubber-tipped blow gun for getting into tight spaces. It works well even at low pressures.

OK.

Interesting. The smaller tanks (probably closer to 5 gallon) I have will fill one completely flat tire to 35psi and another to about 15. Or, so I hear after loaning it once, it'll fill all 4 tires from 5 psi up to 20 psi after a mud stickage.

Indeed!

HF has one FRL on sale for $20 now and they're alright. For simply blowing chips and dust around, you don't have to have a NASA- or scuba-approved $3,000 regulator, pard. ;)

A lot of people get complacent about eye protection when blowing, so the ER gets lots of folks in there from doing something they know better than to do. I hope you're not one of them. Chips are real eye-eaters.

Just use a tiny nozzle on your 100 psi 7 gallon tank. I have one of those and I'll pump it up to 125psi. That will run an airbrush for a really long time. If I try to blow stuff off with a normal nozzle from that source it's good for about 30 seconds. Why don't you vacuum up the chips? Brush them into a pile and suck 'em up.

I do use a vacuum, but especially if I've used cutting fluid, there's always a thin layer of swarf stuck to things. If I've used a lot of coolant, I'll frequently mop that up, but if I've been sparing, I can get rid of what's left with a quick blast of air. I'm mostly interested in making sure there are no chips between the vise jaws that might interfere with setting pieces on parallels & such. As such, I'm only trying to clean a really small area.

Doug White

THat's OK. Don't think I didn't do a cost/benefit analysis of those tanks. That's why they're in the bomb-pruf basement. The only thing that gets it when they detonate is my poor phase converter!

Jim

Yup. That's the route I did before I got a larger compressor.

Another option I haven't seen mentioned...

Get a nice-sized CO2 tank. In addition to all the normal "air" usage, you might use it for MIG, making seltzer (or soda or beer), or putting out fires. Each refill (or exchange) will cost a few $, but since CO2 is liquid in those tanks, the capacity is high for their size compared to compressed air.

sdb