Why have dual tanks on an air compressor?

[Stephen Tashiro]

I think air compressors with dual tanks are impressive looking tools. But I don't understand why they are better than a compressor with one big tank. The only air compressor I own has one big tank. The directions say to open the drain valve (with its very disagreeable and typical tiny t-handle) after you finish using the machine.

Why have dual tanks on an air compressor?

Must we drain both tanks on a dual tank compressor?

Who sells a valve that will replace those tiny t-handle valves and be comfortable to use?

[Jason Roehl]

I think the biggest reason is probably stability. Having dual tanks allows the motor to sit low and beside the tanks, or in between them, so the whole unit is not so top-heavy. A pancake-style compressor is a little more stable than a hot-dog style or vertical-cylinder design, but I think they're still on the tippy side. My P-C twin-cylinder pretty much doesn't tip over--it's heavy, and the motor sits low.

Jason

[harry strasil]

more capacity.

[Dan Friedrichs]

Stability is the only reason.

You can replace those little screw-type valves with a piece of threaded pipe and a ball valve. I believe the fitting is a standard 1/4" NPT, so just go get an elbow and a 1/4-turn ball valve. Much easier :)

[Tony Bilello]

I think the biggest reason is probably stability. Having dual tanks allows the motor to sit low and beside the tanks, or in between them, so the whole unit is not so top-heavy.......
Jason

The dual tank units are largely used on construction sites where portability on wheels and stability are important. They are loaded and unloaded from pick-up trucks daily. Most of the dual tank units I see in use are also driven by fairly large gas engines (5HP) and put out a relatively large volume of air like in the 10CFM at 90lbs of pressure.

[Bill Keehn]

I have the Dewalt D55168 15 Gallon 200 PSI upright air compressor that I got on clearance at the borg. It's got oodles more capacity than my old compressor, which had a single 4 or 5 gallon tank at 120 PSI. Even doubling the capacity of the old compresser wouldn't bring it close to what I have now.

On the other hand the new compressor is VERY tippy towards the front. You don't want to pull on the air hose even a little bit because the thing will fall over. It's handle seems to serve as a crashbar to keep the fitting from being broken off, but even so, it's scary. I'll defintely be adding some sort of kickstand to it.

[Larry Edgerton]

Tanks need to be round. It is the ideal shape as pressure pushes on each square inch equally and perpendicularly making it retain its cylindrical shape. If you want your tank to take up a rectangular space, your only economical solution is two or more cylinders in series. I have a Hitachi that has stacked tanks, same thing.

Think of what happens to your rectangular plastic gas can when you leave it in the sun.

[Chris Tsutsui]

You should drain both tanks of a dual tank system.

Here are my thoughts on why a dual tank setup can be better.

Let's say we are testing two compressors. compressor "A" is a large 2 gallon chamber. compressor "B" is a dual 1 gallon container.

Which container has more tank surface area? The dual tank version right?

More surface area means less force being exerted per square inch on that tank.

Thus, at 150 PSI for both styles you would have less force being exerted on the walls of the dual tank version than on the single tank version.

If the tank was the weakest link in max PSI capacity (which I bet it is not), then the dual tank setups would be better.

But since the tanks can probably hold way more PSI than the compressor motor can pump then the designers probably don't choose dual tanks to increase PSI capacity, but more for other engineering reasons... heh

[Bill Keehn]

Which container has more tank surface area? The dual tank version right?
Yes

More surface area means less force being exerted per square inch on that tank.

Thus, at 150 PSI for both styles you would have less force being exerted on the walls of the dual tank version than on the single tank version.

Sorry, but I'm pretty sure that's wrong. At 150 PSI, you have 150 pounds per square inch of surface area no matter which style tank you use.


All you can really say is that the dual tank version used considerably more steel to do the same job as the single tank version. Lets suppose the one and two gallon tanks are spheres. If you took the same amount of steel used in two one gallon spheres and used it to make a new spherical tank, what capacity do you think it would be? If you said about 2.8 gallons you'd be right.

I think another factor in the two-tank compressors might be it's easier and cheaper for the manufacturer to carry one kind of cylinder to make both 5 and 10 gallon compressors than it is for them to make two kinds of tanks.

[Wes Grass]

Greater area, but the material can be thinner. So maybe a savings in weight of material but I can't imagine you'd make up the costs of fab'ing and welding 2 items instead of one.

[Thomas Syrotchen]

Chris does have an interesting point that I wonder if any mechanical engineers could help answer. While the pressure at any point is 150 psi, the total force on the cylinder wall would be less as it would be the pressure * surface area. I can't think of anything that would be a factor in a pressure vessel, but I'm curious if anyone knows?

[Angie Orfanedes]

Ah-hem. Yes, I have a degree in M.E. from a few years ago - here's my kick at the cat. The shape with the least surface area to enclose a given volume is a sphere. When you leave that shape and enclose the same volume with another shape, you have greater surface area - if you still run the tank to the same pressure per square inch, the force on each square inch is obviously the same, but since you have more square inches (more area), the total force pushing from the inside of the shape is greater. If you use two cylindrical tanks instead of one tank to hold the same volume, the same logic applies. If you run to the same pressure per square inch, the force on each square inch is the same, and the TOTAL force exerted from inside the tanks is greater, but the force exerted inside each tank is lower on a two tank system than a one tank system (given equivalent volumes), because less surface area per tank, etc.

Wow, that made my brain hurt.

[Chris Tsutsui]

Yeah I'm not a mechanical engineer and was just putting a thought out there that I think I worded the wrong way...

I was basically saying smaller containers can hold higher pressures than larger containers.

I really have no idea how the shape affects pressure and whether or not total planar surface area in a container plays a role on max pressure capacity.

[Bill Keehn]

Not an ME, but I still don't buy it. Ever seen above ground swimming pools? The smaller pools use the same flimsy metal as the big ones, because the pressure is the same. As long as a circular shape is maintained, the pools can all be filled to the same depth.

Now one thing that is different is that the big pools are a LOT less resistant to someone caving the sides in by leaning over the edge.

If I had to guess, I'd say that the reason for the small pressure tanks is better dent (point force) resistance, due to a greater curvature, which results in less opportunity for a rupture.

[Andrew Whitson]

In case anyone is interested, the stresses in a cylindrical pressure vessel are:
hoop stress = Pressure * radius / wall thickness
longitudinal stress = Pressure * radius / (2 * wall thickness)

In words, the stress resisting the end caps blowing off is half the stress resisting splittting the cylinder down the long axis. The radial stress is negligible compared to the hoop and longitudinal stresses. For any given pressure, a smaller vessel can utilize a thinner wall section. For any given wall thickness, a smaller vessel is stressed less, and will fail at a higher pressure.

The pool scenario is a bit different, being open on one end, and having a linear pressure gradient down the wall.

Cheers,
Andrew

[Peter Kuhlman]

I work in the oilfield with equipment rated up to 30,000 psi - not an engineer. The choice of a single versus double tank can be an economic choice or a structural choice. For 150 psi - that is 150 pounds of pressure on every square inch of exposed surface - a 10 inch square plate having 100 square inches will have a total of 15,000 pounds against it!!!! That is scary. Many workers have been killed on low pressure vessels and piping with large surface areas. For example, a pipe flange on a large pipe might have 500 inches of surface and with 30 psi, that is 15,000 pounds total pressure on the flange. Easily enough to kill a guy.

For the dual tank, smaller tanks with less exposed surface will have less total internal pressure so the metal can be thinner. The engineers have to figure out if that is more economical to produce than a single tank with thicker heavier metal. As others stated, there are worksite issues where having a low center of mass has great advantages for stability.

You should see the pipe we use for 15,000 psi. It is called 4 inch triple X and while not exact measurements, has about a 1" inside hole in pipe with an outer dimension of a little over 4 " - and this is specially hardened metal where any welds have to be heat stressed and all connections and welds have to be XRayed. At 20,000 psi and above, no pipe is used, only huge blocks of metal so pressure drops need to be as quick as possible off of a wellhead.

[Paul Ryan]

I am not sure I will just throw out my belief.

I think everyone else is over thinking the situation. I don't believe it has anything to do with pressure per square inch or stability. I believe it is a way for the maufacturer to produce a machine that has the same volume and out put as a larger machine. 2 10 gallon tanks would hold the same volume of air as a 20 gallon thank, but could be postioned (stacked) to make the machine more compact and more portable. The machine would produce the same as the large machine but take up less room in the back of a pick-up or in the corner of a room. Just my .02

[Bruce Wrenn]

Many places require tanks over a certain size to be ASME rated. That means HEAVY, too heavy to be portable. I can't remember whether it is fuel, or air tanks that are designed to hold 119 gallons, as the regulations and standards change at 120 gallons.

[Ron Bontz]

Ok. It has been many years since I had any statics or dynamics but I think you guys are saying it is the ratio of wall thickness to volume that determines PSI max. Given the same shape of container. Yes?:confused:

[Ed Kilburn]

Please correct me if I'm wrong, be I've been told a tank with 6" or less inside diameter doesn't need to be ASME stamped, saving a lot of cost.

[Bob Genovesi]

The last time I turned on my compressor it worked..... nuff said...

[Bill Keehn]

In case anyone is interested, the stresses in a cylindrical pressure vessel are:
hoop stress = Pressure * radius / wall thickness
longitudinal stress = Pressure * radius / (2 * wall thickness)

In words, the stress resisting the end caps blowing off is half the stress resisting splittting the cylinder down the long axis. The radial stress is negligible compared to the hoop and longitudinal stresses. For any given pressure, a smaller vessel can utilize a thinner wall section. For any given wall thickness, a smaller vessel is stressed less, and will fail at a higher pressure.

The pool scenario is a bit different, being open on one end, and having a linear pressure gradient down the wall.

Cheers,
Andrew

Thanks for the tip Andrew. After reading about hoop-stress, I think I understand now.

I think Ed's argument about the smaller tank not requiring the same level of certification is probably a big factor.