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View Full Version : Why have dual tanks on an air compressor?



tashirosgt
2009-Jun-14, 01:12 PM
I find my idea in the thread on passive radar detection much more interesting than the discussion of warning labels on extension cords in another thread. However, given the the enthusiastic confusion that arises from talking abouit extension cords, I can't resist asking the following:

Why have dual air tanks on an air compressor? I think air compressors with dual tanks are very impressive looking. However, I don't understant why they are made. What are the advantages of having two small tanks instead of 1 big tank with the same volume?

clop
2009-Jun-14, 01:24 PM
Well how topicist.

Are these two tanks connected together or is there a valve, filter or regulator between them?

tashirosgt
2009-Jun-14, 01:32 PM
On dual tank air compressors found in my local hardware stores, the two tanks are directly connected without any regulator or valve between them.

One nuisance feature of air compressors is that you must drain moisture from the tank(s) after using them. On some dual compressor designs, one tank is above the other and only the bottom tank has a drain. When the tanks are side-by-side, they both have drain valves.

clop
2009-Jun-14, 01:39 PM
On dual tank air compressors found in my local hardware stores, the two tanks are directly connected without any regulator or valve between them.

One nuisance feature of air compressors is that you must drain moisture from the tank(s) after using them. On some dual compressor designs, one tank is above the other and only the bottom tank has a drain. When the tanks are side-by-side, they both have drain valves.

Yes because the moisture in the air condenses when you increase the pressure. Now that I think of it the air compressors on air-brake lorries have more than one tank. When I used to drive them I remember having to bleed two or three tanks depending on the vehicle.

Trebuchet
2009-Jun-14, 02:47 PM
I'd guess the dual tanks are simply because the manufacturer could easily add the second tank to a single tank model and create a larger version. They may have found this more cost effective than a single tank, had a large stock of the single tanks, or some such. It may even just be marketing, because the dual tanks look cool as mentioned above. I can't see any practical advantage to it, and there are probably some disadvantages.

TJMac
2009-Jun-14, 03:23 PM
I think, other than the plumbing involved, there is no disadvantage to going with two smaller tanks vs. one big one. Advantage is being able to get a lower wider profile, which can be an advantage at a construction site, or even in your garage, depending on where you are using it.

That's pure guesswork on my part, not some sort of special pneumatic expertise. :lol:

TJ

grant hutchison
2009-Jun-14, 03:31 PM
I imagine several things might be involved.
1) Laplace's law (http://hyperphysics.phy-astr.gsu.edu/Hbase/ptens.html). You can contain the same volume, under the same pressure, but with reduced wall tension, if you place the gas in a couple of smaller cylinders rather than in one large cylinder. Seems like that might be beneficial in terms of materials technology.
2) Failure mode. If one tank blows the other will empty through it relatively slowly, spreading the energy release over time.
3) Manoeuvrability. A flat array of two cylinders can be easier to move and store than one long or fat cylinder.

Grant Hutchison

PraedSt
2009-Jun-14, 04:53 PM
I imagine several things might be involved.
1) Laplace's law (http://hyperphysics.phy-astr.gsu.edu/Hbase/ptens.html). You can contain the same volume, under the same pressure, but with reduced wall tension, if you place the gas in a couple of smaller cylinders rather than in one large cylinder. Seems like that might be beneficial in terms of materials technology.That'll be it.

I remember investigating wall tension in a rotating orbital when I first started on BAUT. I'd forgotten what I'd worked out. :lol:

You want a higher proportion of sphere to cylinder if I remember. Hoop stress is lower. Again, I forget why.

I presume these compressor tanks have hemispherical caps?

grant hutchison
2009-Jun-14, 05:05 PM
You want a higher proportion of sphere to cylinder if I remember. Hope stress is lower. Again, I forget why.A sphere gives you half the tension of a cylinder, for the same pressure and radius. The double curvature of the surface distributes the load better. Unfortunately, it's a less convenient shape at higher volumes.

Grant Hutchison

novaderrik
2009-Jun-14, 05:41 PM
the only dual tank air compressors i've ever seen were the smaller high pressure models that are designed for contractors to take to job sites.
they have two little tanks on the bottom, and run at about 250 psi- that's a LOT opf stored energy in a small place, and you can run the normal construction site tools all day without the compressor even breaking a sweat.
i've never seen a compressor with one tank on top of another- but there are two stage compressors that use a 2 cylinder pump to increase the pressure in stages, which introduces less heat into the air and strains the motor less than just using a single big pump to pump the air up to 175psi in a big single tank.

mugaliens
2009-Jun-15, 06:46 AM
I imagine several things might be involved.
1) Laplace's law (http://hyperphysics.phy-astr.gsu.edu/Hbase/ptens.html). You can contain the same volume, under the same pressure, but with reduced wall tension, if you place the gas in a couple of smaller cylinders rather than in one large cylinder. Seems like that might be beneficial in terms of materials technology.

Bingo. It's beneficial in terms of weight reduction for the same volume of air compressed at a specific pressure.


2) Failure mode. If one tank blows the other will empty through it relatively slowly, spreading the energy release over time.

Yes, but... With two tanks, you've doubled your liklihood of failure.


3) Manoeuvrability. A flat array of two cylinders can be easier to move and store than one long or fat cylinder.

True!

jj_0001
2009-Jun-15, 07:33 PM
Well, in the old high-capacity ones I used to deal with, one was a low-pressure tank driven by the first two stages of the compressor (3 pistons, 2 stages), and the second the high-pressure tank, also driven by a two-stage compressor, this one of 2 pistons and 2 pistons in each stage, but with different diamaters by a facter of about 4. The whole thing ran via two 5 HP 3 phase motors with separate controllers for low and high, had an arrangement so that both motors would not start at the same time, and spewed heat liberally about. No surprise there.

tashirosgt
2009-Jun-15, 07:56 PM
That must have been a huge apparatus. Where was it?

I understand (or do I?) the utility of multiple tanks if you you do compression in multiple stages. What I was asking about are the typical compressors that I see in local hardware stores such as Lowes and Home Depot.

(In fact I'm tempted to buy a $60 over-and-under dual tank compressor that's on sale at Lowes and Target - another tool that I can store, unused, in my crowded garge. )

jj_0001
2009-Jun-15, 09:55 PM
That must have been a huge apparatus. Where was it?

In a tire recapping shop. It was about 15' long and about 4' deep, and floor to about 5' high. It was surplus and massively overspec'ed for the job it was doing, but not terribly uneconomical (only ran on demand), and you could fill a half-dozen 1200x22's while you were loading a rack full of recap molds at the same time with no noticible twitch in the regulated side pressures.

It was outside under a shed, with rainhats for the few sensitive parts. It could melt snow at 10' in an Ohio winter.

clop
2009-Jun-15, 11:22 PM
In a tire recapping shop. It was about 15' long and about 4' deep, and floor to about 5' high. It was surplus and massively overspec'ed for the job it was doing, but not terribly uneconomical (only ran on demand), and you could fill a half-dozen 1200x22's while you were loading a rack full of recap molds at the same time with no noticible twitch in the regulated side pressures.

It was outside under a shed, with rainhats for the few sensitive parts. It could melt snow at 10' in an Ohio winter.

Probably came from an old church organ.

clop

Valkyrie801
2009-Jun-16, 02:03 AM
One gets two smaller tanks...

So while you are using it, and one tank runs out...

You can call on the second tank for one's work, while you get the other "Smaller" tank refiled with out having to heft the whole damned thing!

Like "DUH?":doh:

Tucson_Tim
2009-Jun-16, 02:07 AM
Like "DUH?":doh:

The thread starter asked a good question. Why do you have to insult him?

Swift
2009-Jun-16, 02:09 AM
Valkyrie801,

You are already on thin ice. I would strongly advise that you not do anything that looks like a rule violation, such as questioning the intelligence of another member.

Tucson Tim,
Please only report the posts, do not respond to them in thread.

Tucson_Tim
2009-Jun-16, 02:26 AM
One gets two smaller tanks...

So while you are using it, and one tank runs out...

You can call on the second tank for one's work, while you get the other "Smaller" tank refiled with out having to heft the whole damned thing!


What are you talking about? Both tanks are being filled by the same compressor. You don't have to take one of the tanks to get it filled ????

Valkyrie801
2009-Jun-16, 02:48 AM
I apologise.

I do need a dose of humility.

Insulting anyone else only insults myself...

and I am just beginning to grasp that this site is international.

All I have known is conflict and adversaries.

I would like to have some friends.

Please forgive me.

pzkpfw
2009-Jun-16, 02:54 AM
http://www.newwoodworker.com/reviews/rgid4.5comprvu.html

See the section "Twin Tanks" on this product page. Here it is clear that both tanks are essentially open to the air line (output) at the same time (in series). The benefits claimed for this product are moisture trapped in the lower tank, and benefits from surge suppression.

Tucson_Tim
2009-Jun-16, 03:04 AM
All I have known is conflict and adversaries.

I would like to have some friends.


Well, I don't always live by it, but a very good start is "The Golden Rule": http://en.wikipedia.org/wiki/Ethic_of_reciprocity

Jerry
2009-Jun-16, 03:25 AM
All I have known is conflict and adversaries.



mmm... must be a Klingon:)

The little side-by sides found in hardware stores are designed to be stable - to not tip over, like a single cylinder would with the filling/dispensing apparatus on the top.

There are a lot of advantages to cascading cylinders if you need to work at high pressure. Pumps can only handled limited pressure differentials without major overheating problems, a few thousand psi at a time, but by cascading to higher pressures, you can use 2000psi gas with a differential pump to boost to 6000 psi; and you can also use an isolated low pressure-high volume source as a boost pump driver. You can easily reach pressures of 20-30,000 psi in three steps.

Another advantage is if you need to raise the pressure of a test chamber to 5,000 psi, you start will a 1500 psi source, then close off the source and introduce higher pressures from other cylinders. This way, only a small percentage of the source gas has to be pumped up to the highest pressures.

A third advantage is heat flow. Most of the delta T is in the low pressure ranges. Cascading the gas flow reduces the pressure drop (in the case above, from 5000 to 1500) while filling at the lower pressure; cutting the adiabatic cooling in half.

Finally, if you need to drain the high pressure tanks, or you don't want to store or transport them at peak pressures, you can dump the pressure into generally larger lower pressure chambers rather than bleeding them off.

mugaliens
2009-Jun-16, 07:40 AM
I apologise.

I do need a dose of humility.

Insulting anyone else only insults myself...

and I am just beginning to grasp that this site is international.

All I have known is conflict and adversaries.

I would like to have some friends.

Please forgive me.

Nice recovery, Valkyrie801.

Welcome back! I think you're still on thin ice with the mods, but keep up the good attitude, and the ice'll thicken in good time.

clop
2009-Jun-16, 11:55 AM
I apologise.

I do need a dose of humility.

Insulting anyone else only insults myself...

and I am just beginning to grasp that this site is international.

All I have known is conflict and adversaries.

I would like to have some friends.

Please forgive me.

Don't worry Valkyrie, I got banned once and it gave me time to think about all kinds of interesting things I might otherwise never have thought about.

clop

rommel543
2009-Jun-16, 01:25 PM
http://www.newwoodworker.com/reviews/rgid4.5comprvu.html

See the section "Twin Tanks" on this product page. Here it is clear that both tanks are essentially open to the air line (output) at the same time (in series). The benefits claimed for this product are moisture trapped in the lower tank, and benefits from surge suppression.

Dang... I just started reading this thread and you had beat me to the the surge protection benefits. Also as Grant already pointed out the decreased tank sizes allows for a higher pressure due to the decreased surface area of the tank. With semi tractor air brake systems the tanks are regulated to have a maximum size. If you need to maintain a higher air levels (pulling > triaxles, trains, etc) you have to add more tanks not get bigger ones. Also if you need higher pressure (overloads) you have to get smaller tanks and get more of them - 2 hp tanks for every 1 standard tank.

jj_0001
2009-Jun-16, 05:04 PM
Probably came from an old church organ.

clop

Err, no. Last time I looked, church organs didn't use 450 psi at some absurd volume I don't recall offhand.

Church organs use in the inches of pressure, 20" is a high pressure for most organs, and that's hardly pushing 45 psi, let alone 450.