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What air compressor CFM and reservoir size needed to run a 50 amp plasma cutter

JimGnitecki

Active Member
Before you build a noise reducing enclosure, this video kind of sums up what I read at the time.

Yes, I predicted the outcome right from the start of the video. The combination of cost and time to reduce the sound level at consumer level is impractical. Plus, he did not mention it, but all the restrictions he added on inatke and exhaust to silence the noise also made it much harder for the compressor to suck in air, AND if that computer fan fails, he'll overheat the compressor pretty quickly. There might even be potential to start a fire. And of course, all these undesirable outcomes are invisible to the user until they manifest in degraded performance or costly failures.

Putting an air compressor outdoors is also not a good solution. That will expose the compressor to weather that will degrade it over time, and will also make it pretty vulnerable to theft. And, any neighbours, whether residential or commercial (anything sbort of industrial) are going to be VERY unhappy.

Face it: compressors are a necessary or desirable thing but are costly and head-rattling loud. if you seek out a quietER one, you will pay more and get less performance, and it will STILL require hearing protection unless your shop is (very) large enough to enable isolating the sucker. And isolating it (as the guy in the video did) unfortunately masks symptoms of its distress when something goes wrong to the point that serious damage could occur to the compressor or to your shop.

In my own case, I have successfully avoided, up to now, creating a NEED for having more than a pretty small and relatively inexpensive one. Topping up my vehicle tire pressures (a truck, a car, a motorcycle, and a small trailer), and feeding my finish nailer, are VERY modest loads for an air compressor. Long bars, the most powerful electric impact tools, and strong electrical power tools have made getting a bigger, louder compressor unnecessary. My radial arm saw, my planer, and my circular saw have been the loudest tools in my shop, but their use is intermittent, not constant, and ear muffs work.

But I underestimated the amount of air that is needed to feed a plasma cutter. I was fooled by the modest PRESSURE requirements that the seller's ad touted, and did not see the CFM statement until I had the machine and its user manual in hand, and found the 6 CFM requirement buried on page EIGHTEEN of the manual, where the manufacturer also confessed that it is probably a good idea to have a notable amount of extra CFM in some situations. One of you on the forum drew to my attention for example that the AFTERflow required on a plasma cutter is often as long or longer than an individual cut uses! So, repetitive short cuts become repetitive LONG CFM consumers.

It would be nice to be able to build up the psi and CFM in advance in a lengthy but quiet process using a small, quieter, and much less costly air compressor. But the math really really works against that. A 20 gallon air tank holds only 2.7 cubic feet at say 125 psi, which you'd consume in a fraction of a minute of plasma cutting. Even a 100 gallon tank holds only 13.5 cubic feet at 125 psi. If you start drawing even the 100 gallon tank down at a rate of 6 CFM at just 50 psi, you'll blow through it ENTIRELY in at MOST 13.5 / 6 x 125 / 50 = 5.6 minutes!

What about renting or buying a large compressed air cylinder? The largest one that Air Liquide apparently offers here in Canada is 230 standard cubic feet. That's 230 / 6 = only 38 minutes of 6 CFM air. I have no idea what an air delivery costs for such a cylinder. At a cutting rate of say 20 inches per minute, that would be 760 gross inches, but actually the NET inches would be far less because of the afterflow requirement.

For example, on 3" cuts, you'd be actually cutting for only 9 seconds, but a 15 second afterflow would mean you'd use up 24 seconds of air or about 2.4 CFM. At that rate you'd burn through the entire cylinder of compressed air in 96 cuts. If you were doing 12" cuts, you'd use up the entire cylinder in 45 cuts (more "efficient" than the 3" cuts because the afterflow remains the same). But at least the air supply would be QUIET versus ear damaging and stressful and annoying. :) But, as I said, I have no idea what a 230 cu ft air refill delivered costs (delivered because the tank is too large and heavy to take in to the air supplier).

Ironically, plasma cutters themselves are getting very inexpensive, but the air to FEED them, properly, appears to be surprisingly costly in terms of either money and/or noise and stress.

Jim G
 

JimGnitecki

Active Member
I found that Air Liquide charges $121 plus tax for that 230 cu ft cylinder refill, so about $127 in Alberta. That assumes you own a cylinder. If not, you need to buy one. Looking at the cost to buy a welding gas cylinder, I THINK a 230 cy ft air cylinder would cost less than a noisy compressor, and FAR less than a "quiet" (say this with a grain of salt) compressor.

Using the above calculated usage numbers for 3" and 12" plasma cuts, that means that using a 230 cu ft compressed air cylinder for plasma cutting would cost $127/96 = $1.32 per cut for the 3" cuts and $127/45 = $2.82 per cut for the 12" cuts.

This is high cost if you do LOTS of plasma cutting. But it is less costly than a compressor if you do plasma cuts at a "hobby" level.

Take for example the cost of that Princess Auto compressor I have looked at: $600plus tax = $630.

That's equal to 630/127 = 5 refills.

Those 5 refills would make a total of 96 x 5 = 480 3" cuts OR 45 x 5 = 225 12" cuts.

Those cuts would cost $127 / 480 = $0.26 each for the 3" cuts, and $127 / 225 = $0.56 for the 12" cuts.

Sure, you have to buy the gas cylinder (ONCE), or rent one (how much?), but hey, gas cylinders don't require oil changes or other maintenance, and, BIG deal, they make NO NOISE! They also consume VERY little space compared to a compressor, since they are vertical and much smaller than any suitable compressor. Consider how much quieter the shop would be if plasma cutting is the primary air usage, as it WOULD be for me.

Just considering out-of-the-box ideas here . . .

Shoot away at the ideas with practical observations . . .

Jim G
 

phaxtris

(Ryan)
Premium Member
Premium Member
To buy a 255cu-ft cylinder is between 500-700 new + fill around here

You may find a used one for cheaper, it may or may not be up for a hydro test soon (depending on the cylinder it is either 5 year or 10 year re-test)

Lease is about 100 a year + fill

A 255 is 200+ lbs, loading then and unloading from a truck is often a 2 man adventure and very awkward, I work as a welder/ironworker...all strong guys...and even in that trade not many guys can load/unload a 255 into a pickup alone, and definitely not recommended, so that is a major consideration
 

JimGnitecki

Active Member
To buy a 255cu-ft cylinder is between 500-700 new + fill around here

You may find a used one for cheaper, it may or may not be up for a hydro test soon (depending on the cylinder it is either 5 year or 10 year re-test)

Lease is about 100 a year + fill

A 255 is 200+ lbs, loading then and unloading from a truck is often a 2 man adventure and very awkward, I work as a welder/ironworker...all strong guys...and even in that trade not many guys can load/unload a 255 into a pickup alone, and definitely not recommended, so that is a major consideration
This is very interesting! For hobby use, a leasing cost of $100/12 = $8.50 per month is very acceptable. Heck, that's not very much more than the costs of oil changes and air filter changes on an air compressor, right, especially one operated in a woodworking shop where sawdust in the air is unavoidable even with dust collection?

I agree about not wanting, or being able, to load, unload, and transport a 200+ compressed gas cylinder. That is why I looked at "delivery" versus pickup on the Air Liquide site. I imagine that if you go to a smaller size cylinder that can be handled by one "aged" man, the cost of a refill gets pretty disproportionately higher.

The Air Liquide webpage, even at a simukated cart order, did not mention any addiitonal charges on top of the $121 (plus tax I assume, to get it to $127).

I can actually see where this gas cylinder approach MIGHT conceivably be "practical" for a hobby level user:
- The cost is reasonable when actually analyzed as cost per cut
- It used very little actual shop floor space
- Its pleasantly quiet
- Its output is more reliably consistent than that of a compressor
- No moisture in the air issues (right?)
- If you need to, or want to, get VERY high CFM rates for certain tasks, you can very easily do so, compared to a compressor
- If you wanted to, you could use a gas mixture other than plain air for performance benefits, but I like the idea of a simple, quiet, and adjustable output air supply that could be used for multiple shop needs, as encountered.

But, learning from my air compressor inquiry experience, what else could blow this idea out of the water from a practical perspective?

Jim G
 

phaxtris

(Ryan)
Premium Member
Premium Member
I don't believe that they will deliver 1 cylinder for an account with only 1 cylinder, if they will I doubt it will be free
 

CWret

Ultra Member
Premium Member
I like the concept of using a high pressure tank like the ones i have for my MIG &TIG gases (my tanks are the mid size type, about 36” tall and easier to handle).

Tanks take up little space, are portable and SILENT!
What i don’t like about that approach is the refill hassle (trip to vendor + Murphy says it’ll run out on the week end). I’m less concerned about the cost because that only hurts once.

I already have enough air for my plasma - as i said elsewhere on the forum- i have a little ultra quiet 2.3 cfm compressor that is used for most tasks and a larger 4.0 cfm guy. I run both in tandem when cutting. They are on separate 110 circuits.

So if i was doing it again, I’d consider this solution:

Get a mid size high press tank with a regulator. Needs 2 pressure gauges - one for tank pressure and one for supply line pressure (guessing about $350 for tank + gauges/ regulator).
Then spring for a small high pressure compressor. They are not only small but also not very noisy. They supply 4,000 to 4,500 psi and cost $400 to $600 on Amazon (see examples below)
Then no trips to the gas vendor. You’d just have to monitor the tank pressure and run the compressor at night to keep the tank topped up.
You’d have to really hate the noise to follow this costly lsolution.

- Velor PCP portable scuba compressor $400 - $450
- GX pump CS2 portable PCP air rifle pump $550
(PCP - pre charged pneumatics)
 

CWret

Ultra Member
Premium Member
My solution above also assumes you are ok with only short duration cutting (hobby) and then have time for the little compressor to do the recharge.
 

JimGnitecki

Active Member
My solution above also assumes you are ok with only short duration cutting (hobby) and then have time for the little compressor to do the recharge.
I hear you, but it sounds too much like replacing a relatively normal and common (although costly) air compressor with an exotic, small one that will have to work much harder and costs almost as much as the big, more standard one. Plus, I am not keen on leaving an air compressor working hard without human supervision in the same room, and as you point out, they may be LESS noisy but still make noise, and in this case, for many hours at a time. So, I am not enthused by that approach. :(

But, I applaud and encourage the "off the beaten track" thinking!

Jim G
 

JimGnitecki

Active Member
I;m going to try to Google what artists who work with plasma arc use for their air supply. Might be some ideas and experience there on the topic.

Jim G
 

slow-poke

Ultra Member
I just watched the video regarding the noise insulating box.

FWIW, I have a very noisy compressor. Our basement has a small dedicated electrical room adjacent to the garage so I decided to hang the compressor from the floor joists with rubber vibration isolators in the corner of that basement room. More floor space but to some extent it made it worse when you walk in that room because the compressor is near ear level. So I built a rudimentary three sided box with surplus plywood and a few sound dampening panels and it helped quite a bit. No fan, I just left the top couple of inches of one side open for cooling. Went from 87dB to 46dB with the door closed, not quiet but tolerable. Closing the gap and adding a fan would surely help, might get it down do 40dB that will have to be a rainy day project.
 

Susquatch

Ultra Member
Administrator
Moderator
Premium Member
What about a big compressor outside - maybe even under the truck hood and a big tank inside. Can't be any worse than cutting the grass......
 

historicalarms

Ultra Member
FWIW, for those looking for more capacity. A friend used an old hot water tank for extra capacity, installed it up in the attic of his garage. Rated for 100psi so might need to tweak the regulator.
My experience would not agree with using an old hot water tank as an air reservoir. My old man was given a renovation removal hot water tank by my uncle to use as an air reservoir...our neighbor told us not to use that damn thing "its a bomb" he claimed "those old tanks rust from the inside out" he claimed....but we didnt listen, needing a big air supply for our impact guns used to change cultivator shovels.
All went well for about a year, that tank was in a shop corner out of our way...but one morning we were both outside the big shop door working on some piece of farm machinery when , all of a sudden there was the god-damndest explosion inside the shop that would have deafened an artillery sergeant and a dust cloud came out the door that you couldnt see through. With only 125 psi in that tank it had bust wide open, starting at a small rust spot the size of a tuney. Not only had thqat tank ruptured, it tore the rubber hose out of the crimp joint and launch itself 1/3 way across a 48 ft long shop....Not gonna ever use any hot water tank again.....
 

slow-poke

Ultra Member
My experience would not agree with using an old hot water tank as an air reservoir. My old man was given a renovation removal hot water tank by my uncle to use as an air reservoir...our neighbor told us not to use that damn thing "its a bomb" he claimed "those old tanks rust from the inside out" he claimed....but we didnt listen, needing a big air supply for our impact guns used to change cultivator shovels.
All went well for about a year, that tank was in a shop corner out of our way...but one morning we were both outside the big shop door working on some piece of farm machinery when , all of a sudden there was the god-damndest explosion inside the shop that would have deafened an artillery sergeant and a dust cloud came out the door that you couldnt see through. With only 125 psi in that tank it had bust wide open, starting at a small rust spot the size of a tuney. Not only had thqat tank ruptured, it tore the rubber hose out of the crimp joint and launch itself 1/3 way across a 48 ft long shop....Not gonna ever use any hot water tank again.....
Interesting, and scary the one I mentioned is still in service must be 10 years plus. I do think he turned the regulator down to 100 PSI. I would have thought if it started to fail it would simply start leaking not explode. I'm going to warn my buddy.
 

JimGnitecki

Active Member
What about a big compressor outside - maybe even under the truck hood and a big tank inside. Can't be any worse than cutting the grass......
Cutting the grass IS pretty LOUD! I am SO glad we now finally live in a neighbourhood where rock ground cover is strongly preferred over grass, and no one near us actually cuts grass! :)

Jim G
 

Susquatch

Ultra Member
Administrator
Moderator
Premium Member
Interesting, and scary the one I mentioned is still in service must be 10 years plus. I do think he turned the regulator down to 100 PSI. I would have thought if it started to fail it would simply start leaking not explode. I'm going to warn my buddy.

I did a short stint right out of school designing pressure vessels. @historicalarms is absolutely right. Doesn't take 125 psi either. 30psig will dismember, decapitate, and destroy a nice shop. Not a good plan at all to use a water tank even if the water tank is rated to 250. The difference is that water is uncompressible. A crack only leaks. Gas on the other hand is compressible. 30 psig is three times the regular air volume all looking for a way out through whatever is in the way.

Ya, it's possible to use a water test on a tank inside another water tank, but I can't recommend it. Pressure vessels are highly regulated for a reason. They are basically a bomb waiting to go off. Please don't do it.
 

JimGnitecki

Active Member
Interesting, and scary the one I mentioned is still in service must be 10 years plus. I do think he turned the regulator down to 100 PSI. I would have thought if it started to fail it would simply start leaking not explode. I'm going to warn my buddy.
Yes! DO warn him. Yesterday, I watched a video taken in the few minutes after a delivery truck loaded with proper, professionally maintained gas cylinders collided with a car on a freeway. There was an IMMEDIATE and LARGE fire (because the gases in the cylinders were flammable), but what was even much scarier is that as the minutes after the collision ticked by, individual cylinders popped off explosively, sending not just steel cylinder fragments, but entire full size cylinders flying at least 50 feet into the air, and going far enough horizontally that even police cars dispatched to the scene and parked a couple hundred feet away had to quickly back up to get out of range of the "flying torpedoes".

I would not mess with 100 psi or higher potential missiles.

Also, any damage caused by a tank or cylinder incident would be the responsibility of the person who created the (illegal by the way) hazard, and if anyone gets hurt or killed as a result, I can see a DA contemplating reckless endangerment or even manslaughter charges.

WAY too many risks.

Jim G
 
A truck maintenance shop in town here (although maintenance might have been a strong term) had their compressor blow up a few years back, it was in a brick room off the side of the main shop. Fortunately it went off in the middle of the night and nobody was there, there was a massive hole in the the outside wall and the tin roof was opened up like an angry monster wanted out in the worst of ways........ Always a good idea to drain the water out from time to time, and don't mess with the pressure switch to save a couple bucks.......
 

JimGnitecki

Active Member
This entire segment of discussion on the dangers of compressed air or gases not being understood or researched before actions were taken brings to mind an incident that a friend of mine was involved in a couple of decades ago.

He and another friend were renovating a house that my friend had bought, hoping to turn a quick buck. Their impulsive plans included taking out some interior walls. Neither of them understood some key house reno concepts, such as shear walls, box beams, and loadbearing versus partition walls. As they were working, INSIDE the house, they heard some creaking begin, which they attributed to the gusty wind outdoors. Fortunately, they took a lunch break, sitting out in the yard.

The creaking slowly intensified, and then, the entire house collapsed!

My friend's buddy looked over at my friend, and said simply "THAT's a bummer."

My friend in the meantime was reflecting simultaneously on the concepts of timing a lunch break just right, a sudden dramatic DEcrease in his net worth, and the fragility of life.

Jim G
 

curmudgeon

(Steve)
Pressure vessels are highly regulated for a reason. They are basically a bomb waiting to go off.
What are your thoughts about repurposing a 100 lb. propane tank as a compressed air storage tank? Google claims, "At 100°F, a 100 lb propane tank has 172 PSIG of pressure." I would install it upside down on a stand to make it easy to install a water drain and would regulate the supply pressure to 120 PSI (or less). I've converted a 20 lb. tanks into a lead melting pot, so am familiar with the challenges of removing valves and purging old contents.

Somewhat related question, do commercial shops ever recertify their air storage tanks?
 
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