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

[Doggggboy]

Obstacles are made to be overcome. People asked me why learn how to TIG weld at age 72. Answer: Why NOT?

Plasma cutting: same.

Jim G

I'd like this a hundred times if I could.
Go hard Jim.

 

[trevj]

Above my pay grade but more for curiosity. Is plasma cutter gas requirement kind of like TIG torch where its relatively low pressure & flow rate at the regulator so it delivers I guess just above atmospheric pressure blanket gas? Or is it more analogous to like a power tool where it needs elevated pressure AT some flow rate to (I dunno) blow the molten material out of the kerf?

Nope, not a shielding gas. Requires flow at pressure, in order to push the molten material out of the kerf, and keep the plasma stream cutting new material.

Which is pretty much why a decent sized air compressor is the norm. Cheapest way to provide high volume, at reasonably high pressures.

 

[trevj]

I'd like this a hundred times if I could.
Go hard Jim.

Not me. It's one thing to learn a new skill, another entirely, to attempt to reinvent it, before you have a real clue how it works when you do it the way that is accepted as 'normal' first.

It feels at time like talking to one of the apprentices, who isn't near as clever as he thinks he is, trying to find short cuts, that end up taking days of extra time....

Best advice I have for Jim, when he eventually decides that an air compressor is in his future, is to watch the motor RPM ratings. A belt drive 1725 rpm motor will run a lot quiter than those gawdawful 3000+rpm direct drive units that get flogged of to folks that either don't know any better, or simply can't be bothered to care.

 

[JimGnitecki]

Above my pay grade but more for curiosity. Is plasma cutter gas requirement kind of like TIG torch where its relatively low pressure & flow rate at the regulator so it delivers I guess just above atmospheric pressure blanket gas? Or is it more analogous to like a power tool where it needs elevated pressure AT some flow rate to (I dunno) blow the molten material out of the kerf?

Excellent question, and one that I have been researching with limited success until yesterday. Most of the websites you go to, including even the ones by organizations like Hypertherm, Miller, etc, are too vague in answering this question, implying that the right "combination" of amperage and psi is needed to get a reasonably successful compromise among cutting speed, cutting quality, and cutting dimensional precision.

Yesterday I found an online video on Youtube by an individual experienced plasma cutter practitioner in which he progressively varied amperage and air psi:

He confirms what the other websites have said, but shows that air psi is more important than amperage, for cutting speed, quality of cut, and aesthetics of the cut. And, his presentation seems to show that a fairly high air pressure is needed.

The user manual that comes with my specific small "lunchbox sized" plasma cutter with only a deceptively optimistic 50 amp rating, seems to also say that you need to have a high CFM as well, like 6 CFM, even though the manual calls for only 30 to 45 psi for many typical thicknesses of material, AND cautions you to not feed any more than about 70 psi through it (through, not to - it has its own built-in user adjustable regulator).

So, I again saw what I have been suspecting and saying throughout my recent investigation of what plasma cutters REALLY require: They appear to need an amperage range appropriate to the thicknesses and types of metals you will be cutting, a psi range that encompasses approximately 30 to 100 psi for small typical metal "workshop" (not metal "industry" which can need much more), and AT LEAST 6 CFM of air VOLUME which is not adversely reduced by too-small air delivery lines.

Your question "Or is it more analogous to like a power tool where it needs elevated pressure AT some flow rate to (I dunno) blow the molten material out of the kerf?" is pretty much right on. The air supply DOES in fact "blow the molten metal out of the kerf".

To me, this all suggests a pretty high powered air compressor - one that gets plenty of starting current 240 volt power, has a large tank (like 80 gallons, which is barely over 10 cubic feet) requires notable floor space, is only marginally portable, and makes a lot of noise to serenade you while you work.

Sure, we see lots of Youtube videos where guys cut metal with small plasma cutters, but the cut rate is slow (way below the 20 inches per minute and higher that is recommended), and quality is poor, and the metal ebing cut is thin.

And yes, the "good well known" brands of plasma cutters do even offer models that have a "builit-in" air compressor within a surprisingly small machine, BUT those machines do thin gage metals only, AND their manufacturers have preset the combination of internal air compressor CFM and psi and machine amperage to work within a carefully controlled thickness range. Their specs look feeble compared to the Chinese "50 amp" units because of the limitations imposed by that tiny built-in air compressor.

The more I learn, the more I am convinced that my approach of a gas cylinder supply versus an air compressor is the right solution for my specific circumstances, which I have described pretty fully in the course of this thread. Your best solution might vary from mine, since my objectives are much more hobby and art oriented than a typical small shop metal practitioner.

Jim G

 

[JimGnitecki]

Excellent question, and one that I have been researching with limited success until yesterday. Most of the websites you go to, including even the ones by organizations like Hypertherm, Miller, etc, are too vague in answering this question, implying that the right "combination" of amperage and psi is needed to get a reasonably successful compromise among cutting speed, cutting quality, and cutting dimensional precision.

Yesterday I found an online video on Youtube by an individual experienced plasma cutter practitioner in which he progressively varied amperage and air psi:

He confirms what the other websites have said, but shows that air psi is more important than amperage, for cutting speed, quality of cut, and aesthetics of the cut. And, his presentation seems to show that a fairly high air pressure is needed.

The user manual that comes with my specific small "lunchbox sized" plasma cutter with only a deceptively optimistic 50 amp rating, seems to also say that you need to have a high CFM as well, like 6 CFM, even though the manual calls for only 30 to 45 psi for many typical thicknesses of material, AND cautions you to not feed any more than about 70 psi through it (through, not to - it has its own built-in user adjustable regulator).

So, I again saw what I have been suspecting and saying throughout my recent investigation of what plasma cutters REALLY require: They appear to need an amperage range appropriate to the thicknesses and types of metals you will be cutting, a psi range that encompasses approximately 30 to 100 psi for small typical metal "workshop" (not metal "industry" which can need much more), and AT LEAST 6 CFM of air VOLUME which is not adversely reduced by too-small air delivery lines.

Your question "Or is it more analogous to like a power tool where it needs elevated pressure AT some flow rate to (I dunno) blow the molten material out of the kerf?" is pretty much right on. The air supply DOES in fact "blow the molten metal out of the kerf".

To me, this all suggests a pretty high powered air compressor - one that gets plenty of starting current 240 volt power, has a large tank (like 80 gallons, which is barely over 10 cubic feet) requires notable floor space, is only marginally portable, and makes a lot of noise to serenade you while you work.

Sure, we see lots of Youtube videos where guys cut metal with small plasma cutters, but the cut rate is slow (way below the 20 inches per minute and higher that is recommended), and quality is poor, and the metal ebing cut is thin.

And yes, the "good well known" brands of plasma cutters do even offer models that have a "builit-in" air compressor within a surprisingly small machine, BUT those machines do thin gage metals only, AND their manufacturers have preset the combination of internal air compressor CFM and psi and machine amperage to work within a carefully controlled thickness range. Their specs look feeble compared to the Chinese "50 amp" units because of the limitations imposed by that tiny built-in air compressor.

The more I learn, the more I am convinced that my approach of a gas cylinder supply versus an air compressor is the right solution for my specific circumstances, which I have described pretty fully in the course of this thread. Your best solution might vary from mine, since my objectives are much more hobby and art oriented than a typical small shop metal practitioner. Unlike many (most?) of you on this forum, I have no other need for a high CFM air compressor.

Jim G

 

[Susquatch]

I'd like this a hundred times if I could.
Go hard Jim.

@JimGnitecki

At 77, X 1M for me too!

You never know when you logged in for the last time.

Smell the roses, enjoy your shop, go hard!

 

[phaxtris]

Isn't it an Einstein quote; " once you stop learning, you start dying "

We're gonna live forever !

 

[JimGnitecki]

Trevj: I hope you did not regard my comments as disrespectful in any way. They were certaily not intended that way. You and a number of others on this forum have been SUPER friendly and helpful to me as I explore and learn about this new metal affliction that has possessed me. You have in fact done a LOT of analysis and looking at air supply options on my behalf. You are very experienced in many areas, and your advice has been solid, and very much appreciated.

I do confess that I often look at technologies, needs, and workable solutions from a "zero-based" perspective. I'm always looking for ways to maximize my capabilities and enjoyment while keeping the amount of money I spend within reason, and within my very patient and indulging wife's tolerance level. I appreciate her tolerance and active support even more than I appreciate your kind help.

Sometimes my proposed approaches and solutions are way off the mark and I eventually see that and try a different solution. But sometimes, the ideas prove to be very successful.

One example: About 35 years ago, my wife and I put together a way to produce full colour, pantone certified electronic desktop published statistical report documents (on 1980s personal computers and new technology Seiko desktop full colour printers) replacing super-costly midframe computer systems, for a company that produced such reports for Fortune 500 companies. We revolutionized that company's business. All of a sudden they could do way more effective reports for a fraction of the cost of what they and their competitors had been doing. We became their most important supplier, producing reports that went to Blue Cross, General Motors, Hancock Insurance, etc. We had those personal computers and Seiko printers running almost 24 hours a day for several years until others finally mimicked our solution.

I don't know how well the Nitrogen gas supply will work until I get the system assembled and have acquired some expertise with the balancing of psi and amperage on my tiny plasma cutter to get that desirable 15 to 30 degree slanted spark stream coupled with low dross and narrow cuts with upright versus tapered cut edges. But, I am excited about the possibilities, and I know I will enjoy the entire testing and learning process.

Jim G

 

[CWret]

Many of us talk about doing something - Jim followed through and got the N tank - a big WELL DONE! -- and I loved his "why not" reasoning for learning plasma at age 72.

The N tank totally solved his noise issue and has a very small footprint as an added bonus (thanks for the pic, the tank looks great in your garage). He also has no cfm or psi limitations. Another plus is that the life of his plasma consumables will be extended since he'll be using clean dry bottled gas. N will improve his cut quality, especially when cutting difficult materials like Al or SS. That's a big plus. If he really wanted to eliminate the issue of his tank going empty on the weekend - he could just rent a second tank (but that's not a big issue for us retired hobby guys).

Here's my comment about the cost - I like to fish but I'm not an enthusiastic fisherman. My fishing rod cost $75 and suits me just fine. My neighbour is an enthusiastic fisherman. His fishing rod looks just like mine but it costs over $300.

Another plus for Jim's tank solution - he can try it for a year with very little commitment other than the $205 regulator setup (which likely has some resale value).

 

[JimGnitecki]

My welding supplier just called me. They love what I am trying to do here, and they are going to adjust my Nitrogen gas refill charge down to make it more affordable so I can use as much as I want.

Jim G

 

[CWret]

Even though the cost may not have been the most important factor for you - them reducing your cost is a smart business move on their part. Be sure to document how long a bottle last, cutting time, material thickness, lengths of cut, etc. If they promote your method they might even improve your pricing some more.
Besides them - the Forum would appreciate that info.

 

[PeterT]

He confirms what the other websites have said, but shows that air psi is more important than amperage, for cutting speed, quality of cut, and aesthetics of the cut. And, his presentation seems to show that a fairly high air pressure is needed.

The more I learn, the more I am convinced that my approach of a gas cylinder supply versus an air compressor is the right solution for my specific circumstances, which I have described pretty fully in the course of this thread.

I'm going to come at it from a slightly different angle & admittedly I'm on uncharted waters here. Maybe we have some experts who can weigh in. The pressurized gas cylinder is relating its volume of N2 at atmospheric conditions. So one would think in round numbers 299 CF / 6 cfm = 49 min flow time by completely depleting the tank. That's why I asked if plasma cutting was more like TIG blanket gas which would have a low regulator setting, it just has to meet a bit above ambient pressure.

But I think when you have a combined high flow demand AT higher pressure delivery in a fixed volume vessel that isn't getting recharged like a compressor, a few things happen. This is just a generic internet chart for illustration purposes, maybe its an oxygen tank, so ignore the units. The vertical red lines represent constant flow rate increments. The black curve shows resultant tank pressure through depletion. Notice it's curved as a function of its physical volume & initial charge pressure. Increased flow rate corresponds to wider step widths (higher volume per unit time), which means you traverse down the same black curve, just faster. The horizontal lines represents regulator discharge setting. The remaining tank pressure must exceed set pressure in order to flow. So blue might be analogous to low pressure atmospheric/blanket gas - say at 5 psi, it can deplete 30 volume units. Green is elevated pressure requirement like a tool or whatever. So using same example, 15 psi setting equates to only 15 volume units. The remaining volume stays in the tank so cannot be applied to the job time. ie. the 299 would be something less.

The question is how much less. I tried to find an online calculator but its amazingly tough. There are welding calculators & oxygen mask/scuba calculators & compressor recharge calculators, but amazingly no generic blowdown depletion calculators where one can input the variables. Maybe its as simple as a Boyles law thing like link? Hope this makes sense. I'm not saying this is the situation but it kind of strikes me as relevant.

7.2: The pressure-volume relationship

Boyle's law is described, i.e., the volume of a given amount of gas varies in inverse proportion to the pressure of the gas at a constant temperature. The breathing process, i.e., inhalation and …

chem.libretexts.org

 

[phaxtris]

I thought about the Boyles law thing as well, but I believe cylinders are rated at say 244cf at 2400psi...or whattever that cylinder is rated, I believe that to mean at 2400psi that cylinder contains 244cf, but the relationship between pressure on the gauge and volume in the tank is not linear....probabaly the reason tanks seems to take longer to go from 1000 to 0 vs 2000 to 1000

 

[PeterT]

Maybe we are saying the same thing, maybe not LOL. The volume of the physical tank is of course fixed, so I think what they are specifying is the equivalent gas volume at standard (atmospheric) conditions. More tank pressure = more available gas volume. Larger physical tank volume at same pressure = more available gas volume.

I found this welding excerpt which I think is like the 'simple' calculation I mentioned. But again, this is atmospheric delivery pressure so can go a long ways to deplete the tank vs elevated pressure delivery. Also welding at 20 CFH (cubic feet per hour) is only 20/60=0.33 cf/min. That's very low compared to any kind of tool demand.

 

[JimGnitecki]

Yes, I will be doing some quantitative testing. Here are 2 photos of the Nitrogen setup at this time:

This is the dual gage & regulator set before I replaced the output nipple with a female NPT 1/4" fitting that accepts a standard air hose. I'll restrict the output psi to 80 psi or so since my tiny plasma machine says not to use more than 65 or 70 psi (I forget the exact psi) inside the machine (It has its own regulator with the adjustment gage and control on the front panel). So, no point in letting more than 80 psi come out of the Nitrogen regulator.

Here is the Nitrogen cylinder and hose setup ready to use. The cylinder safety chains are hooked onto wall hooks that are the biggest and strongest I could find and still be able to fit the chain links over them.

The hose fittings are 1/4" since that is what both my plasma cutter and my finish nailer use. But the hose is 3/8" ID, for less restriction. The hose is rated for 300 psi service.

The only issue I ran into insetting this all up was with proper sealing of the various 1/4" NPT air fittings. This is the first time I have actually needed to INSTALL fittings, as opposed to merely using already-installed hoses and fittings. I carefully read multiple online instructions on how to install and seal them "properly". Those instructions all said to use only 2 layers of the white pipe sealing tape, avoid covering the very first thread that engages (so that you can't get any of the tape into your air system!), to then tighten BY HAND and then apply only light torque for a small fraction of a turn. Test using dishwashing detergent brushed on the fitting while under pressure and look for air bubbles.

But those instructions must have assumed high quality fittings, not the cheap ones commonly sold now at big box stores. Those fittings probably have looser thread tolerances. 2 layers of tape was not sufficient. I got small air bubbles, and faint sound when I placed my ear right beside them, until I had 4 layers in place. I also needed to use a wrench for more than a small fraction of a turn. But i finally got no bubbles at any of the fittings with the output psi of the N2 cylinder set at 80 psi.

Notice that I used a quick disconnect fitting even at the nitrogen end of the hose, and of course a female QD at the hose end that connects to the plasma cutter water filter.

My welding supplier also reinforced that I will have no moisture problems since I am using nitrogen versus compressed air from a compressor.

Thge N2 cylinder pressure is starting out at about 2300 psi, and the starting volume is apparently 299 cubic feet.

We'll see how the testing and learning goes. I've read enough online now to realize that getting that reasonably perfect combination of:
- amperage
- gas psi actually delivered to the torch
- travel speed
- that perfect spray output at 15 to 30 degrees from vertical, below the material being cut
- NO upward spark spray
- a visible final "cut grain" slanted at the same 15 to 30 degrees from vertical, visible on the cut edge of the material
- Dross on the underside that is minimized and easy to knock off
- Little or no dross on the top side of the cut material
- a clean (no blob) beginning to the cut and clean end to the cut, achieved via proper torch rotation at the beginning and then at the end of the cut, along the axis of the cut both times, but in opposite direction (pull torch handle rearward at start and then forward at the end

will be quite challenging at first, as I juggle the various things to remember and to adjust as needed.

Jim G

 

[JimGnitecki]

I thought about the Boyles law thing as well, but I believe cylinders are rated at say 244cf at 2400psi...or whattever that cylinder is rated, I believe that to mean at 2400psi that cylinder contains 244cf, but the relationship between pressure on the gauge and volume in the tank is not linear....probabaly the reason tanks seems to take longer to go from 1000 to 0 vs 2000 to 1000

Let's be just ab it more specific and accurate. In your exmaple, the "244 cf" means that the gas contained in that full cylinder would occupy 244 cubic feet if it was uncompressed and at "standard" temperature, sea level, etc". When compressed in the tank, it is obviously a much smaller volume.

Since standard atmospheric pressure is 14.7 psi, if the pressure inside the full cylinder is (accurately) 2400 psi, then the interior volume of that cylinder is 244 x 14.7/2400 = 1.5 cubic feet! That passes the sanity check, since the cylinder is "about" 8.5 inches INside diameter, and maybe roughly 4 feet high (since the top is tapered). My rough math comes up with 1.57 cu ft, which is close enough!).

Now the gas law says PV = nRT where:
P = pressure
V = volume
n= a constant whos evalue depends upon the units being used
R = the universal gas constant
T= temperature

N, R, and V are fixed.

So, the variables are Pressure and Temperature.

With "AIR" in a compressor tank, the temperature changes a lot as the air is being compressed, and it apparently changes enough to have water vapor in it either condense or evaporate depending on what is happening at the moment with the compressor and the amount of humidity in the air to begin with.

I am no chemist, so I don't know what happens with Nitrogen in a cylinder, but the experts tell me that you get no moisture issues. I have no idea what happens to temperature as Nitrogen gas is used, and at differing rates.

Jim G

 

[phaxtris]

Maybe we are saying the same thing, maybe not LOL. The volume of the physical tank is of course fixed, so I think what they are specifying is the equivalent gas volume at standard (atmospheric) conditions. More tank pressure = more available gas volume. Larger physical tank volume at same pressure = more available gas volume.

I found this welding excerpt which I think is like the 'simple' calculation I mentioned. But again, this is atmospheric delivery pressure so can go a long ways to deplete the tank vs elevated pressure delivery. Also welding at 20 CFH (cubic feet per hour) is only 20/60=0.33 cf/min. That's very low compared to any kind of tool demand.

View attachment 30731

I get what your saying now....is 5cfm @ 20psi going to the same volume at 5cfm at 70psi

That's a good question.......

 

[historicalarms]

In your case of trying to maximize bottle usage & as much pressure retention throughout the bottle life that you can , I would shorten that air line with the quick coupler to the absolute minimum you need ( position your plasma cutter as close as you can. The optimum thing would be to do away with that quick coupler and join them solid....every time you disconnect there is going to be an escape of gas equivalent to that long hose ID...that PSSSHt you hear every time you uncouple in this case is money not just air.

 

[JimGnitecki]

I need to correct a statement i made:

Earlier, I said there is no point in providing more than about 80 psi at the output of the Nitrogen regulator since my plasma cuter is designed to handle only up to 65 or 70 or 75 psi internally (can't remember what exactly off the top of my head). BUT, if the CMF (versus psi) requirement apaprently needs to be an honest 6 CFM for some cutting, then I may need to provide considerably MORE psi all the way before the plasma cutter's internal regulator, so that the 6 CFM DELIVERY RATE can be met.

I have no idea how MUCH more psi migt become necessary in that scenario. That will have to be part of my experimentation.

Jim G

 

[CWret]

My shielding gas tanks are right beside my welder. Shutting down the welder and turning off the gas supply line is always done at the same time. That's another good reason to have your tank close to the cutter - therefore easy to shut them both off at the same time.