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

[Six O Two]

I remember watching some Mythbusters episodes with exploding water tanks that were quite eye-opening to me at the time.

If you're set on the horizontal 20 gallon form factor for a compressor, you might want to check the used market as well. I see craftsman and devilbiss 3 & 5hp 220V models rated for ~10cfm@90psi come up every now and then selling for a few hundred dollars. Last year, I picked up a 2nd hand Speedaire much like this one which is rated for 10cfm for a fraction of this listed price. It's still loud, but the low rumble is a lot more pleasant than the high pitch whine of my previous 3 gallon mastercraft compressor.
https://www.grainger.ca/en/product/AIR-COMPRESSOR-3-0-HP-240V-135-PSI/p/GGF1NNF7

 

[Chicken lights]

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?

You could get a used air tank from a heavy truck wrecking yard, just make sure the ports are pipe thread

 

[Susquatch]

Propane tanks are a different beast because they are designed to hold liquid gas. When they are close to empty you could see mostly gas at that pressure if a puddle of liquid at the bottom remained.

But I can't in good conscience recommend it. It's still very dangerous.

All of the compressed air tanks here at the farm need to be recertified or discarded on a given date stamped on the tank. I bet nobody does it though. Even my little portable CanadianTire air tank has a discard date on it, but no certification date.

I don't know for certain, but I'd bet commercial tanks are supposed to be recertified. I'm not sure how many actually are.

I am of the view that this is not something to cut corners on. It's kind of like doing the brakes on your truck. It's just not worth taking chances to save some money. Regrets after an accident is not something any of us want to look forward to.

 

[JimGnitecki]

I had a good talk with an experienced salesman at my local welding supply place, which equipped me with the gas and other supplies I needed for my TIG welding. I described my objectives and constraints, and he said he would call me back with the best solutino he could find.That turned out well.

He called me back and said the best solution he could provide that would work and still be not very costly was a Nitrogen cylinder with the following breakdown for initial cost:

$125​	Gas fill
$205​	Dual gage regulator setup
$95​	Annual lease on tank
$425​	subtotal
$21.25​	tax
$20​	delivery (and he might be able to skip the delivery charge as I'm close to their regular route
$466.25​	total initial setup
8.45​	cubic meters of Nitrogen in the filled tank
=299​	cu ft
approx 140 lb​	Total filled tank weight

He looked at the idea of a high pressure (4000 psi) tank, but the killer there was the cost of a suitable safe high pressure gage/regulator setup, which would exceed $1100!

The Nitrogen setup by comparison is as you can see far less unusual and pretty "standard" in comparative cost, availability., and cost competitiveness.

In the Nitrogen solution, the "one-time cost" is only $205 (the dual gage / regulator setup). The annual lease on the tank is $95, or $8 per month, which is reasonable for having the instantly available and reliably consistent gas psi and CFM "on call" as needed.

The $125 fill cost for 299 cu ft is reasonable.

Nitrogen is apparently a good gas to use with Aluminum and stainless steel plasma cutting especially (which is most of what I plan to do) for superior cut quality. And it works fine on mild steel as well. Apparently, it provides a particularly smooth cut.

I won't have to handle transporting the 299 cu ft = 140 lb tank myself. The supplier will deliver it. My rear garage has a 3 foot driveway to the back lane behind our home. Pretty nice.

The Nitrogen cylinder regulator assembly connects to a standard 1/4" air fitting hose.

Running some math on how many cuts I could make, in varying lengths, before needing a refill, it looks like POSTFLOW is going to be ther biggest variable to optimize if many of the cuts are short. In fact, for example, on cuts of 3", it looks like postflow would consume more Nitrogen than the cuts themselves, if postflow is set at the machine's maximum of 15 seconds! I have no idea yet how much postflow would actually be appropriate for cuts of only 9 seconds duration.

A nice thing about Nitrogen in a cylinder is I can run VERY high CFM or very high psi if the circumstance ever suggest that would be a good idea for a given material or material thickness. That's a big advantage over a fixed capacity compressor.

Overall, this seems like a far better solution than buying a large compressor for which I have no other need.

So, I am going to try this. I need an Argon refill for my TIG welding right now anyway, so the shtop will deliver both a replacement Argon cylinder and the nitrogen cylinder tomorrow, and I'll then do some experimenting.

Jim G

 

[phaxtris]

just an observation but by the second fill you are at the cost of a compressor, anything after that you are going to be in the red

 

[Susquatch]

Overall, this seems like a far better solution than buying a large compressor for which I have no other need.

So, I am going to try this.

Good. I'm glad you are happy.

I confess those costs would have stopped me cold. But at least now I'll never lust after a plasma cutter again. So I'm grateful to you for your journey.

 

[Chicken lights]

just an observation but by the second fill you are at the cost of a compressor, anything after that you are going to be in the red

also just an observation, but you can always sell an air compressor if you don’t need it any longer

 

[Six O Two]

... a large compressor for which I have no other need.

I mean, now that's just crazy talk. You're working with metal, you need an air compressor... think of all the air-powered tools you could buy to go with your compressor! Die grinder, small belt sander, cut off wheel, nibbler... The list goes on and on! And they're cheap compared to electric ones...

 

[DavidR8]

It occurs to me that a air nibbler may be just the tool for the intended cutting tasks.

 

[historicalarms]

But at least now I'll never lust after a plasma cutter again.

Not me, now Im in the other camp...been looking at plasma again ever since Jim started this thread....but I am in your camp on the bigger compressor...kinda like a gun...better to have one and not need it than really need one and not have one!!

One thing that is gonna differ me from Jim when i get that damn plasma now...I'll regulate my cutting speed expectations to the compressor limitations that I already have...I have limitless time to wait for the compressor if i wasnt waiting for it, I'd just be sitting in a chair looking outside.

 

[farmer888]

I admit I didn't read all the replies. I think I'd still look into a bigger compressor. The cost to get the cylinder of nitrogen and reoccurring costs for refills and the annual lease are well on your way to a bigger compressor. Even if you have to get a water and oil separator. We have always used our plasma with air from oiled compressors with no issue.
Sometimes good deals on used, just have to have the concern about rusted tanks etc.

 

[Susquatch]

Not me, now Im in the other camp...been looking at plasma again ever since Jim started this thread....but I am in your camp on the bigger compressor...kinda like a gun...better to have one and not need it than really need one and not have one!!

Darn good point! I'm in no rush to go anywhere either!

I'll start lusting again!

Thanks for getting those snowshoes of mine back on the ground.

 

[JimGnitecki]

just an observation but by the second fill you are at the cost of a compressor, anything after that you are going to be in the red

That is true, BUT:
- The plasma cutter is the ONLY tool I have that needs a bigger compressor than I already have
- The Nitrogen cylinder can produce both PSI and CFM much higher than any compressor I can afford or justify - if I ever need them, and ONLY when I need them
- The Nitrogen gives superior quality results over air on the aluminum and s.s. that will be the vast majority of what I work on
- My work volume for plasma cutting will be low compared to any other process in my shop (but the plasma cutting capability will allow me to do more interesting work!)
- No NOISE from the Nitrogen cylinder. This is a HUGE benefit to me personally
- The cost of the Nitrogen, whether viewed as an annual cost or as a cost per piece, is acceptable to me, and the negative of longer term cost penalty over a compression is less important to me than the benefit of NO NOISE.

Jim G

 

[JimGnitecki]

I mean, now that's just crazy talk. You're working with metal, you need an air compressor... think of all the air-powered tools you could buy to go with your compressor! Die grinder, small belt sander, cut off wheel, nibbler... The list goes on and on! And they're cheap compared to electric ones...

I know that air tools make sense for a production shop, but for me, they are unnecessary beyond the air finish nailer I already have and do run off my current small air compressor.

Jim G

 

[trevj]

How far to the house from the shop? And, is the Missus, if there is one, usually in the house when you use the shop?
My first thoughts would be to look at an upgrade to the shop power, so as to handle a decent 3-5 HP compressor, along with the cutter. Shop for good name brands, used. Or, watch for sales. Pay attention to the expected operating life of the compressor head, to get a decent idea where it fits in the grand scheme.

If you cannot get more power to the shop, if you have the space in your basement or in a utilities room, run the compressor there, and feed a large diameter hose through the wall, and run a flex line out to the shop when you need the volume. Only really viable if SWMBO, is not going to be bothered by the noise of it running.
Use large diameter HVLP Painting recommended Air Lines and Fittings.

A gasoline powered Contractor's Compressor may be an option too. Rough rule of thumb, is to have three times the rated horsepower of gasoline motor, to get similar performance to a given HP Electric motor.

If noise is a particular concern, put the comressor in a dog house, outside the shop. Have seen more than one guy that actually painted the front to look like the opening on a dog house, too.

I have used a Nitrogen bottle on a high flow regulator, to run air tools, but it burns through a lot of bottled gas. A compressor is cheaper! And, waiting for it to fill back up, is still way faster than waiting for the truck to come by, sometime next week...maybe... Finding tha the compressor leaked down over the weekend, isn't going to blow any serious holes in your plans, either!

 

[JimGnitecki]

How far to the house from the shop? And, is the Missus, if there is one, usually in the house when you use the shop?
My first thoughts would be to look at an upgrade to the shop power, so as to handle a decent 3-5 HP compressor, along with the cutter. Shop for good name brands, used. Or, watch for sales. Pay attention to the expected operating life of the compressor head, to get a decent idea where it fits in the grand scheme.

If you cannot get more power to the shop, if you have the space in your basement or in a utilities room, run the compressor there, and feed a large diameter hose through the wall, and run a flex line out to the shop when you need the volume. Only really viable if SWMBO, is not going to be bothered by the noise of it running.
Use large diameter HVLP Painting recommended Air Lines and Fittings.

A gasoline powered Contractor's Compressor may be an option too. Rough rule of thumb, is to have three times the rated horsepower of gasoline motor, to get similar performance to a given HP Electric motor.

If noise is a particular concern, put the comressor in a dog house, outside the shop. Have seen more than one guy that actually painted the front to look like the opening on a dog house, too.

I have used a Nitrogen bottle on a high flow regulator, to run air tools, but it burns through a lot of bottled gas. A compressor is cheaper! And, waiting for it to fill back up, is still way faster than waiting for the truck to come by, sometime next week...maybe... Finding tha the compressor leaked down over the weekend, isn't going to blow any serious holes in your plans, either!

Thanks, Trevj, for taking the time to do the very thorough analysis! Here are my responses to your questions:

The rear garage, which is the shop, is located about 10 feet from the rear of the house. The kitchen and living room are both located at the rear of the house, and my wife spends her time there! She, like me, is retired, so is ALWAYS there when I am in the shop. She has expressed her displeasure about the CURRENT, too small compressor running in the shop!

So has our 10 year old cat "Sophie". Here ears are WAY more sensitive than human ears. She cries when we load or unload the dishwasher, so we isolate her in a bedroom when doing the dishwasher loading / unloading.

Upgrading shop power is not an affordable OR allowable solution. That would require a permit, at least 50 feet of VERY expensive no. 6 underground rated electrical cable (our current no. 8 cable is at the code limit for even a welder (let alone any other usage like a higher powered compressor - welders get a special dispensation only because they work in such short bursts of current), the labour cost to dig and lay the cable to code depth, the cost to restore the dug up landscaping, and the electrican's fees. Not going to happen, not only because of the raw cost, but also because, again, I have no other need than the "cheap" plasma cutter to justify it!

The wife, the cat, our adult disabled son, all our neighbours (50 foot lots with 10 feet between homes) and city code would all veto any thought of running an electric or gasoline compressor in the house or yard!

Then there is ME. I run the current too small compressor only when I absolutely need to, as I like my hobby time to be pleasant, not a noise endurance session. With the air finish nailer, or topping up the various vehicle tires, the compressor runtime is very short. Not so with a plasma cutter.

It's basically an avalanche of "no" on the higher power compressor . . .

Jim G

 

[JimGnitecki]

Here's a photo showing the "floor space"occupied by the Nitrogen tank:



I still just have to buy 2 hooks and 2 lengths of safety chain to secure it to the wall. It apaprently holds 299 cubic feet, and is only 9" in diameter. Its location right by the overhead garage door made it very easy for the delivery truck drivers to simply "walk it" into its location.

Now I just need the right adaptor to connect a normal 1/4" air hose to the output of the twin regulator assembly. I'll stop in at the weld supply place tomorrow to get them to equip me with the right adaptor.

Jim G

 

[trevj]

Thanks, Trevj, for taking the time to do the very thorough analysis! Here are my responses to your questions:

The rear garage, which is the shop, is located about 10 feet from the rear of the house. The kitchen and living room are both located at the rear of the house, and my wife spends her time there! She, like me, is retired, so is ALWAYS there when I am in the shop. She has expressed her displeasure about the CURRENT, too small compressor running in the shop!

So has our 10 year old cat "Sophie". Here ears are WAY more sensitive than human ears. She cries when we load or unload the dishwasher, so we isolate her in a bedroom when doing the dishwasher loading / unloading.

Upgrading shop power is not an affordable OR allowable solution. That would require a permit, at least 50 feet of VERY expensive no. 6 underground rated electrical cable (our current no. 8 cable is at the code limit for even a welder (let alone any other usage like a higher powered compressor - welders get a special dispensation only because they work in such short bursts of current), the labour cost to dig and lay the cable to code depth, the cost to restore the dug up landscaping, and the electrican's fees. Not going to happen, not only because of the raw cost, but also because, again, I have no other need than the "cheap" plasma cutter to justify it!

The wife, the cat, our adult disabled son, all our neighbours (50 foot lots with 10 feet between homes) and city code would all veto any thought of running an electric or gasoline compressor in the house or yard!

Then there is ME. I run the current too small compressor only when I absolutely need to, as I like my hobby time to be pleasant, not a noise endurance session. With the air finish nailer, or topping up the various vehicle tires, the compressor runtime is very short. Not so with a plasma cutter.

It's basically an avalanche of "no" on the higher power compressor . . .

Jim G

Honestly, given all the obstacles in your path, I am more than a little surprised that you chose anything to do with metalworking, as a hobby!

I hope the tank works out for you. As a long term prospect, it seems a money sink, from my perspective.

 

[JimGnitecki]

Honestly, given all the obstacles in your path, I am more than a little surprised that you chose anything to do with metalworking, as a hobby!

I hope the tank works out for you. As a long term prospect, it seems a money sink, from my perspective.

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

 

[PeterT]

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?