Compressed air supply required for 50A plasma..

[hotponyshoes]

Hi all,
I have just ordered a machine torch for my Cebora Prof 55 plasma cutter and I am half way through building the table.

Now, I am looking for a air compressor to run the thing. The small one I have will only cope with a 2' cut before the pressuer drops to low.

The confusing bit is this:
My plasma manual tells me I require 140 liters/min at 4.7 Bar.

All the compressers I se advertised are rated in CFM and given a maxium working pressure in bar.

How do I work out what size compresser I need? It will only be used for the plasma and nothing else.

Also, Would it be possible for me just to buy a head and run it from a motor?
Would I need a reciver tank if I could just pump 140l/m of air at 4.7bar?
Thanks!

[millman52]

This link will take you to a PSI-Bar conversion chart.

http://www.britishmetrics.com/html/pis-bar.htm

& 1 CFM = 28.31682 Litre/min.

Hope this helps

[Weldtutor]

Originally Posted by hotponyshoes Would it be possible for me just to buy a head and run it from a motor? Would I need a reciver tank if I could just pump 140l/m of air at 4.7bar?

An air receiver tank is usually needed to provide sufficient pressure to your plasma machine to allow it to sense that it can "start"
Most plasma machines will not start without air pressure.

The air storage will also allow correct volume & a smoother working transition from no air to working pressure/volume.

[Geof]

Originally Posted by hotponyshoes ...Would I need a reciver tank if I could just pump 140l/m of air at 4.7bar? Thanks!

You need a tank because it is practically impossible that the L/m provided by the compressor will exactly match what the plasma needs. A storage tank provides a buffer capacity so that if your requirement is only 70% of what the compressor can provided the compressor will only run 70% of the time.

You also need a tank so you can set the cut-in pressure for the compressor at a pressure a bit higher than the maximum you will ever need for the plasma and have a regulator on the outlet from the tank. This way it does not matter if the tank pressure cycles up and down between the cut-in and cut-out pressures the plasma supply is kept constant by the regulator.

And you will probably also need oil and water traps in the outlet line.

[The Inventor]

Originally Posted by Geof And you will probably also need oil and water traps in the outlet line.

That is a "MUST HAVE" piece of equipment! (And don't go cheap on this! A Good
water/oil trap will save your machine)

If you were to come off of a head 'only' setup, you would be pumping a ton of oil
and water vapors thru the line right into your plasma machine.

As for air supply... the bigger the holding tank the better. Think along the lines
of VOLUME, (stored Cubic Foot)... the more you have the more constant your
air flow will be.

I worked in a factory that installed 2 or 3 - 500 gal holding tanks and could now
shut off their back up compressor. (Saved a ton of money on electricity as
well.)

[hotponyshoes]

Thanks for the info!

I have a full filter setup attached, filters oil and water etc.

According to the table I will get 140 Litre/Min from a 5cfm compressor.

The thing is, I have been trying to run it from a 'hobby' compressor (the idea was to make it portable). According to the plate on the compressor it's 7CFM with a max pres of 8 Bar.

If I let it charge up fully I can cut for about 10secs until the compressor starts running again and carry on cutting for anotheh 20secs until the 'low air' light comes on and the plasma shuts down.

After trying to do some maths I have worked out that the 7cfm rating must be free air (ie at 1 bar) so I would need a compressor with a rating of 33cfm to produce 140L/M at 4.7bar.

This size of machine costs about 3 times the plasma cutter!

The compressor I have only has a tiny (25L) tank so a bigger one would help. But If I had a 150L reciver I would still only get 2 mins of cutting if it was charged up to 9 Bar.

As I am making a table could you guys tell me what size compressors you use to cut non-stop?
Thanks!

[Geof]

If you are looking at getting something with a 33 cfm rating I strongly suggest you consider a Screw Compressor. Piston compressors most times only have a duty cycle of 30 to 50% otherwise they are likely to overheat. In other words if you are consuming 33 cfm for several minutes continuously you need a piston compressor rated at maybe 50 cfm or a really big tank.

Screw Compressors can run at 100% duty cycle 24 hours a day without problems. They are more expensive than piston type but they are smaller for the same output capacity and they are more efficient so their operating cost is lower. They also last much longer; I bought a used 60 cfm screw type that has done 45,000 hrs and can still put out 45 cfm.

If you are setting up a business get a screw type. I wasted half the ultimate cost of my screw compressor buying one, then two, then three piston compressors first.

[millman52]

Most compressors, In the USA anyway will show ratings of Free air (CFM @ 0 PSI) Then usually (CFM @ 40#) & (CFM @ 90#)etc.

[millman52]

By my calculations you need a compressor that will produce a minimum of 5 CFM @ 4.7 bar. or 5 CFM @ 70 PSI. At that the compressor would want to run all the time you are cutting with the plasma.

As someone else stated you don't really want that situation.

Personally I would want a minimum of 60 gal tank, single stage compressor in the 3-5 Hp Most of them will give around 11 CFM @ 90 PSI or about double what your plasma cutter needs.

Something else you will have to give thought to is distance of your compressor away from your table. Air needs to cool for the moisture/oil to condense back to droplets from vapor. Most standard oil/water traps will not catch vapor.

[massajamesb]

I run a 60 gallon tank, 7.5 Hp belt driven Husky compressor. I think it is in the 10 CFM range at 90 psi, 12 CFM at 40 psi. It runs frequently, but it not only keeps up, but will fill and shut off while I am cutting.

I believe I can go buy another one for about 450 USD.

As far as oil/water traps, go with a good Devilbiss (or equivalent) dessicant dryer. Normal water/oil traps don't compare to a dessicant filter. They are rather pricey, but well worth it. You will notice a difference in cut quality and tip life. These are the same filters used in most professional automotive paint boothes. A good dessicant dryer will run from 150-450 USD.

[millman52]

Originally Posted by massajamesb I run a 60 gallon tank, 7.5 Hp belt driven Husky compressor. I think it is in the 10 CFM range at 90 psi, 12 CFM at 40 psi. It runs frequently, but it not only keeps up, but will fill and shut off while I am cutting. I believe I can go buy another one for about 450 USD.

LMAO Yeah sounds like the Campbell Hausfield I am running. 7.5 HP My A** I think mine says 7.5 Peak HP I'm sure that's while the start caps are in kicking it up to start. In reality that is somewhere in the 3-4 HP range @ 15Amp draw 220V. I think it was 399.95 At the local Harbor F store. The compressor pump lasted apx 3 years before the flywheel came loose & ruined the crankshaft. I replaced the compressor pump with a single stage Quincy pump. Full lubrication internal oil pump. Rated for continual run.

There is lots of inflated propaganda out there on HP on the homeowner small shop type compressors. As well as minimum air requirements on air tools.

[millman52]

My 2 stage compressor has a true 5HP 29A @220V GE motor. The motor weighs around 150# itself. The entire single stage unit tank & all probably doesn't go much more than 200# That alone should tell you something.

Air compressors are like most anything else you buy. You pretty much get what you pay for. Compressors are about the most demanding piece of shop equipment there is on a motor.

My definite list of dont's include:
1) don't buy an oil free
2) don't buy a direct drive
3) don't believe HP claims on advertising
4) don't set them up inside your shop. Unless there is no alternative
5) don't buy just what it takes to just get by