air compressor size

[pcilse33]

How big of an air compressor are Y'all using? I have a hypertherm 45xp. I have two small compressors (40 gal and 60 gal) then an old 80 gal storage tank, then running thru a desiccant dryer and finally to a 20 gallon reservoir tank (for clean dry air) but still run low on air. One compressor is rated at 3.5 scfm @90 and the other 4.0 scfm @ 90.
I have since learned that air compressors are rated higher than they actually are and the Powermax 45 is rated to us 7-8 scfm "unless in constant use" but most of my cuts the cool down flow at the end of the cut just barley stops before the next cut starts.
What size and manufacturer are you using?

[weldguy]

Not sure the brand but mines nothing special. Twin piston pump on a 60 gallon tank with a 5hp motor and rated for around 11-12 cfm and it has no problem keeping up. Pump volume is a little low but you certainly have enough reserve air in all your tanks. Are you sure you don't have a partially plugged air filter that's not allowing air to pass quick enough to keep the plasma running? Easy way to check is have a pressure gauge before and after your filter system to compare pressures, shouldn't be much pressure drop on the clean side of your filter system.

[CGT80]

Pressure is key!

My machine is a Westinghouse Air Brake Company 2AVC and the design is 70+ years old and the pump could easily be 40-50 years old, but maybe newer as well. It is a 2 stage V configuration and it looks roughly the size of a harley engine. The pump weighs 200 pounds without the flywheel and is run on a 5hp baldor motor (a real 5hp motor, not some over rated POS). The pump is pressure lubed and will work up to 275 psi. It is spinning at 840 rpm roughly and cycles from 135-170 psi and takes 60 seconds to top off an 80 gallon tank. It is rated 17.2 cfm at 175 psi and is averaging 20 cfm based on tank fill time.

Full tank pressure is run through 125' of hose, mixed 3/8" and 1/4" and then goes into a regulator, 5 micron filter, and 0.01 micron paper filter. It never drops below 110 psi at the hypertherm 65. The pressure allows the small hose for that run and it means any pressure lost from the filters is no big deal.

The hypertherm 65 is about 7cfm max, according to Jim Colt at Hypertherm. The fine cut tips use less, but I also occasionally use it with 65 amp tips for heavy material.

OP, that is only 7.5 cfm if both compressors are living up to the specs. The problem I had with an old cman 20 gallon advertised 5hp that made 10 cfm is that the pressure dropped too much before turning on. It is right next to the table, so a short run of 1/4" hose. The tank pressure and pump pressure max out around 135 psi, so I couldn't set the switch higher, and the low side of that switch would often end up around 100 psi or less. Basically, there was no overhead/reserve. It would have kept up with a different switch, but it is so noisy and annoying even with ear pro that I just ran a hose to the big compressor.

I got a 1947 WABCO 1BYC and restored it for that shed, as it feeds the garage next to it for blowing off parts for our repair/sharpening business, but it only makes 6 cfm with the old GE 1.5hp motor. It is very quiet compared to the old craftsman. Most likely, I will keep using the big machine for the plasma table, but at least when I am in there and not cutting, the other compressor will be tolerable to listen to.

So, what is the low and high pressure on the compressors and what is the pressure after the filters and while the plasma is running? More tanks will only buy a little time, if the pumps can't make as much volume or pressure as the plasma needs.

[SegoMan DeSigns]

I have an Eaton 20HP 3 ph dual stage w/120 gal tank / auto tank drain / Big Joe regulator(125 max line pressure). 1"main manifold w/ 1/2" feeder lines, All 4 corners of the building has drip traps. Motor Guard cartridge type filter plumbed directly to both the Hypertherms

[pcilse33]

Thanks for the feedback. I will have to upgrade my compressors.

[ScottF]

Sounds like you need a compressor with higher SCFM output. If you want to read more gee wiz info about compressed air systems keep reading. If not....STOP.

Compressed air is measured in SCFM, PSI and storage capacity. Each of these metrics are important and interconnected but a deficiency in one cannot be solved by an excess of one of the others. Compressed air systems are not efficient but there are ways we can make them more efficient.

SCFM (standard cubic feet per minute) is the measure of air flow corrected for pressure and temperature. It is the industry standard for measuring air flow. Understand that each manufacturer will do anything they can to make their machines look better than the others. So air compressors might say they can provide 14 SCFM @90 psi but you may not see that in actual usage. The higher the rated output (SCFM) at a higher pressure (PSI) the higher the cost of the compressor. This is where you get to balance your needs and desires with your budget. If you use the Powermax 45 at 7 cfm all day long make sure that your compressor supplies more than that otherwise you will have to stop using the plasma while you wait for air to build. I size my own needs at double the demand. So if my continuous demand is 7 SCFM @ 90PSI, I have a compressor that will supply 14 SCFM at that pressure. This allows the compressor to not only supply my demands but also build a surplus and put it into the storage tank.

PSI in rough terms is the amount of work that the air can do. One gallon of air at 60 psi can do X amount of work and one gallon at 120 psi can do more work. Most air tools need roughly 80-100 psi of air to work per the manufacturer. As an example/analogy say you need to file some paperwork (requires 80psi). You can hire a clerk to do it at minimum wage (100PSI) or you can hire a MBA level employee (175 PSI). Both will get the work done but you will pay more for the ability to do more work both in the upfront cost as well as the in the operating costs.

Not all compressed air systems have or need storage. Storage or receiver tanks allow for a reserve of air for brief periods of higher usage (above the SCFM output rating of the pump) and allow the pump to cycle off and on if they are not rated for continuous duty. If you are using a tool that has a SCFM lower than that of the pump the larger amount of storage you have will allow for a longer period of use before the pump cycles on at the low pressure cut in. A tank also allows you to use a tool with a very high air consumption rate (grinder, sander, etc.) for a brief period of time even though your pump can not supply the demand continuously. The larger the tank the longer these periods are.

To get the most efficiency out of they system set the regulator that supplies your shop air to provide the pressure you need to operate the desired tool while under flow. Creating the pressure differential is key to efficiency (higher pressure in the storage tank supplying the working lines) but you will over spend on electricity if you run your system at 250 PSI system when all you need is 100psi.

If you are still reading this I hope that I have done a decent job explaining.

[pcilse33]

Thanks
That helped. I need to get a compressor rated closer to 14scfm.

[DXF]

I originally used a Sears 5hp 25 gal. air compressor (very loud) and then a Porter Cable about the same size. They both created a lot of heat and moisture and both burned out and had to be rebuilt. Finally spent the money on a Campbell Hausfeld 4hp 80 gallon rated at 12.2 cfm. Best decision I have ever made.

Dave Hanks

[2015cmax]

Scottf,
I think that your discription and explanation is very well written and very clear for most of us with zero background in this often complex subject. The lack of foundation pushed me in the direction of that old saying, "more must be better". My visualation when deciding on a compressor some 30 years ago was that of water through different diameter pipe and pressure. A larger diameter pipe may have a higher rate of flow but without a bigger pump it will not have the desired pressure needed to complete or sustain the operation. As crazy as this sounds, it has worked for me. My 30+ year old Craftsmen 5hp 30gal has never let me down. Admittedly, I am treading on thin ice using it for my plasma table running a PM65. I have only run one complex operation where out of caution I piggy backed a second gas driven compressor to supplement the system. That operation, several months ago, gave me pause to start considering an upgrade compressor.

Your explanation will give me greater decision making confidence. Thank you for that!!

Mike