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What size air-compressor is needed for continuous duty?

I'm trying to use a 90psi needle scaler to remove paint layers from an outdoor wrought-iron trellis that has lots of nooks and crannies an abrasive pad and angle-grinder cannot reach. My brand-name 6-gallon air-compressor doesn't have enough capacity to support continuous use: the scaler works well only for about 5-6 seconds and then its ability to remove the paint rapidly peters out. Then it takes about 15-20 seconds for the compressor to get back to 90psi. So the job is taking forever.

  • Your tool's documentation will state its requirements. There's no rule for this sort of thing. – isherwood Dec 18 '19 at 16:54
  • The tool does not state its requirements. It just sits there and says nothing. And there's nothing written on the box. And the manufacturer probably does not speak English. So I guess my question is, what is the typical rating of that kind of scaler? – mr blint Dec 18 '19 at 19:13
  • I'd be willing to be that we could find specs for it if you care to post the model. Here's an example of a spec chart that lists 6CFM @ 90psi. You can then search for a compressor that claims to provide that flow volume, such as this one, for example. There is no "typical rating" since all these tools use air more or less efficiently. – isherwood Dec 18 '19 at 21:22
  • Is a compressor rated at 6CFM at 90psi capable of running a tool continuously if that tool consumes 6CFM at 90psi? Is that a correct interpretation of the CFM rating? Or is there a"duty cycle" rating in addition to the CFM rating? – mr blint Dec 18 '19 at 21:56
  • There is a duty cycle, which means, essentially, that you can run x% of each hour. – isherwood Dec 18 '19 at 22:33
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Your tool and your compressor both have (or should have, but they probably do) ratings in "Standard Cubic Feet Per Minute" (SCFM) possibly stated just as CFM.

The compressor SCFM needs to be equal to or greater than that of the tool to operate the tool continuously.

Small "homeowner-grade" compressors are often not rated for continuous operation at all (they will overheat if asked to supply their maximum SCFM for more than 10 or 20 minutes at a time.) Industrial compressors are more typically designed to operate continuously for long periods without failure.

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  • All compressors can supply their stated SCFM continuously. The problem is that most homeowner types provide very low amounts of air, typically below 2 SCFM. – Matthew Gauthier Dec 19 '19 at 0:03
  • Thanks. My no-name needle scaler had no documentation but I've found another one at a big box store which says it's rated at 6CFM. – mr blint Dec 19 '19 at 15:55
  • First one I happened to look at was 15 CFM. Your undocumented import may vary ;-) – Ecnerwal Dec 19 '19 at 16:34
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For continuous duty on the compressor end you need a rotary screw rather than a piston compressor; you've probably seen road crews towing them behind the truck. For continuous tool use what you need is a compressor which provides a higher CFM than the tool requires. (And technically an air set to inject oil into the line, because if you're stopping to lubricate it manually the tool isn't in continuous use.)

The first problem you're running into is that portable compressors provide very small amount of air. A typical six gallon pancake provides about 2.5 CFM, which suffices to run staplers and finish guns all day. Anything using more air like a framing gun or your scaler and you'll be waiting on the compressor. They can still be useful with large tools you use intermittently.

The second problem you'll encounter is physics. Enthusiastic horsepower claims aside you can only draw a finite amount of power from a standard outlet. Ingersoll Rand's Garage Mate is a popular portable compressor retailing around $600, which gets you a whopping 5.5 CFM. (Portable is a bit of a stretch too, since it has a twenty gallon tank.) That's about all you're getting from a regular outlet.

Unless you're running a large assortment of tools it's probably more economical to stick with corded electric ones if you need to use them continuously.

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  • Thanks for the info. Unfortunately, the electric needle scalers cost about $1,000. – mr blint Dec 19 '19 at 15:53
  • You may get better suggestions if you post a question about removing the paint. Air erasers are about $90 and have minimal air requirements. A chemical stripper and wire brush is probably faster. If you can remove the trellis dipping or electrolysis are options. – Matthew Gauthier Dec 20 '19 at 12:40
  • The trellis is cemented in place, unfortunately. I am using chemical stripper, wire brushes, angle-grinder with roloc disks and knotted brush, and a needle-scaler. I don't think a small air-eraser would be up to the task, since there's about 75 years worth of paint layers on this metal trellis. Where the paint isn't chipping and flaking away, it is very resistant to abrasion and chemical stripping. They made some very tough paint back in the 1950s. – mr blint Dec 31 '19 at 14:36
  • Since the trellis is metal rather than wood you can use a caustic stripper rather than a solvent-based one. Lye will work, but not being able to move the trellis complicates things. Googling caustic vs solvent strippers may be an interesting read. Depending on where you live it may be economical to have someone media blast it. Soda blasting is pretty common on the coast, probably more expensive inland. – Matthew Gauthier Dec 31 '19 at 18:58
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I used a 90 PSI needle scaler (Ingersoll-Rand Model 125) for two weeks, four hours a day of nearly continuous scaling. This needle scaler consumes 8 SCFM. My compressor was an Ingersoll-Rand Model 30 with an 80 gallon tank, rated for 24 SCFM, with a 7.5 horsepower motor that consumes 40 amps at 230 volts. The compressor cycled on and off every few minutes during the entire experience and it got blazing hot, but it was able to keep up with the air demand.

In my opinion, a needle scaler is the wrong tool to clean a wrought iron trellis. Use an angle grinder with a knotted wire wheel to get into 99% of the nooks and crannies, and finish the last 1% with the needle scaler using the compressor you already have. If you're just touching up random spots that you missed with the wire wheel, you can tolerate brief delays while the compressor pumps up.

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  • Thanks for the real-world experience and data/sizes. I am indeed using an angle-grinder. I'm also using a smaller rotozip tool with 2-inch roloc disks to minimize the use of the needle-scaler. Where they fit, the grinders are truly far better than the scaler, as you say. But your sense that the grinder can handle 99% of the task is far too optimistic. Maybe on some trellises. But with the one I'm cleaning up, it's closer to 50/50 because the trellis has a diamond pattern which makes it very difficult to fit an angle-grinder in. – mr blint Dec 31 '19 at 14:27

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