Air compressor ignorance

[OpaDC]

BIL came by earlier today and dropped off 2 air compressor he says don't work and thought I might want. I said suuuure. Now the good part, I have never used one before. One is this one: http://www.harborfreight.com/cpi/cta...emnumber=90234
other one I have to get the grease of labels first to get any idea what it is. The Central one I got working (reset switch). Other one bigger but older and appears to need lots of work. In trying to see what tools I can get for it I am getting confused because of this: 5.6 CFM @ 90 PSI; 7.2 CFM @ 40 PSI. More PSI = less CFM?
Any help would be appreciated.

[Tom Slick]

Bernouli's principle states that when flow goes up, pressure goes down and vice-versa. so you hit the nail on the head!

http://en.wikipedia.org/wiki/Bernoulli's_principle

[OpaDC]

Bernouli's principle states that when flow goes up, pressure goes down and vice-versa. so you hit the nail on the head!

http://en.wikipedia.org/wiki/Bernoulli's_principle

Understand it in principle if not in theory. That being said, what do you look for in buying tools vs CFM (SCFM) of compressor? (This is relative to woodworking, not mechanics.) Sander, sprayer, nailer?

[Uncle Cracker]

It's like this: Your compressor has a motor which is capable of doing a certain amount of work. It takes work to pump volumes of air. It also takes work to pressurize that air. So, your motor can devote all its energy moving air at little or no pressure, or all of its energy building pressure while moving little or no air, or it can compromise somewhere in the middle. The more air moved, the less energy left to build pressure, hence the performance numbers move up and down in opposite directions.

[drumpriest]

I only have a tiny pancake for driving nails. I used electric for sanding and turbine for spraying, and it seems to be working. I've often wondered about air sanders though, are they more efficient? Faster? build up less heat on the paper??

For driving nails you can use a small compressor so long as the air expended doesn't out run the motor's ability to compress the air. My 6 gallon PC compressor drives several nails from the framing nailer before having to charge, and as nailers get smaller, it drives more and more.

[LCHIEN]

its complex buying tools vs. compressor capacity in CFM.

First thing, you have to realize the Continuous CFM is limmited by the power and piston size and RPM of the compressor mechanics.

Second the CFM deliverable from the compressor should not rely on the compressor running 100% of the time, they usually shouldn't be used where the motor runs more than 50% of the time, which involves the tank capacity - still you shouldn't run more air than the motor/psiton can provide averaging 50% of the rated capacity.

IOW, if you have 5.6 CFM compressor then you should not run more than 2.8 CFM continuously.

Now what is continuous? if you work in bursts, like a nailer, that's not continuous.
even a ratchet wrench is not continuous, you migth tighten a nut or bolt for a few seconds. Continuous is long enough to drain the tank down and start the compressor motor running and keep it running more than 50% of the time over a period of time. Sprayers usually run for minutes on end, sanders and surface grinders and the like due to their intended uses also take continuous energy. So these are hardest on a compressor.

[just started]

I use a random orbit air-powered sander for wet-sanding so I don't light myself up. I do have to stop for a few to let the compressor catch up, but it's not too bad as it gives me time to rehydrate or return the products of such rehydration.

[cabinetman]

This answer to your question may sound too simple. Air operated devices list the PSI and CFM that they operate at an optimum efficiency. PSI (pounds per square inch) is a pressure rating, and CFM (cubic feet/minute) is an air flow (volume) rating.

A compressor has both as an optimum rating when running. If an air device is hooked up that requires less PSI, more CFM is available.

Not to get too confusing with this general description, but there is an order of output that comes from the stored air in the compressor's air tank. The compressor will run continuously until the tank fills and reaches its maximum set pressure. If an air tool is attached and operating, and its demand is less than the output capacity of the compressor, the air tank will deplete to run the device, and when the pressure in the tank reaches a set point, the compressor will "kick on" to replenish the tank. This cycle can be ongoing.

If the air device's demand is greater than the compressor's output, the tank will deplete, the compressor will "kick on" to replenish and if it can't it will run continuously to both replenish the tank and run the air device. When this happens the tool will operate at the total output of the compressor, which may be less than its efficiency. For example, a high demand drill may slow down.

The general rule of thumb in getting a match up is to check out the air devices to be used and their operating demands (CFM & PSI) and then to see if the compressor has that capability.

A saving grace for a compressor is that some devices are intermittent, like a nailer, that only uses a short burst of air at a time. And then there are devices that use a continuous supply of air over longer periods, like spray guns and air sanders.
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[OpaDC]

Thanks to everyone. The light clicked on finally. That's what I get for trying to think after 8 pm. Now to go buy some more tools.