Air tool question

[Norton155]

First off this is a great forum, I've been snooping for a while but this is my first post. Anyway, my dad got me an early present this year, 33 gallon Craftsman compressor and air tool set. I've been wanting one for a long time. I have a question about matching up tools with psi and cfm. If a cut off tool calls for 3 cfm @ 90 psi, and my compressor puts out around 5 cfm @ 90 psi, is this ok? How about if a sander requires 5.5 cfm @ 90 psi, is 5 close enough? I am pretty proficeint with hand tools, and can not wait to start using air tools on my cars. Thanks in advance!

 

[krusty the clown]

you can run a sander but you be taking lots of breaks waiting for the compressor to build back up! if you plan on doing a lot of sanding you may want to trade up.

 

[timgr]

Your compressor can deliver 5 cfm of air at 90 psi running continuously. The 33 gallon reservoir lets you exceed that delivery rate for short periods.

Ok, some science content. The basic gas law says that P1*V1 = P2*V2, where P1 and P2 are pressures and V1 and V2 are volumes. If you ask for 5.5 instead of 5 CFM at 90 psi, you have to reduce the duty cycle of operation by 1 - 5/5.5, or about 10%. That means that for every 9 minutes sanding, you need 1 minute of recovery time. Of course, that assumes all this equipment actually operates at its ratings.

How long you can sand before you have to pause depends on the size of the backup reservoir. Here it's 33 gal, and I presume the standby pressure is 135 psi. So, there's 0.134 cf per gallon. That's 4.4 cf in your tank. At 135 psi (using the gas law above) that's 6.6 cf at 90 psi, the delivery pressure, or 2.2 cf of excess capacity.

This tells us two things. First, the recovery time from 90 psi to the standby pressure of 135 psi will require 2.2 cf / 5 cfm = 0.44 m or about 30 seconds. Next, you should be able to sand until the capacity runs out; the reserve capacity divided by the excess demand is 2.2 cf / (5.5 - 5) cfm = 4.4 minutes.

Note that we have not accounted for changes in air temperature with compression, which can only make things worse. Plus, the compressor's standby and startup pressures are different, which complicates things ... but this simple model should be good enough to estimate performance. Ideally, you should be able to sand for about 5 minutes, then pause for something like a minute. When the sander starts to slow down, you'll need to wait until the compressor tops up the tank.

Hope this isn't an overload, but that's how you make a BOTE (back of the envelope) calculation of such things.

 

[wythors]

Note: Go out and get yourself some good ear protection before you start your next project. Those oil-less Craftsman units are ungodly loud. I had that exact unit and took it back. I traded it up to one of the "Professional" models with an oiled motor. MUCH quieter.

 

[Norton155]

timgr thank you that all makes perfect sense now. The tank actually has a max pressure of 150 psi, but I can figure out the math on that. That should give me about 3 cf of excess volume.

I think I'm about to answer my own question here. Running a tool that is rated to use less cfm then your compressor puts out just lets you run longer between refills. Correct?

wythors I would love the 60 gallon IR in the Craftsman club flyer. That is going to have to wait, I'm still in college living at home. I graduate next year and plan on staying with the folks for a couple of years becasue I would rather save for my own place then rent. I can't wait to have my own shop/garage to work in. My family members refuse to keep our garage uncluttered!

 

[Danglerb]

I just bought the Sears I think 26 gallon, and its no louder than the average jet engine, but I don't think even a quiet rated one would be quiet enough for me to use when this one would be too loud, except maybe in the evening hours during the summer when neighbors tend to have windows etc. open. OTOH none of the air tools I plan to use are very quiet, so whats the point.

As for air use, it just depends on what you will be doing a LOT of. If you frequently want to use a high air use tool, a smaller compressor is going to be a pain. If most of the time you aren't using an air hog, then the several hundred bucks in difference can happily stay in your pocket. I'd also give serious thought to using electrical versions of some tools instead of buying a huge compressor. If you need more air, it won't be a small step, most would go directly to something like 15 scfm and 80 gallons or larger.

 

[timgr]

Running a tool that is rated to use less cfm then your compressor puts out just lets you run longer between refills. Correct?

Yep, that's right. You can even have a 100% duty cycle (running continuously) if you are using air at less than the compressor output rate. The compressor will still cycle as you draw down the tank, but the compressor will always catch up since it produces air at a faster rate than you are using it.

 

[Charles (in GA)]

The problem is, the cut off tool and the sander probably use alot more CFMs of air than the specs are telling you. 3 CFM is nothing, I cannot imagine a cut off tool using that little air. I suspect they are not telling you the whole story, and have fudged some low numbers for marketing. I'm willing to bet the compressor will be running continuously and not able to keep up at that.

You will hate the noise. After about ten minutes of it you will be wanting to shove it off the nearest cliff. I think Sears gets lots of returns on those oilless compressors due to the noise, I know of a couple of people who have bought them and taken them back after they heard how loud they were.

Charles

 

[johnny1290]

I'll chime in. I'm using a 3hp 15gal craftsman compressor, with a cut off wheel it runs out of breath quick, with a 90 degree angle grinder its been OK.

The noise...I'd say it's more like a jet at idle than at takeoff speed, but maybe that's just cuz its stashed under the house

Will never buy an oilless again, and will get a huge tank. Unless you have understanding neighbors, the noise makes it almost unuseable, outside of a farm or something.

 

[Danglerb]

I suspect two things make the noise, higher operating speed, and no sound reduction on the intake. Nothing to do about the first thing, but I am betting some kind of better intake filter could reduce the noise a bunch.

 

[MarkH]

I have the same the same compressor at the house in town. I had to draw down a air conditioning system on my wifes car. I ran it up to 150 lbs, using a good not HF vacuum pump rated at 4.7 CFM to 70 psi here is what happened. I set the tool side at 70 psi, the tank side started at 150 psi at 135 the pump kicked in with the tank psi dropping to 90 where it stabilized with the pump running constantly.

Very noisy 2 hours and a warm pump. I had taken the cowling off it and had a fan running on it to keep it cooler. For runs that long, I believe it could have damaged something it I did them all of the time. It is great to have a compressor at the house and this works well for most of the things I do. Still was looking at the farms to see if there was a chance we would be closing any shops this year, because if we did I was going to have the compressor shipped to me.

 

[Ign]

The problem is, the cut off tool and the sander probably use alot more CFMs of air than the specs are telling you. 3 CFM is nothing, I cannot imagine a cut off tool using that little air.

3 cfm is no-load I'm sure. Start to actually cut something and it likely jumps to the mid 20's. Reputable tools and suppliers provide both no-load and max consumption figures. Even impact wrenches -like say the IR2131- consume in the neighborhood of 24cfm at full load. Fortunately w impacts it doesn't matter much 'cause you're rarely on the trigger for more than a few seconds. But with something like a cut-off tool you'll work a little 26gal compressor hard. Keep a beer handy for all the breaks.

 

[TNToy]

Someone got it.

Most impact guns are listed at around 2-6 - which is what they use when freewheeling with no load. They actually consume 15-20 CFM at full load. So they'll hit your compressor hard, but it's usually okay because they aren't at full load once the fastener breaks loose, so they don't use massive amounts of air for very long.

Air sanders, and grinders are the biggest air hogs, and are well up above 20 CFM. However, they're used under heavy load pretty much continuously (unlike impacts) and they **** a LOT of air.

All that means is that if you're swapping in an engine and using an impact, it'll run every 5 minutes, and small compressors like yours are very loud. If you're using a grinder or cutoff tool, you'll be waiting around for the compressor to refill.

But that's okay, because you'll never understood how people lived without air tools, now that you can use them.

I have a 12CFM 80-gallon upright, and it baaaaarely keeps up with a small die grinder, and runs nearly continuously.

 

[wilbilt]

Air sanders, and grinders are the biggest air hogs, and are well up above 20 CFM. However, they're used under heavy load pretty much continuously (unlike impacts) and they **** a LOT of air.

I'll second that. I have a decent compressor (4 cyl., 2-stage Emglo on an 80-gal. tank, real 5HP 23A motor), and it works hard to keep up with a single DA running, or a little bitty die grinder.

Impact wrenches, tire machines, etc., are not a problem.

 

[Ign]

At the bottom of the description here, 24cfm at load for a 2135:
http://www.tooltopia.com/index.asp?PageAction=VIEWPROD&ProdID=8427

 

[hoarder1212]

Congrats on the new compressor.I think I have the same one.It is a bit loud but it recovers fast,so it doesn't run long.I use mine mainly for impact wrench and I love it.If this came with tools and fittings do yourself a favor and throw the fittings away and buy good ones.the fittings that came with mine are cast aluminum and they break easy,and that can be very dangerous.