How do I choose a stationary compressor?

[Ben Cadotte]

I have had very good luck with my 3.5 hp (actual) 45 gallon Kobalt. Longest waranty of any large department / borg store. It has a respectiable CFM capacity, little higher than even some 60 gallon units. It will run down with an air sander held wide open though. But I don't use it all the time so I felt the 45 gallon tank was a good size for me. Much quieter than my Craftsman 25 gallon noise maker. Not silent but wont scare you when the shop is quiet and all of a sudden it kicks on.

[Joe Jensen]

In my quest for my latest compressor, I read a ton on the subject. I think there are some myths in this thread.
1) 2 stage is necessary for PSI over 100-120. If you only need less than 100 PSI, a similar price point 2 cylinder 1 stage will make more air per HP than a 2 stage. For example, Quincy makes a 2 stage model 325 that takes 3-5HP. Their comparable 1 stage is a model 240. It also takes 3-5HP, and it makes more air at 100PSI than the 325 with the same HP.
2) Tank size is not the critical factor. In any high volume application the tank will empty very quickly. This is easy to prove to yourself. Fill your tank and then see how quickly the pressure drops below your working pressure. I read that you want at least 1 gallon of tank for every CFM the compressor produces. Larger tanks let the compressor cycle less often but as you approach the max CFM the pump can produce, tank size becomes irrelevent.

Here is a good link to truth in compressors.
http://www.truetex.com/aircompressors.htm
I like the rules of thumb listed here from the link.

Rules of thumb:

• A good compressor, per true HP, will deliver about 4 true CFM at 100 psig.
• The tank should be sized to be at least 1 gallon of volume per CFM of the compressor.
• Uncooled compressed air is hot, as much as 250 to 350 deg F!
• Many tools require more CFM at 90 psi than what is physically possible to get from the power available through a 120 VAC outlet. If you don't observe this physical reality, then either your tool won't run right, or you won't be able to run it at a decent duty cycle.
• Beware also, that the CFM figure given as the required air power on many tools (e.g., air chisels/hammers, sandblasters) is for an absurdly low duty cycle. You just can't run these constantly on anything but a monster compressor, but the manufacturer still wants you to believe you can, so you will buy the tool.

[Brian Elfert]

Dang it! The guy sold the compressor literally 10 to 15 minutes before I would have been there. I was on the road over to his house when he called to say it was sold. He did tell me when I called that someone else was also on the way over. I should have called right away Sunday evening. Oh well, at least I wasn't too far from home.

Maybe the malfunctioning traffic light and the accident on the freeway were omens that I shouldn't buy a compressor right now. I probably spent 15 minutes or more sitting in traffic.

I should probably just buy a compressor at Home Depot for $400 as I have a need for a compressor in the next week or two. I could probably search Craig's List until next fall and never find another deal as good as the one I missed. With my luck all I would see on CL is gold plated compressors or ones that are rusty and worn out.

[Brian Elfert]

Yes, I realize any decent compressor will require a 220 volt circuit. I can fairly easily run one out to the garage where my shop is for now.

[Jacob Reverb]

I recently bought an Eagle 60-gallon 150-psi-max compressor that puts out a whopping 18.5 CFM at 100 psi (three-cylinder compressor). It came with a 5hp continuous-duty motor. I've been real happy with it so far.

[Randal Stevenson]

In my quest for my latest compressor, I read a ton on the subject. I think there are some myths in this thread.
1) 2 stage is necessary for PSI over 100-120. If you only need less than 100 PSI, a similar price point 2 cylinder 1 stage will make more air per HP than a 2 stage. For example, Quincy makes a 2 stage model 325 that takes 3-5HP. Their comparable 1 stage is a model 240. It also takes 3-5HP, and it makes more air at 100PSI than the 325 with the same HP.
2) Tank size is not the critical factor. In any high volume application the tank will empty very quickly. This is easy to prove to yourself. Fill your tank and then see how quickly the pressure drops below your working pressure. I read that you want at least 1 gallon of tank for every CFM the compressor produces. Larger tanks let the compressor cycle less often but as you approach the max CFM the pump can produce, tank size becomes irrelevent.

Here is a good link to truth in compressors.
http://www.truetex.com/aircompressors.htm
I like the rules of thumb listed here from the link.

Rules of thumb:

• A good compressor, per true HP, will deliver about 4 true CFM at 100 psig.
• The tank should be sized to be at least 1 gallon of volume per CFM of the compressor.
• Uncooled compressed air is hot, as much as 250 to 350 deg F!
• Many tools require more CFM at 90 psi than what is physically possible to get from the power available through a 120 VAC outlet. If you don't observe this physical reality, then either your tool won't run right, or you won't be able to run it at a decent duty cycle.
• Beware also, that the CFM figure given as the required air power on many tools (e.g., air chisels/hammers, sandblasters) is for an absurdly low duty cycle. You just can't run these constantly on anything but a monster compressor, but the manufacturer still wants you to believe you can, so you will buy the tool.

Only thing I will add is that you will need between 10 - 20cfm to run many autobody tools continuously. They are much worse than woodworking tools since most are open ports and you need to run non-stop. Having tried running one on less than required I can say it is a royal pain to do so.

Brad

Brian, I have used a Speedair true 3HP cast iron single stage for 18 years. It will last a long time, but it's too small for the HVLP conversion gun I bought.

I've been wanting a Quincy Industrial compresser for 20 years. They are very heavy duty pressure lubricated compressors run very low RPM so they are very quiet and last forever. Quincy now makes a cheaper compressor series too. The pump on mine is a Quincy model 325 which is a 2 stage that takes a 3 or 5HP motor. With a 3HP motor it only runs at 400RPM.

The problem is that new they are prohibitively expensive, over $3500 new. I've been watching Craig's list and I was finally able to find a single phase one on a smaller (60gal) tank. The one I bought came out of a Dentist office and other then being stored outside (not hooked up) for the past 5 years, it is like new. The motor needed a new start capacitor, and I'm repainting it (vanity) but I will end up with less than $400 in it.

Moral - look at used...joe

Your gonna have to look at your use. I know too many hot rod guys who buy all the tools, build the car, then the tools set (expensive, not good on some stuff, and one use specialty tools). I stick by my look at your specs, then go larger (its not like we have never wanted a new/more power tool, right?) Commercial shops (autobody, dealerships, etc), are MUCH more apt to run a rotory screw type compressor (REQUIRES THREE PHASE), if it weren't for that, I would have bought the one I found a few years back for $1000. More air supplied then a homeowner SHOULD ever need.

[Joe Jensen]

Your gonna have to look at your use. I know too many hot rod guys who buy all the tools, build the car, then the tools set (expensive, not good on some stuff, and one use specialty tools). I stick by my look at your specs, then go larger (its not like we have never wanted a new/more power tool, right?) Commercial shops (autobody, dealerships, etc), are MUCH more apt to run a rotory screw type compressor (REQUIRES THREE PHASE), if it weren't for that, I would have bought the one I found a few years back for $1000. More air supplied then a homeowner SHOULD ever need.

I agree completely with your logic but unfortunately, or maybe fortunately for my wife, I'm space limited to most of a 3 car garage. If not, I'd have a 37" wide belt, a CNC, a large slider, etc etc. I keep looking for a 5HP screw compressor with a 60 gal tank, but short of a $5K new unit, I haven't found one. If space and AMPs were not a problem, I'd have a $1000 10HP screw compressor. They are on EBAY and Craig's list all the time..joe

[Josiah Bartlett]

Look for a compressor with a 1725 rpm motor instead of a 3600 rpm one. That will tell you that it runs quieter, has bigger pistons for the given CFM rating, and is likely bulletproof.

I run a horizontal 2 cylinder single stage Quincy I got of Craigs' for $250. It has full oil pressure lube like a car engine (even has an oil pressure gauge), a nice 4 pole (1725 rpm) Baldor motor, and it puts out 8 cfm at 100psi. The best thing about it is that it is really quiet. I can carry on a normal conversation right next to it while its running. Unless the tank rusts out I think I'm set for life.

[Joe Jensen]

Look for a compressor with a 1725 rpm motor instead of a 3600 rpm one. That will tell you that it runs quieter, has bigger pistons for the given CFM rating, and is likely bulletproof.

I run a horizontal 2 cylinder single stage Quincy I got of Craigs' for $250. It has full oil pressure lube like a car engine (even has an oil pressure gauge), a nice 4 pole (1725 rpm) Baldor motor, and it puts out 8 cfm at 100psi. The best thing about it is that it is really quiet. I can carry on a normal conversation right next to it while its running. Unless the tank rusts out I think I'm set for life.

Killer stuff, sounds just like the one I found on Craig's list. Quincy #325 compressor, 6o gal horizontal tank, and a 3HP motor. I don't need 175 psi so I'm considering lowering the pressure switch to say a max of 120 psi to keep the air cooler as it compresses. You have one up on me as mine does not have a Baldor motor...joe

[Tom Veatch]

Brian, you gotten a lot of good advice in this thread. To sum up and add my thoughts on the matter:

1. Look to your tools. Which tool, currently owned or potential acquisition, has the highest air consumption. Assuming a single user environment, that is the absolute minimum CFM output compressor you should consider and going higher by 20-25 percent or more is good.
2. What is the maximum pressure you must have available. Unless it's greater than 120-130 PSI, a single stage compressor is sufficient. Greater than that, you should strongly consider a 2 stage unit.
3. A secondary pressure consideration is that higher pressure (within reason) is good since it effectively multiplies the capacity or your tank.
4. Larger tank size is good, but is a secondary consideration to the CFM output of the compressor. A large tank (or a smaller tank at higher pressure) serves two purposes. First, it bridges the gap when usage *temporarily and monentarily* outstrips supply. Second it reduces wear and tear on the compressor motor because it reduces the number and spreads out the timing of the start/stop cycles. Frequent motor starts are much harder on the motor than continuous running.

Remember CFM is king!!! If economic conditions demand it, sacrifice all else before you give up any CFM.

[Rich Engelhardt]

Hello,

How do I choose a stationary compressor?

Look at the Ingersoll Rand line.
Then compare apples to apples with everyone else.

(IMHO - only) there's IR, and there's everyone else.

A stationary compressor is a significant investment in money as well as time and effort to buy, transport and install. You only want to do it once - is what I'm getting at.

I don't need 175 psi so I'm considering lowering the pressure switch to say a max of 120 psi to keep the air cooler as it compresses.

175 psi in a 60 gal tank will deliver roughly as much air as 120 psi in an 80 gal tank.
More psi "squeezed" into a smaller tank roughly equates to having a larger storage capacity, with a smaller footprint.
Don't concern yourself with trying to "false economize" - a quality pump can handle any slight increase (if there even is any) in temperature with ease.

[Rob Will]

Lots of good advice here.
Woodworking tools are not a big deal but if you start doing auto body work you will want at least a 5 hp compressor.

I would not consider those light duty compressors from the BORG.

My machine shop has a 3hp Westinghouse industrial compressor that was built in 1958 !!.......I bought it used for $300......and it is still going strong.

If there is any way you can, look for a higher quality compressor. Tractor Supply sells Ingersol Rand and Rural King sells Quincy. In my wood shop, I have a Curtis.

Rob

[JayStPeter]

It takes ALOT of air (CFM) to keep a paint gun going continuously for an hour or so.
A 60 gal. compressor won't handle what your want to do.

Paint guns are easy compared to air sanders, sandblasters, and heck even air ratchets and impact guns. I use an automotive HVLP spray gun with a different needle to spray WW finishes and have never had to wait for the compressor. Every so often it kicks in and everything keeps going. There is plenty of air at the <30psi the gun runs at. High quality guns with lower air usage are easy to find. However, about 1 min. with a sandblaster and I have to wait another couple for the tank to refill. No such thing as a low air use sandblaster. Blast off a few stubborn exhaust bolts with an impact gun and I have to bleed off some air to force a refill so I have enough pressure to blast off another few.
For the amount of stuff I do, my cheap 60 gal is OK. But, if I had a big car resto project going I'd want something bigger. I consider my 60 gal merely adequate for maintaining my 18 year old track car but fantastic for WWing (including spraying).

[Dan Lautner]

"The problem is that new they are prohibitively expensive, over $3500 new"


Im from the philosophy that if it is only 1% of your income go for it.

Dan

[Jude Tuliszewski]

I went with an Eaton comp. 7.5 hp with 33 amps running, and the 80 gal. tank. Search compressors to find my thread on it. I have been using it to sand blast my house, and it puts out plenty of air. It was competitively priced with the other pro style comps. What sold me on the Easton was the weight (witch means cast iron everything) and it has a continuous run feature that keeps the motor from cycling on and off during high air usage. I have had it for about 6 months and it has been a solid performer. I could have bought a less expensive comp. and gotten by, but I knew if I did I would probably regret it later. I know it is tempting to go the economy rout but after having done that a couple of times and not been happy with the outcome, the old saying holds true “ buy quality cry once buy cheap and cry all the time”.


If you do look for my thread search for Eaton compressor to find it faster than just searching compressor.