So Confused About What Size Air Compressor I Need

rangerenthiusiast · Aug 4, 2018

Hey, all.

I won’t go into all the details, but my current project has been so disheartening. At the moment, I could really use some good advice on air compressors. I’ve been unable to buy a lot of the air tools that would have made life much easier (sheet metal cutters, wire wheels, etc) as well as a small sandblaster that I desperately need, because my air compressor is too weak to push them. It’s a stupid little 6 gallon pancake Bostich compressor, with a 0.8 HP motor that I bought brand new, which is only good for filling the tires on my vehicles or pushing a nail gun.

I’ve already dumped more money into this project than the truck is worth, so I’m trying to find a better compressor on Craigslist. The problem is that it’s incredibly difficult to figure out what the most important factor is when deciding if a particular compressor will do the job. Those few air tools that actually list the compressor requirements to run them use different criteria. Some will go by the volume of the compressor (ie - “35+ gallons = continuous use,” “10-35 gallons = intermittent use,” “1-6 gallons = do not use,” etc). Others (including a small sandblaster currently for sale at Harbor Freight), claim that they will work with any compressor over 1 HP, which I find highly unlikely. Still others have a CSFM requirement, which can be tough to find listed on a new compressor, much less a used one on CL. Further complicating things is the fact that I find all kinds of mixtures of HP and volume on different compressors (2.5 HP motors paired with a 25 gallon tank, 5 HP motors paired with a tiny 10 gallon tank, etc). I would tend to think that higher HP motors would always be paired with bigger volume tanks and that somewhere along the way, you’d hit the size that would fit the bill for your needs. Obviously, this is not so.

So what is more important, HP or volume? Is there a ballpark HP/volume combination that will work to run a small sandblaster and basic air tools (impact wrench, metal cutoff tool, etc) without breaking the bank?

Thanks a bunch for any insights you may be able to provide. :icon_thumby:

scotts90ranger · Aug 5, 2018

somewhere around 20 to 30 gallon is the sweet spot for most compressors, I have about a 30 gallon, it's the older version of THIS which I got of craigslist for $200 about 6 years ago... It won't keep up with a air cut off wheel, haven't tried a sand blaster but for intermittent use you should be fine (blast for a minute, wait a minute, etc...). To an extent for the normal garage use volume can make up for power, and a high pressure compressor makes up for some more of that... my compressor is only rated at around 2hp but has the same SCFM ratings as most oilless 5hp compressors. oiled compressors are much quieter than the oilless compressors that are direct drive off the motor and generally last longer.

I know of a few people that are happy with the ~20 gallon harbor freight unit... they are slightly louder than the belt drive compressors since they are direct drive, but work.

ericbphoto · Aug 5, 2018

^^^ He said it pretty well. I would say that CFM rating is most important, BUT.... For intermittent use, a large tank can make up for a small compressor in many cases. Most air tools are designed to run at 90-100psi at a certain CFM. If you have a compressor with a large tank that can supply 150psi, then you put a regulator on the outlet to deliver 90psi to your tools. The compressor can then fill the tank to 150 and rest until the pressure falls down to the pressure switch setting, then refill. The rest period let's the compressor and motor cool so they don't overheat and burn up.

A compressor with a larger motor up rating will normally have a larger compressor piston. That allows it to bring the pressure up faster and have a longer cooling period. That makes it last longer. Smaller motor/compressor combos will have to run a lot more to deliver the needed air volume/pressure. It all depends on what you will be needing from the compressor.

When looking on Craigslist, you need to be a detective. Use the info given and Google the rest. Sometimes, if an ad looks promising but is lacking info, you might need to go and see it in person.Take anrag and flashlight so you can read the label. If all it has is a manufacturer name and model number, Google it on your phone to find the ratings. Even if the compressor is a cheap brand, if it exceeds your needs by a decent factor, it may last you a long time simply because you will rarely use it at Max capacity.

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alwaysFlOoReD · Aug 5, 2018

CFM is the best indicator of power. Most sandblasters I've seen advertise they work at ~5 to 25 cfm. At 5 cfm you will work for 10 or 20 seconds and then wait for the tank to fill up. A bigger tank will let you work longer, BUT you will also wait longer for it to fill again [ if a bigger tank lets you work 5 times as long, the fill time will be 5 times longer too]. Most goodish consumer grade compressors are in the 10 to 12 cfm rating. They will have a 5 hp motor. Be careful, a lot of companies rate the HP at "peak HP", which is misleading. Look at the amp rating on the motor, it should be at about 21 amps for a 120 volt motor. This means you need a slow blow 20 amp fuse, and 30 amp would be best. You can't just change a fuse for a higher rated one without also upgrading the wiring. I have run 2 portable 2-3 hp compressors into one big tank, that can work out cheaper if you have the room.
A commercial sandblaster will be using a compressor with CFM rating north of 150cfm
Be aware that I'm NOT a licensed electrician and you need to use your due diligence and check out what I've said.

adsm08 · Aug 5, 2018

Right. A big tank can make up for a small compressor, which is why it is not a linear progression of tank size and compressor power.

I have a 60 gallon tank, with a 2.5 or 3 horse compressor and no regulator, just a 155 PSI cutoff switch. It keeps up with everything except my orbital sander. I run impact guns, air ratchets, drills, grinders, a pneumatic press ram, fill tires, and paint with it. The only thing I don't do is sand blast, but I have a friend with an identical compressor and a small blast cabinet, and his setup works just fine.

CFM is the best rating to use, if you can find it. I know my system is rated for a sustained flow of 13 CFM at 90 PSI and 11.5 at 150 PSI.

alwaysFlOoReD · Aug 5, 2018

^^^ I forgot to mention that most compressor heads have a range of HP that will run them. A smaller HP motor will have smaller pulleys so the compressor head slows down and the cfm rating goes down too. It still builds pressure to the cut-off rating, be it 90 psi or 150psi. I could be off on the 5 HP needed for a 10 to 12 cfm rating, perhaps it is 3 HP like adsm08 says..

adsm08 · Aug 5, 2018

alwaysFlOoReD said:
^^^ I forgot to mention that most compressor heads have a range of HP that will run them. A smaller HP motor will have smaller pulleys so the compressor head slows down and the cfm rating goes down too. It still builds pressure to the cut-off rating, be it 90 psi or 150psi. I could be off on the 5 HP needed for a 10 to 12 cfm rating, perhaps it is 3 HP like adsm08 says..

CFM is going to be determined by tank and compressor size. A large tank with a large compressor will be higher than a small tank and a small compressor, but a small tank and a large compressor may be able to get to a higher CFM than a large tank and small compressor. It is all about how much volume it can flow.

I know my system can actually sustain about 10 CFM, for an hour or so. I learned this while sanding my truck down for new paint.

I also learned that at the end of that hour the air hose and tools start getting hot, because the air coming from the compressor is very hot. At that point I switched to the electric sander.

alwaysFlOoReD · Aug 5, 2018

I would say that CFM is only a product of the compressor head. The volume of the tank has nothing to do with CFM other than if it is a restriction. The tank is simply a tank. If you had no tank at all the compressor would still put out it's same CFM. If you had no compressor head the tank would put out 0 CFM.

And advertised CFM is theoretical CFM found by piston piR^2 x stroke / minutes. Actual CFM is less, as you found out.

adsm08 · Aug 5, 2018

alwaysFlOoReD said:
I would say that CFM is only a product of the compressor head. The volume of the tank has nothing to do with CFM other than if it is a restriction. The tank is simply a tank. If you had no tank at all the compressor would still put out it's same CFM. If you had no compressor head the tank would put out 0 CFM.

And advertised CFM is theoretical CFM found by piston piR^2 x stroke / minutes. Actual CFM is less, as you found out.

Right. But storage capacity will dictate how long that flow rate can be maintained and should figure into advertised capacity. A compressor running at full capacity with no storage will provide 10 cfm for a shorter period than one with a tank that can allow storage and thus duty cycle. A tank might not contribute to peak flow but the advertised numbers are usually constant flow.

Also, turns out my 60 gallon is 3.7 hp. Its my 20 that is 2.5.

alwaysFlOoReD · Aug 5, 2018

adsm08 said:
But storage capacity will dictate how long that flow rate can be maintained and should figure into advertised capacity.

No, it does not figure into advertised CFM rating. The tank will dictate the [can't think of the correct term] the percent in time that you can use the air, similar to a welder. If the tool you're using is the same or less than the running CFM, then you will never run out of air at the pressure the compressor develops, so 100% working time. [ but the compressor and tank get hot...] If your tool uses more CFM than the compressor can provide, then you will have a lower than 100% working time. However, a tank can prolong the time you can use a tool, such as a sandblaster, but you will have to allow the compressor to catch up eventually. The best way to size a compressor is to find the biggest CFM user of the tools you have and buy a compressor with more CFM than that tool. If you size the compressor CFM 2x as much as your tool, then the compressor will run half the time and won't get as hot. Not everyone can afford that so sometimes you just wait for the compressor to catch up :annoyed:

.

ericbphoto · Aug 5, 2018

alwaysFlOoReD said:
No, it does not figure into advertised CFM rating. The tank will dictate the [can't think of the correct term] the percent in time that you can use the air, similar to a welder..

Duty cycle

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alwaysFlOoReD · Aug 5, 2018

^^^ That's it. Thanks.

planeflyer21 · Aug 5, 2018

Since you are looking used compressors on CL, make sure to check them over really well.

I've seen a couple of videos of older compressor tanks coming apart, with not very pretty results.

rangerenthiusiast · Aug 6, 2018

Thanks so much to everyone who chimed in with so much great data and opinions!! I’ll have to read things over a few more times, but I already feel a lot better informed than when I started.

You guys are just the best for taking the time out of your day to provide those of us who are less knowledgeable, but trying their best to get by. Kudos to you all! :icon_thumby:

adsm08 · Aug 6, 2018

alwaysFlOoReD said:
No, it does not figure into advertised CFM rating. The tank will dictate the [can't think of the correct term] the percent in time that you can use the air, similar to a welder.

Right. It dictates the duty cycle, which will have DRASTIC effects on how long you can run tools before your compressor starts to overheat. That means it effects how long it can maintain that advertised "constant flow".

A compressor running 100% duty cycle, so on all the time, will over heat and have to be shut down much faster than one running a 50% duty cycle, which is roughly what I get with my setup if I am running the grinder.

So Confused About What Size Air Compressor I Need

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