Air compressors: SCFM vs tank size

Air compressor 1: 3.7 SCFM @ 90 PSI with a 25 gallon tank
Air compressor 2: 5.2 SCFM @ 90 PSI with a 5 gallon tank

Critique my thinking....

Compressor 1 will take a long time to fill the 25 gallon tank. But once it is filled, you'll have the required pressure to run an impact wrench. As you use the impact wrench, you'll drain the pressure in the tank and the motor will kick on. It will take a long time to refill the tank. The motor runs seldom but for long durations.

Compressor 2 will fill up its tank in no time. As you use the impact wrench, the motor will be kicking on a lot to keep up with the demand. The motor runs often but shorter durations.

Am I on track?

If I am sanding down a car, Compressor 1 would be better. If I'm just changing a tire, Compressor 2 would be the one. Yes?
 
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I have a 60 gallon tank stand up compressor I use at work alot, and it seems to kick on just as much as the 20 gallon I have in my garage. They both fill up to around 140psi, and seem to do the exact same job. I don't even notice much of a pressure drop change between the two. It's all in how you have them adjusted, really. The only difference, aside from the 60 gallon being 11.3cfm@90psi compared to my 10.2cfm, it's also 230v 3 phase.

Now, the compressor we use to run some conveyor at work, is about the best I have ever seen. It's a (twin)15hp, 250 gallon, 98cfm@175psi 1,045RPm bad mamma jamma. Of course, it's running on 460 3 phase. That thing almost sent one of my air wratchets to heaven.

I'd say just go for what you have the space and money for. It's what I did, and it worked out great.
 
I have a 60gal craftsman, before that I had a 30gal craftsman...I'll probably upgrade again to an even bigger tank and way better flowing lines. However, I do all my own work, and basically, have a full shop in my garage.

like Shiroelex said, get what you can fit.
 
It's not about getting the biggest and baddest out there. I'm trying to understand compressors, their ratings, and real world use. Specifically, scfm and tank size and how they affect the end user.

Well, I can help you out with this one. I got into painting last year so I know all about this crap.

You are correct with your thinking ... the second compressor listed will kick on/off more often and probably work better ... however, there's another factor is important to weigh into the equation. What is the MAX psi of each compressor?

Things like a die grinder or air sander, paint gun, etc., need a good bit of SCFM ... food for thought ... But post the max PSI if you know it.

Also, if you are gonna use it for tools only (not paint guns) then I recommend a compressor that uses oil. Oil-less compressors are good for painting because they keep the outgoing air cleaner. Oil compressors can take a beating though and will outlast an oil-less. Just get a good filter to filter our air/water. I'll help you with that after you decide what you want to get.
 
CFM, or air consumption, is probably the most important thing to consider. Most air tools will list their minimum recommended CFM requirement. Most 3/8" impact wrenches will need at least 4-5 CFM @ 90 PSI. 1/2" impact wrenches will normally require anywhere between 5-8 CFM depending on the tool.

Things like sanders, die grinders, and air drills will consume the most air within a given time period.

You don't want the compressor to constantly be cycling on/off. You will get fluxations in flow and pressure and your air tools won't work as well. And it's annoying.:)

You really want to try to get at least a 60 gallon/10 CFM compressor, or your impact wrenches will feel weak, and your compressor will constantly be cycling. That 5 gallon tank won't work at all. That's is for roofers and fencers using air nailers.

This compressor kit is a nice compromise for price and grunt, and would probably make a good starter setup:

Craftsman Compressor Kit
 
I don't see myself using a sander or paint gun or any tool for long periods or an air drill. My use would be simple tire changes, exhaust components, shocks, etc. just short sporadic use. So I just want one that's enough to run an impact wrench for short bursts. The size and weight of the entire unit is important to me as I plan on putting this in the attic above the gargage eventually and just running some plumbing down into the garage. That is why I am considering the one linked below. It is rated at 5.3 scfm. The highest in any of the small compact units in the $200-$300 price range. My concern is that it might not be able to keep up with a 4-5 scfm impact wrench when I have to sit there and just hammer on a stuck bolt. On the other hand, I doubt I'd really sit there hammering for 60 seconds. I'm sure the noise and vibration will be annoying enough to where I will hammer a stuck bolt in 15-30 second bursts.

http://www.northerntool.com/webapp/wcs/stores/servlet/product_6970_200197074_200197074

I can get a 3.5 scfm one with a 21 gallon tank but that doesn't meet my size and weight requirements. It is too big to place in the attic and I don't want it sitting in the corner of my garage.

Then there are the 10 gallon ones.... :shrug:
 
If the tool says 5 scfm@90 and the compressor says 3 scfm, won't that still work if I have a medium to large tank? The 3.5 scfm pump will store up say 5, 10, 20 or 60 gallons of air at 115, 120 or 150 psi and away I go. The larger the tank, the longer it will take to bleed it down to the TurnOnPressure. Once the pump kicks on, you're now looking at the pump's rating. The lower the pump rating, the longer it will take to fill it up to the TurnOffPressure. So the scfm of the compressor isn't directly tied to the scfm of the tool, no? The tank serves as a buffer so you don't have to match the pump's scfm rating to the tool's scfm rating. Yet, everyone says get a compressor with more scfm than the tool. That's where I'm disconnected. Is that just so you can keep going? Or like be able to use the tool immediately without waiting for the tank to fill up?
 
If the tool says 5 scfm@90 and the compressor says 3 scfm, won't that still work if I have a medium to large tank? The 3.5 scfm pump will store up say 5, 10, 20 or 60 gallons of air at 115, 120 or 150 psi and away I go. The larger the tank, the longer it will take to bleed it down to the TurnOnPressure. Once the pump kicks on, you're now looking at the pump's rating. The lower the pump rating, the longer it will take to fill it up to the TurnOffPressure. So the scfm of the compressor isn't directly tied to the scfm of the tool, no? The tank serves as a buffer so you don't have to match the pump's scfm rating to the tool's scfm rating. Yet, everyone says get a compressor with more scfm than the tool. That's where I'm disconnected. Is that just so you can keep going? Or like be able to use the tool immediately without waiting for the tank to fill up?

You are pretty much on the money.

The flow rate at which a compressor can deliver a specific volume of air is written in CFM. Because atmospheric pressure plays a role in how fast air moves into the cylinder, CFM will vary with atmospheric pressure. The measured CFM will also vary with the temperature and humidity of the air. To level the playing field, the compressor manufacturers calulate standard cubic feet per minute (SCFM) as CFM at sea level with 68° F air at 36% relative humidity. SCFM ratings are given at a specific pressure; 3.0 scfm @ 90 psi, for example. If you reduce pressure, SCFM goes up, and vice versa.

You would be surprised how fast you can go through 20-30 gallons of air with just one tool operating. So the largest volume tank you can feasibly get up into your attic, do so.
 
You are pretty much on the money.

The flow rate at which a compressor can deliver a specific volume of air is written in CFM. Because atmospheric pressure plays a role in how fast air moves into the cylinder, CFM will vary with atmospheric pressure. The measured CFM will also vary with the temperature and humidity of the air. To level the playing field, the compressor manufacturers calulate standard cubic feet per minute (SCFM) as CFM at sea level with 68° F air at 36% relative humidity. SCFM ratings are given at a specific pressure; 3.0 scfm @ 90 psi, for example. If you reduce pressure, SCFM goes up, and vice versa.

You would be surprised how fast you can go through 20-30 gallons of air with just one tool operating. So the largest volume tank you can feasibly get up into your attic, do so.

You, too are on the money. I agree that a 10 scfm 60gal (even 20gal) would be the best but that is more expensive than his budget (i think)...

The compressor linked to, will work fine for taking tires off and even beating on a shock top nut and things like that. Mainly because the max PSI is 135. The only time that CFM matters is when you plan to run something for a continuous amount of time. And so if you have a stuck nut, you can crank the regulator to 135psi and for a short amount of time, it will be plenty strong.

For example, my compressor is a 9.0 @ 40psi and like 5.6 @ 90 psi with a 20 gallon tank but the max PSI is 100 so It won't do ***** with an impact. It works good for painting, EVEN THOUGH the scfm is lower than the guns I use! The reason I don't have a problem is because I'm not painting full cars, just parts. But that's getting off subject kinda ... My step dad has a Porter Cable with basically the same specs as the one you posted the link to "PSI: 7.7 CFM @ 40 PSI; 5.3 CFM @ 90 PSI" and it works very good with my impact because I can crank up the pressure.

Having said all that, I agree with LaserRed that you should get the largest tank you can fit because the constant cycling can get annoying ...

Hope that is more helpful than confusing. Just based on my experience. :)
 
Hey propellarhead, check on Craigslist under your city and the tools section. I have purchased many tools and equipment off of Craigslist before. The cool thing is you can go check out the item(s) for sale in person before you buy.

I'm sure you could find a steal on a compressor on there.;)
 
You're actually touching on the areas I had in question. I like to understand the principle behind it instead of just getting the one with the biggest numbers. Thanks.

No prob ... I know what you mean ... I like to understand why things work and how in order to make informed purchases for specific needs. IMO, buying a compressor should be directly related to what you intend to use it for. So buying a monster compressor is pointless if you aren't going to be using many air tools or just using tools for a long period of time (constantly running I mean).

I went by homedepot today to get a chainsaw file and saw that they had a lot of compressors on sale. I don't know what the deal was but I do know that homedepot sometimes sells returned items for a LOT less. That's how we got my brothers compressor for Christmas. Just a thought .....

Also, here's kind of a rundown on tools and PSI:

Air Tool Description - Average CFM @ 90 PSI
Angle Disc Grinder - 7" 5-8
Brad Nailer 0.3
Chisel/Hammer 3-11
Cut-Off Tool 4-10
Drill, Reversible or Straight-Line 3-6
Dual Sander 11-13
Framing Nailer 2.2
Grease Gun 4
Hydraulic Riveter 4
Impact Wrench - 3/8" 2.5-3.5
Impact Wrench - 1/2" 4-5
Impact Wrench - 1" 10
Mini Die Grinder 4-6
Needle Scaler 8-16
Nibbler 4
Orbital Sander 6-9
Ratchet - 1/4" 2.5-3.5
Ratchet - 3/8" 4.5-5
Rotational Sander 8-12.5
Shears 8-16
Speed Saw 5


Sorry to threadjack but how much quieter is a 2 stage than a 1 stage? I live in a very strict neighborhood, and any excessive noise can get us in trouble.

I don't really know the answer, but the only "quiet" compressor that I've seen (one that wouldn't get annoying after 20 cycles) was a small porter cable. It was like a 2 gallon ... But for a nail gun it would have been sweet.
 
The pump CFM really dictates how fast will the pump fill the tank. The tank size dictates how long you can use a tool before the pump has to run again. The tool CFM dictates how quick the tool uses up the compressed air. If you have a low rate pump, you better have a big tank and patience. If you're gonna run a grinder for long periods, you better have a high rate pump.

I'm cool with all that... Now there's PSI. The higher the max PSI, the more rusty bolt breaking torque the impact wrench can apply. 100 PSI is low? 135 is good?
 
I'm cool with all that... Now there's PSI. The higher the max PSI, the more rusty bolt breaking torque the impact wrench can apply. 100 PSI is low? 135 is good?

This is based SOLELY on MY experiences with compressors .. My compressor has a 100 psi max, meaning that the tank will only fill up to 100 psi of pressure which also means that the air output max psi is 100 (make sense?) ... There is a screw on the power switch that you can adjust to allow more/less air to fill the tank. In other words, if you turn the screw back some then the compressor will kick off before it reaches 100 psi (or whatever the max is) and if you try to over-screw (increase pressure) then on mine, the inlet tube that the air goes into the tank, will pop off due to the pressure being too high.

That may not make any sense and if not, I apologize but that's the only way I know how to describe it IMO. Now, my stepdads compressor tank size is way smaller than mine but his compressor's max psi is 135.

OK, so if you are still with me, cool ... One time I was trying to remove the top nut from my shocks using an impact and my compressor (100 psi max). I had the line regulator set to 100 psi (really only gonna give you like 95 +/-) and it wouldn't do anything. Just made a lot of noise. So I hooked up to my stepdads little porter cable and cranked the psi up to 135 on the output regulator ... So I was getting "35" psi more off the start. Keep in mind though that it isn't going to run at 135 for long because of the lower scfm (as you previously described correctly). But the initial torque was much higher using his compressor and it instantly spun the nut off with NO problem. And remember my compressor wouldn't do anything but beat on it with no luck.

I hope that helps some.. :shrug:

FWIW, the reason I have my compressor is because I got a really good deal. (this is still potentially relevant so hang with me) .. My freinds dad runs tractors and he was unable to fill the tires with this compressor because the tires need 100 psi or so. So, he bought a compressor that had like a 150psi max because he needed more psi so that he could actually fill the tires. When he was using mine (the one I bought from him for 70 bucks), the tires wouldn't fill because he was basically just equalizing the pressure at 100 psi instead of actually 'filling' the tire.

Just another example to maybe help with the comprehension of "real-life" type of situations.