Air Compressors and Tools 101

My first suggestion is to avoid oilless compressors. They are loud, cheap and don't last. I would also avoid Porter Cable compressors, especially the JetStream, as they have proven troublesome. Porter Cable do make decent power tools, unfortunately, their compressors are not among them.

They may not be sexy, but the best units are the heavy, twin cylinder, belt driven oil lubricated compressors. Since it is likely that you will not be going to any industrial compressor suppliers, in which case I would suggest buying a small Champion or Quincy (the best there is), I would recommend either a twin cylinder Campbell Hausfeld, or a twin cylinder Coleman Sanborn unit, both available from Canadian Tire. Husky compressors are re-badged Campbell Hausfelds, as are some Ridgid's. I refer to the cast-iron pumps, as most aluminum pumps (excepting Champions) are not very well built, but use the metal for its casting and cooling properties

Please note, my recommendation of the Campbell Hausfeld twin, oil lubed unit does not imply any further recommendation of CH products, most of which is throwaway junk. Same goes for Coleman. DeWalt compressors are good units, as they are the old Emglo's. I think some of them use the CH twin cylinder pump as well. Can Tire used to sell some nice twin cylinder IR units awhile ago, but I don't see them anymore. I guess they were a bit pricey. Probably why they don't sell Milwaukee tools there anymore either. They are really selling a lot of crap there nowadays. They used to be a half decent store once, now it's all Chinese crap, some of which is the same stuff you can get at Princess Auto. (For the Americans on this site, Princess Auto is a lot like Harbour Freight, but it has a lot of other stuff geared towards farmer's and tradesmen; like welding equipment, large fans, stuff to build trailers etc. They also carry a lot of stuff bought up from other companies liquidating stock, and bankruptcies and the like, so you never know what you'll find when you go there. It's an addictive shopping experience. Sometimes they have weirdo stuff like military generators that run at 400Hz and stuff. If you are near the border, you might find something thats worth getting shipped to you. They have a bi-weekly flyer at their website www.princessauto.com).

If you must, you can get small compressors from Princess Auto for dirt cheap. Longevity is questionable, as is workmanship. I do know that PA does not stock many parts. Anyhow, it is useful to remember a few things:

Ratings on air tools are usually base on a 25% duty cycle, so actual air consumption under heavy loads can be up to four times greater than advertised. Also, it is physically impossible to draw 5hp worth of electricity out of a 110V outlet, and very difficult to do out of a 220V home service. It works out to something like 40amps @ 110V, which, if you can manage it, will certainly put you on a first name basis with the local fire chief. Also consider that compressor can draw up to 10 times the rated amp draw on start up, so if you need air far from the compressor, lengthen the hose, not the power cord, indeed, avoid using ANY extension cords with an air compressor, as they are one of the highest current drawing machines there is on startup and need as much voltage as they can get.

Disregard any output ratings at 40psi. You will rarely, if ever do anything at that low of a pressure, except possibly HVLP spraying, in which case it might be relevant. Rule of thumb is 4cfm per ACTUAL hp @ 100psi. So, by reverse logic, you can take the CFM output @ 100 psi and divide by 4 and you will be able to roughly calculate what the actual HP output of a compressor is. There is a lot of wild exaggeration on power outputs in this end of the compressor market, with companies using some pretty dubious methods to measure HP. Some use Peak Power, which is basically how much power the motor can develop with a locked rotor and someone holding the breaker on with his thumb. Others use the amp draw at startup Watts = Volts X Amps, 1 hp = 746 watts etc. Others use the "potential energy equivalent" available from a tank of air @ 100 psi etc.., none of which have any bearing whatsoever on real world conditions. Campbell Hausfeld was actually successfully sued a few years ago for inflating (pardon the pun) the HP ratings of their machines and were forced to give customers who had bought their products a $50 voucher towards the purchase of a new CH unit. They are far from being the only ones, but I guess they were the biggest target.


One last note, single stage compressors lose a lot of their efficiency at higher pressures and make a lot of heat if worked had.

Decent Machine at Can Tire

I soaked that up like a sponge!
Stay away from any compressor with "Hot Rod" on it too! An absolute waste of money, materials, and labour!

So now that you picked up your compressor your ready to go, all thats left is the air tools, but which one to get (CFM numbers)? Well... it depends!

Im using a HVLP spray gun as an example (just got a new hvlp gun yesterday so its on my mind ), but the general idea is the same for any air tool as they are all rated. I'm also using a 8gal tank as an example as its usually the size a newbie DIY starts with. Note that 8 gallons is approx 1 cu. ft. at 40 psi, it will hold the equivalent of 2-2/3 cu. ft. /min at atmospheric pressure.

CU. FT. /MIN = CFM

The first question you have to ask yourself is; "How long does it take the compressor to fill the tank to the point of shutting off, *STARTING*FROM* zero pressure?" This will tell you what the 'actual' capacity of the compressor is.

For the sake of argument, let's say it take about 3 minutes. This works out to roughly 1 cu. ft. /min at atmospheric pressure. Now lets assume you have found a gun that needs only 5 cu.ft/min at atmospheric pressure. The entire capacity of your tank gives you about 30 seconds of spray time. In which period, the compressor has been able to provide another 1/2 cu. ft. worth. so you get a grand total of 36 seconds of spraying time. Now wait 3 minutes for the compressor to recharge the tank, and you can spray for another 36 seconds. WOOPS! forgot about the 50% duty cycle. make that 3 minutes for the compressor to recharge the tank, and another 3-1/2 minutes for the compressor to 'cool off'. or roughly 36 seconds of painting, every 7 minutes.

But wait, it gets worse. the HVLP gun probably specs it's air requirements at something above 'atmospheric pressure'. If it needs air at a mere 15psi, cut the 'painting' time to somewhat less than half the number above. You'll get about 17 seconds of painting time, and then have to wait the same 6-1/2+ minutes before you can paint for another 17 seconds. If the specified air requirements are at a higher pressure, things get EVEN WORSE. Not to mention what happens if it takes more than 3 minutes to fully charge the tank.

Again, I used a HVLP paint gun as an example, but the idea is the same. Hopefully some of you will remember my notes when you go out to purchase a nice quality impact gun, die grinder, ect.. or you will soon find yourself running out of air.

Nice writeup! Lots to learn about this equipment, but I had read in the past that there are basically 2 main manufacturers of the pumps used on these, and they are just marketed under the various brand names. Here's the post:

"Almost every consumer grade compressor out there is made by Devilbiss, I put a list below. I got myself an Ingersoll Rand 2 stage commercial model and it is a VERY nice compressor, very quiet and very smooth running. I threw it up in the attic in my garage to save noise and space I also ran a pipe for the air intake outsie which cuts the noise in half almost. If possible I would make a little bump out shed onto the garage and put the compressor in there.

So if you end up getting one at Home Depot or Sears try and get one with a screw air filter so you have the option of mounting it outside, check out the rpm's it works at, the slower it spins the quieter. DO NOT buy an oiless model no matter what. Other then that pick a color you like cuz the rest is just the same anyways. If you go for a higher end model I would recommend an IR or a Princess Auto 2 stage.

Campbell Hausfeld
CH Extreme
Farmhand
Husky (models beginning with FP, WL, HS, VS, VT)
Husky Pro (models beginning with HS, VS, VT)
IronForce
Maxus
Power Pro

DeVilbiss' air compressors were sold under the following brand names:

2X4
Accuspray
Air America
American IMC
Blue Point
Charge Air Pro
Companion
Craftsman
DAPC
Delta
Delta Shopmaster
Delta Workshop
DeVilbiss
Ex-Cell
Force International
Husky
Impact
Ingersoll-Rand
Matco
Porter-Cable
Power Tools
Pro 4000
Pro Air
Pro Air II
Rand 4000
Senco
Snap-On
Speedaire
Steel Driver Series
Superior Fastener
Tradesman
W R Brown
Westward"

You seem to know alot about this stuff, what are your comments?

Seeing as I haven't updated this thread in awhile I figured it was about time. I was asked the following question not to long ago;

I do not seeing myself spray painting just wrenching and ratcheting, probably some grinding and of course blowing all the leaves from the garage. The question is how much air flow do I need for these tools? How do the tank size and flow relate to each other?

My response;

Actually, spray painting is only a moderate air flow requirement. Wrenching and ratcheting is low flow requirement for the compressor, because it's intermittent usage and the tool takes air from the tank when it needs a burst. Continuously blowing leaves would be the largest demand on the compressor, as it basically just opens the end of the hose and lets all the air out of the tank, after which all you get is the output flow of the compressor, which won't be much for blowing unless you have a Caterpillar engine driving the compressor. Short bursts of blow air are taken from the pressure tank, and the compressor doesn't even have to be running except to repressurize the tank afterwards.

Grinding is the highest air demand application. This takes lots of continuous air flow at high pressure, like maybe 16 CFM at 90 PSI, and very few home use compressors can keep up. Here you calculate the possible "trigger time" from the volume of the air tank. As you're grinding the tank pressure will drop from 110-120 PSI to less than 90 PSI (trigger time), and then you have to stop grinding and wait for the compressor to refill the tank back to the 110-120 PSI level (dwell tine). The ratio of these two numbers is duty cycle. For example, if the compressor puts out 7 SCFM at 90 PSI, and the tool consumes 16 SCFM at 90 PSI, the duty cycle will be 7/16=44%. To calculate trigger time you start by subtracting the compressor output from the tool demand, and the difference is how fast the tank will be depleted (16-7=9). A larger tank will give you a longer trigger time, and then you have to wait longer for it to refill. The duty cycle is dependent on compressor output, while the trigger time is dependent on both compressor output and tank volume. A 20 gallon tank may give you 20 seconds trigger time and a 40 second wait, while a 30 gallon tank with the same compressor might give you 30 seconds trigger time and a 60 second wait to refill. Either way, it's still 33% duty cycle, and the only way to improve that is with a larger compressor.

If you do a lot of body grinding or sand blasting you need a very large compressor. If you do these things only occasionally, then maybe the wait cycle won't bother you so much, and a large tank may be more important for longer trigger time. If you do more short cycle things with large consumption, like filling car tires with air, then a larger tank would be good, so you don't have to wait for the compressor before you finish filling the tire. Whatever you do, be sure the compressor has enough capacity to keep up with your largest continuous flow requirement. You don't want to stop to wait in the middle of a painting a car, so if the paint gun requires 7 CFM continuous at 40 PSI, the compressor had better be capable of 7 CFM at 40 PSI. In other words, first consider carefully what air tools you will be using, and then source a compressor to handle the need. Too little compressor won't get the job done, while too much compressor is just a waste of money (and maybe garage space too).

Compressor output and tank size are not necessarily related, except that larger tanks are often mated to larger compressors (but not always). It often boils down to marketing technique, which is designed to take advantage of the uninformed buyer. Sometimes they just install a very large motor (sounds powerful) on an otherwise wimpy unit, and you still have a wimpy unit that can only use a fraction of the motor capacity. Sometimes they put a wimpy unit on top of a large tank to make it look impressive (big), and you still have a wimpy unit, but it then has a little more trigger time. One thing they almost never do is to install large compressor with a small motor, even though it's more efficient that way. A larger displacement compressor running at lower speed consumes less electricity and runs quieter, but it costs more to produce. You generally only find this combination in industrial quality tools (expensive).

However, a two cylinder compressor will be more efficient than a single cylinder unit, especially at higher pressures. This means a 2-HP twin cylinder unit can put out more air than a 2-HP single cylinder unit. This is usually accomplished by making it a two stage compressor, which is seldom even mentioned in advertising. The first cylinder pumps into the second cylinder, which in turn pumps into the tank. This allows the first cylinder to be pumping against less back pressure, which is why it flows more air. If both cylinders were pumping in parallel it would act like one cylinder twice the size, and it would pump more air at low pressure, but flow would fall off dramatically at higher pressure. This is pretty easy to spot by checking the specs on the label on the compressor. Flow is always stated at two different pressures, usually 40 PSI and 90 PSI. When those numbers are widely different it will pump more air at low pressure and much less at high pressure. When those two numbers are closer together it is doing less flow at low pressure but has better flow at high pressure. High pressure is where it counts for the more demanding applications such as grinder, impact wrench, sand blaster, etc. Anyone can build in good flow at low pressure, but it's the flow at high pressure that really counts.

One big caution is not to fall into the modern day horsepower trap. If you see an advertised "5 horsepower" motor with a 120 volt 15 amp line cord plug, you are being bamboozled. A 15 amp circuit can reliably only put out 12 amps continuously without risk of blowing the fuse or circuit breaker, so anything with a 15 amp plug will likely not consume more than 12 amps continuously. Check the power consumption rating on the motor. 12 amps times 120 volts is only 1440 watts, which is about 2 HP of electrical consumption, and with a little efficiency loss the motor will put out a little less than 2 HP continuously. The way they get away with stating the high power number is by stating the maximum momentary power output. If the motor is coupled with a large flywheel (or has a heavy armature) it could put out 5 HP for a second or two as it looses speed and dissipates the energy in the flywheel, but that is absolutely meaningless for a compressor that demands continuous level power to operate. These guys should all be tossed into jail for the deceptive advertising practices.

So you need to look at the air performance numbers, not at the hardware. Pay no attention to the "rated" motor power. There may be some small interest as a tie breaker to look for the larger displacement compressor in the interest of electrical efficiency, but not at the expense of air output. A larger tank is sometimes a little help with trigger time, but otherwise there is no substitute for AIR FLOW OUTPUT, which is the all important factor.

The direct coupled units with the compressor connected in line with the motor shaft will have low speed motors and high speed compressors. These are less efficient and generate MUCH MORE NOISE. YOU CANNOT TALK TO A FRIEND IN YOUR SHOP WITH ONE OF THESE UNITS RUNNING and you may well want to use ear plugs. With a belt driven unit the compressor runs much slower than the motor, so the compressor is quieter and the motor is more efficient at higher speed, usually 3600 rpm (or 3450) vs. 1800, and the compressor will be larger displacement which is also more efficient.

One other consideration is the input line voltage. Small compressors of 1.0-HP or less run quite nicely on 120 volts. Running a real 2.0-HP motor (or an advertized "5.0-HP motor) on 120 volts is pushing the limits of your house wiring. This draws so much current, especially on startups at high pressure, that it can cause momentary brown-out conditions. This probably won't blow a fuse, but if the house wiring is small or long it will cause the lights to dim and flicker, and it might even drop the line voltage enough momentarily to reset electronic clocks and computers. In severe cases a 2-HP compressor may stall and not start up at all until you bleed the tank pressure down to a low value. Also the 240 motor draws only half the current under all conditions, and it will be a little more efficient running, but you have to have the 240 volt line in the shop to provide the power.

For myself, I would go out of the way and pay the expense to install a 240 volt circuit in the house/shop if necessary. Many motors in the 1.0 to 2.0 horsepower range (and "advertised 5-HP" motors) can be rewired inside of the end cover to run on either 120 volts or 240 volts. The 120 volt operation actually requires a heavier line cord, because it draws twice the current.

So...do I get my Air Compressors & Tools 101 certificate now?

Originally Posted by IanB

You seem to know alot about this stuff, what are your comments?

Funny that you mention it, there was a huge recall on Porter-Cable brand compressors last year (which were/are made by Devilbiss). You may also note that they make alot of pro-grade pressure washers aswell.

Devilbiss was also involved in a class action lawsuit back in 2004 as they were over inflating HP numbers. If I recall correctly they refunded each person $70 or so.

Campbell Hausfeld was in even hotter water over this issue.