The MGA With An Attitude
Air Compressors, Introduction -- AR-101

AR-101 is a review of everything you need to know about air compressors before buying one. Key thoughts here are air delivery at given pressures, and you can just about forget about anything else. Particularly do not get distracted too much by the size of the air tank or the advertised horsepower of the motor.

On 11/27/2000 Steve Petrosky wrote:
.... "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."

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 depleated (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 whimpy unit, and you still have a whimpy unit that can only use a fraction of the motor capacity. Sometimes they put a whimpy unit on top of a large tank to make it look impressive (big), and you still have a whimpy 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 runnning 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 contiuous level power to operate. These guys should all be tossed into jail for the deceptive advertising practices.

ADDENDUM May 26, 2004:
Thanks to a notice from Greg Lanz, Tucson, AZ. There is now a class action settlement related to this practice of overrating air compressor motors. The economic return for the affected buyers is minimal, but at least it appears that this practice of deceptive labeling of motor power is about to stop.

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.

I use a 2-HP  2-cylinder belt driven compressor mounted on a 20 gallon horizontal tank on wheels that fits nicely underneath the workbench. This puts out 9.0 CFM at 40 PSI, and 7.5 CFM at 90 PSI, and it does everything I need quite nicely. Except that I do only occasional air grinding (30 second trigger and 30 second wait), and I don't own a sand blaster (yet). If I would buy a sandblaster I would have to limit it to one that works with 7.5 CFM at 90 psi. Incidentally I chose this unit specifically because it does do 7.5 CFM at 90 PSI, which is about the best you can find with only a 2-HP motor. This is (slightly) better output that the current "5-HP" direct drive 120 volt units sitting on the vertical 30 gallon air tanks.

And since someone has asked specifically, it's a Craftsman brand I picked up a Sears about 20 years ago. It is 2hp belt drive 2 cylinder on a 20 gallon air tank on wheels. It was then a $500 unit on sale for $400, and we have had considerable inflation in the ensuing years. Sitting beside it was a 3 hp unit with the same compressor on a 30 gallon tank for about $100 more money. That one only put out about 10% more air at the same pressure, and it was exclusively a 240 vac unit with no option to wire it for 120 vac. Today it's hard to find something with similar specs in a consumer product department because of modern marketing strategy that dictates charging the most money for the least cost to produce while trying to fool the customer. Most homeowner targeted compressors are now the noisey direct drive units with grossly inflated power ratings and somewhat lesser air output.

These days you may have to check into the industrial or professional supply market to find something similar to mine. Look for a real 220 volt AC 2 HP motor with belt driven compressor. Air output should be close to 9.0 cfm at 40 psi, and 7.5 cfm at 90 psi. Tank size might be anything from 15 to 30 gallons either horizontal or vertical. Motor power ratings might still be inflated to as much as 5.0 to 6.5 stated "peak" horsepower, but the belt driven units are more likely to state real motor power, because the belt drive motors are made separately as standard motors for lots of various applications. Check this slightly smaller 1.5 hp 20 gallon unit at $675 for example:   (This link worked on 11/9/02).

An option to wire the motor for operation on 120 vac input power is a dead giveaway that it's not over 2 hp. If you happen to find a compressor something like this, please let me know. I have searched widely and find slim pickings in current inventory. For this you might expect a retail price in the range of $700 to $1000, which is why the consumer market is now burgeoning with the cheaper direct drive units.

One prime 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 advertised "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 (by itself), 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 shop if necessary. I have a 3-wire 6-gauge feeder line running from the main box in the house out to a 60-amp 240-volt remote breaker box in the garage. The 2-HP compressor is on a 20-amp 240-volt circuit, draws 10.5 amps maximum (more like 6.0 amps while running continuously), and never causes any noticeable flicker in the lights on startup. I also have a 50-amp 240-volt circuit there for an electric welder, as for that you really do have to have the heavy duty wiring. 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.

Barney Gaylord

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