The MGA With An Attitude
Air Compressors, Advanced -- AR-102

AR-102 is an advanced course in air compressor tech. Fair warning, it gets a little deep in here, and you might want to drop out in mid course. But some people thought the first course wasn't enough.

"I just finished reading your tips on compressors .... it was great. Do you have any updates? Have you bought a new compressor? Any additions to the compressor saga?"

No, no, and no. I've been using the same compressor since at least 1986. It suits my needs well enough. I have only restored one car and done body work and repainted a few others (and my trailer as well). In addition to some low consumption paint guns, I have an air ratchet, a couple of impact wrenches and a air chissel, all of which are high draw but low duty cycle tools. I also have a dual action air sander, which is a high consumption and high duty cycle tool, so that has a limited duty cycle on trigger time, and I use it primarily for finishing work. For really heavy duty sanding and grinding jobs I have electric tools. I have a small open air (no cabinet) sand blasting gun that is not very fast, but I only use it on rare occasion.

The one really huge air hog of note is a good sand blaster, and there the sky is the limit on air consumption. A professional media blaster guy in my town has a 75 HP (real 75 HP) electric compressor on a 300 gallon air tank. He runs a heafty bead blast unit with 100% duty cycle when blasting a complete car body down to bare metal in a few hours. He thinks 75 HP and 300 gallons is not large enough, even though he pays more than $6 per hour for the electric bill when it's running. That's a little out of my league. If I ever have a blasting job that big I'll farm it out.

"I hate to do this to you, but can you tell me what make of compressor you're using?"

Sure. It's a Sears Craftsman unit, and I paid a little too much for it when it wasn't on sale. It was one of those days when I felt rich and felt the need and had a tight schedule all at the same time.

"I am looking to spend between $200 and $300 for a decent one. Found a PUMA, 4HP, 20 gallon tank, electric motor with belt, 115/230 VAC, with the following specs:
MAX PSI: 115
7.5 CFM @ 40 PSI
6.5 CFM @ 90 PSI"

Buy it. It's all you need for a home shop. For commercial professional use you might justify the cost of a 20HP 75 gallon unit, but I personally wouldn't have the space to install one of those. My 20 gallon horizontal belt drive unit fits under the work bench.

With the 4 HP motor (inflated number if it's running on 120-VAC) you may be able to up the capacity some just by changing drive pulleys to run the compressor faster. But beware that some of the cheaper units use a small compressor that is running pretty fast to begin with, so this may not be very effective. My 2 HP unit is very efficient because it has a fairly large and (relatively) slow running compressor (which may be why it was more expensive), but it may also be near the limit of what a 2 HP motor can do. These days it is common to find a combination with a larger motor and a smaller compressor (for the same money) running faster (for the same output) which is not so good, but they sell well by advertising the motor size.

The important thing to remember is not to get sucked in by ads touting 5.0 to 6.5 HP compressors that will run on 120 VAC. In reality there ain't any such thing. A 15 amp outlet is generally limited to about 12 amps continuous duty, and at 120 volts that's only 1440 watts, which is just under 2 HP. The legally honest lie about the power is derived from the flywheel effect of the motor under stalling conditions from top speed to zero, which has absolutely nothing to do with its ability to run a compressor.

A 240 volt 20 amp circuit can handle up to a real 5 HP motor with continuous duty cycle, which would be pretty near the limit for home use. My home shop has a 220V 60A remote service box, I have one 20A x 240V circuit for the air compressor, and I even have one 50A x 240V outlet for an electric welder. That outlet could run a compressor up to 12.5 HP (real power rating) continuous duty (if I should ever feel the need). 20 HP would require a 240V 100A remote service box, or if you're near an industrial area you might bring in a three-phase line and get by with 240V 3-phase 60-amp service (just for this one tool). In other words, forget about anything larger than about 5 HP (real power rating) for a home shop.

"I plan to use air tools needing 4 CFM @ 90 PSI, and would love to get a small sand or bead blaster, but am now trying to get "informed" about the different types."

For painting you would need to spray continuously until the job is done, so the compressor must have at least enough output to run the paint gun in continuous mode. But don't get scared by a paint gun that claims to need 8 to 12 CFM, because most guns will spray automotive paints (relatively low viscosity) at much lower pressure and volume that the stated numbers. I never run my paint gun above 40 psi, and I nearly always use 1/4" hose for the last 10 feet. The compressor does run continuously, but the pressure never falls below 40 at the regulator. In other words, that 7.5 CFM at 40 PSI is plenty enough to run most paint guns.

If you ever use a tool that sucks more air than the compressor can put out, then you start calculating duty cycle and trigger time. Duty cycle is supply/consumption, so if you have a 9 CFM tool and a 6 CFM compressor, it can only run 67% of the time at full power, and has to rest the other 33% of the time. The rest of this is a tough lesson.

Trigger time (how long the tool can run continuously) is determined by the size of the air tank and the magnitude of the swing between maximum and minimum tank pressure, and the relationship between cunsumption and supply. A 20 gallon tank (2.66 cubic feet) bleeding down from 100 PSI to 40 PSI will give up about 4.7 Standard Cubic Feet of air (see formula below). This can be supplimented by flow from the compressor while the tool is running. As such, the tool draws all of the flow from the compressor, and the excess flow required is drawn from the tank. In this example the 9 CFM tool would be drawing 6 CFM from the compressor and the other 3 CFM from the tank, so it could run with full output for 4.7/3 = 1.57 minutes (while the tank pressure was falling from 100 PSI to 40 PSI). That's the allowable trigger time. Divide this by the duty cycle (.67) and you get the total cycle time (2.34 minutes). As such this tool running with this compressor and tank can run full bore for 1.57 minutes out of any 2.34 minute period (assuming it is shut completely off for the rest of the time).

This example might be reasonable for a low speed air sander doing rough sanding on body filler or primer paint (not grindind steel), where you can work for a minute and a half and sit it out doing cleaning and inspection for the next 45 seconds while the compressor recovers back to 100 PSI.

The next example is much tougher. Suppose you have a big air grinder or a sand blaster that needs 16 SCFM at 80 PSI. The same 20 gallon tank bleeding down from 100 PSI to 80 PSI will give up only 1.45 Standard Cubic Feet of air. The tool will take 6 CFM from the compressor and 10 CFM from the tank, so the trigger time will be only 1.45/10 = 0.145 minutes, or just about 8.7 seconds before the pressure falls below the required 80 PSI. Duty cycle is 6/16 = 37.5%, so the total cycle time will be 8.7/0.375 = 23.2 seconds. This combination can then run only 8.7 seconds out of any 23.2 second period. Can you live with a tool that can only run 9 seconds and then requires a 15 second rest? If you were to increase the tank size from 20 gallon to 75 gallon these numbers would increase to 3.75 times, so then this same tool could run 34 seconds and rest 56 seconds. This might be okay for grinding if you don't do too much of it, but what should be a one hour continuous sand blasting job would take 2 hours and 40 minutes (while you get lots of one minute breaks).

The trigger time can be increased substantially by boositing the the maximum pressure to 125 PSIG, but the duty cycle remains the same. Duty cycle can only be increased by increasing the output of the compressor. The other problem is that you need an air hose large enough to handle the required air flow for those more air hungry tools. A 1/2" drive air impact wrench won't do diddy on the end of a 1/4" hose, and only moderately well on a 3/8" hose. And not many people have a 1/2" hose in their home shop, or plumbing and regulator that large on the air tank.

Now your question below (far below) is "What is SCFM?" A cubic foot is a well known volume. A Standard Cubic Foot of air is one cubic foot of air at 14.7psi and 60dF (standard atmospheric conditions at sea level). In this case 14.7 PSIA (A is for Absolute) is equal to 0 PSIG (G is for Gauge).

When speaking of the air consumption for air powered tools, the CFM numbers are actually SCFM numbers, so always add the "S" in front of the "CFM" on the tool.

When speaking of the pressure required for the operation of the tool, this is always assumed to be Gauge pressure (PSIG), so always add the "G" after the "PSI" on the tool, and remember that the gauge is always reading 14.7 PSI less than Absolute pressure.

When air expands or is compressed quickly with no means to gain or lose heat, the changes are governed by this formula for adiabatic expansion:
(P x V) / T = 53.3
where P is absolute pressure in pounds per square foot, V is volume in cubic feet of one pound of air at the given pressure and temperature, and T is Absolute temperature in degrees Rankin.

Tricky stuff here. Atmospheric pressure is 14.7 PSI (standard conditions at sea level). A pressure gauge (common type) will read zero when the pressure on both sides is the same. So the gauge will read zero when the pressure in the tank is equal to one atmosphere. This condition is 0 PSIG and 14.7 PSIA. To use the expansion formula above you always have to add 14.7 to the indicated gauge pressure. As such, a change of pressure from 100 PSIG to 40 PSIG is actually 114.7 PSIA to 54.7 PSIA, and you have to use Absolute pressure numbers to make this calculation work.

You may recall that for temperatures, degrees Kelvin are degrees Centigrad above absolute zero. Similarly degrees Rankin are degrees Farenheit above absolute zero. Don't let this bother you, as there is a way to avoid temperature considerations all together. And you don't have to know how large a pound of air is, because there is nothing in these formula about weight.

From the equation above one can derive the following six basic equations. The carrot sign (^) denotes power (or exponent).
V2/V1 = (P1/P2)^0.71
P2/P1 = (V1/V2)^1.41
T2/T1 = (V1/V2)^0.41
V2/V1 = (T1/T2)^2.46
P2/P1 = (T2/T1)^3.46
T2/T1 = (P2/P1)^0.29

As we are not immediately concerned with the change in air temperature, all you need do is to ignore any equation with a T in it and use the others. The two remaining equations are simply reciprocals of each other, so by properly swapping the 1s and 2s you can use just the first formula, and now you're down to just this:
V2/V1 = (P1/P2)^0.71

Now when you release air from your 2.66 cubic foot tank to run a tool at 40 PSIG, you first calculate how much the air expands going from 114.7 PSIA to 54.7 PSIA, and you get this:
V2/V1 = (114.7/54.7)^0.71 = 1.69
meaning that the 2.66 cu ft at 100 PSIG becomes 4.50 cu ft at 40 PSIG. But 2.66 of that remains in the tank while the rest, 1.84 has escaped (at 40 PSIG). Then you can do the calculation again to find what happens to the 1.84 when it expands down to one atmosphere of pressure, like this:
V2/V1 = (54.7/14.7)^0.71 = 2.54
meaning that the consumed air will expand to 1.84x2.54 = 4.67 cubic feet as it drops to one atmosphere of pressure. This is where I got the 4.7 about 12 paragraphs earlier. Now if you can follow all of this you can do any of the calculations yourself.

"I am also looking at blast cabinets, but they have a range of specs. In particular, one model from Central Pneumatic calls for
Working Pressure: 80-120 PSI
costs $99.99"

Depending on the size of the output air nozzle, this could take any amount of air, and you can't tell how much is required by the givein information.

"Another from to Central Pneumatic calls for
Air Consumption: 5-8 SCFM (what's SCFM?)
40-80 PSI working pressure
costs $69.99"

This should have about a 75% duty cycle at high pressure with the compressor noted above. It may run continuously at about 60 PSI, but that can slow down the action a LOT. For blasting in short time you need high pressure. Reducing pressure reduces the effectiveness of the blaster dramatically. Cut the pressure back to 40 PSI and it may not even remove paint at all. Lower pressure is generally only used for delicate materials (like aluminum door or bonnet skins). Otherwise high pressure is king for sand blasting.

"Another from Central Pneumatic states
Maximum air pressure: 125 PSI
nothing on CFM rating
costs $119.99"

No way to tell how much air this one needs without asking. These problems arise either from the failure of the sales group to understand the needs of the customer, or from the needs of the sales group to fool the customer.

For sand blasters you need to know the air consumption numbers to calculate (or estimate) the allowable trigger time and duty cycle. You should plan on running a blaster near maximum pressure for tough jobs. And you would like to have a high duty cycle available, so you don't have too much waiting time on a big job when you would like to go continuous. Better to use a smaller nozzle for less air flow and a smaller working spot and be able to run a very high duty cycle. If you run out of pressure you're into wait time.

Wheeew! I guess you got $.03 worth today.


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