In Part 2 of Wayne Scraba’s series on compressed air, he looks at the practical aspects — power needs and power delivery.
In our last segment we examined the basics of shop air compressors. If you spin your browser back to that article, you’ll recall we looked at tank configurations, sizes and compressor types.
This time around, we’ll zoom in on the power needs of the compressor along with the air quantity needs of various power tools. You might be surprised to find some seemingly innocent looking tools have a voracious appetite for compressed air. But other seemingly huge air users are actually quite frugal.
Power
There are lots of ways to rate an air compressor. Some folks make decisions based upon the pump output PSI while others rate the compressor by electric motor horsepower. The best compressor performance gauge is the output CFM at a given PSI. The rate at which a compressor can deliver a volume of air is noted in cubic feet per minute (CFM). Because atmospheric pressure plays a role in how fast air moves into the cylinder, CFM varies with atmospheric pressure. It also varies with the temperature and humidity of the air. To create a standard, manufacturers calculate standard cubic feet per minute (SCFM) as CFM at sea level with 68 degrees F air at 36% relative humidity. SCFM ratings are given at a specific pressure, for example, 20 SCFM at 100 psi. If you reduce pressure, SCFM goes up, and vice versa.
You also may run across a rating called “Displacement CFM”. This figure is the product of cylinder displacement and motor rpm. In comparison with SCFM, it provides an index of compressor pump efficiency. The CFM and PSI ratings are important because they indicate the tools a specific compressor can drive. When choosing a compressor, make sure it can supply the amount of air and the pressure your tools need. The bottom line here is to select an air compressor that exceeds the CFM of your most powerful air tool.
Just how much air does a given pneumatic tool require? Check out the following chart. Keep in mind this is only a general guide, since some brands and configurations of air tools use more or less air:
| Tool | CFM | PSI |
| Upholstery Stapler | 2.0 | 30 |
| Ratchet 1/4″ | 3.0 | 90 |
| Air Hammer | 4.0 | 90 |
| Die Grinder 1/4″ | 4.0 | 90 |
| Drill 3/8″ | 4.0 | 90 |
| Drill 1/2″ | 4.0 | 90 |
| Impact Driver 1/2″ | 4.0 | 90 |
| Ratchet 3/8″ | 4.0 | 90 |
| Spray Gun, Commercial | 4.0 – 7.0 | 30-70 |
| Jitterbug Sander | 6.0 | 90 |
| Board Sander | 7.0 | 90 |
| Impact Driver 3/4″ | 7.5 | 90 |
| High Speed Grinder | 8.0 | 90 |
Electrical Requirements
The electric motors found on air compressors range from simple 110-volt 1/2 HP models all the way up to giant three-phase, 25+ HP 220-240-volt monsters. The electrical service available to your house or shop dictates the largest compressor you can install (we’re talking 220-240 volt models here). The electricity supplied to the majority of residential properties consists of a single phase, or voltage signal. But the electricity used by heavy industries is generally three-phase power. Few, if any, residences have three-phase electrical capability (and neither do some commercial buildings). There are converters available that allow you to use a three-phase motor in a building that is wired for single phase, but if buying a new compressor, it’s simply more practical to begin with a single phase model.
When shopping for a compressor, keep an eye out for the amperage rating of the electrical motor. Electrical service is measured in amps with the standards being 100, 150 and 200 for a typical residence (usually, the older the residence, the smaller the electrical service). Amperage is like the size of the water pipe that feeds the residence – the bigger the pipe the more water that can be delivered to the house. So far so good, but what about the electrical motor? All compressor motors should have a tag that indicates the amperage of the motor. The 5 HP, 230-volt Baldor motor on the Devair compressor shown in the accompany photos has a 23 amp rating. To accommodate the compressor motor, a dedicated 30-amp circuit is used in the shop panel. If you do the math, a residence that has a 100-amp service will be hard pressed to give up 30 amps just for an air compressor. You can see the need for 200-amp service in a situation such as this. As the electric motor size (HP) goes up, so does the amperage requirement. In other words, it is definitely possible to overwhelm the electrical service in your workshop with a monster compressor. Finally, remember that there is only so much one can accomplish with a simple 110-volt air compressor. Most hobby compressors are really overrated (for marketing purposes) and operate on a normal 15-amp 110-volt circuit that is only capable of running about 2 real HP or less.
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A final consideration when purchasing an air compressor is service. There are dozens, if not hundreds of air compressors on the market today. Some are made in North America. Plenty of others aren’t. Some manufacturers have credible parts and service “chains” while others have zero maintenance or service items available. What you have to consider when buying a large item such as a shop compressor is if the manufacturer will be around long enough to supply parts if they are ever necessary. Otherwise, the air compressor you purchase will have a true value of $1.50 per pound (or whatever the going rate for scrap metal is today). When it comes to air compressors, you definitely get what you pay for. For a closer look at air compressors as well as important compressor accessories, check out the accompanying photos.
A 230-volt, 5 HP Baldor electric motor, powers this Devair compressor. More in the next photo: 
What really critical here is the amperage. The total amp load (23) dictates the size of the electrical service you require in your garage or workshop. 
What’s this? It’s a heavy-duty magnetic electric motor starter made by Square “D”. Every large compressor 220-230 volt compressor has such a device as standard equipment or at the least, optionally available. It’s designed as an overload device, providing protection for the electric motor. It’s wired into the air compressor pressure switch, and in turn, wired to the main fuse panel by way of a disconnect. 
You’ll need a switch of some sort to start the compressor. I use this manual motor controller between the fuse panel and the compressor. In essence, this is a heavy-duty switch, and perhaps overkill for the application. It doesn’t have to be mounted right beside the compressor, but it should be reasonably close (this one is roughly 5-feet away). FYI, there are similar switches that can be locked out by way of a pad lock if necessary. Another big issue when choosing an air compressor is the type of electrical service you have in your shop or garage. The text offers more insight into the needs of a large air compressor. 
This is a Wilkerson combination filter (moisture trap), pressure regulator-tool oiler. In my personal situation, I don’t paint cars in our shop, but I do plenty of mechanical work with air tools. Air tools need oil, and it’s just a bunch easier to use this combo unit to provide a mist of oil to the tools as they’re used (rather than removing a plug on each tool and oiling it individually). Should there ever be a need to paint something with this compressor, and then the modular nature of the Wilkerson assembly allows for the temporary removal of the oiler. 
There are plenty of methods to plumb an air compressor, but I used AN hose and aluminum AN fittings. The reasons were simple: It looked clean, I had the hardware left over from old racecar projects and it functions perfectly. You can do the same with ordinary brass fittings and conventional hose or hard line, but be absolutely positive the lines you use are capable of withstanding high pressure. 
You have a couple of choices when it comes to airline. As mentioned earlier, you can run hard line to various points in your workshop or you can run hose off the end of the combo regulator. I’ve always hated tripping over air hose in the workshop. Because of that I chose to use a 50-foot auto retracting Prevost hose reel (this is a heavy duty, high flow, 3/8-inch line assembly). 
A carpenter came up with the idea of mounting the 55-pound hose reel on a plywood plate that is anchored on two wall studs. The plywood plate is doubled up 1/2-inch plywood (which, in turn is approximately 1-inch thick) and is fastened to the studs by way of 8 hefty wood screws. In turn, the Prevost hose reel is bolted to the plywood. We mounted the reel as high as possible, and since it’s roughly in the middle of our workshop wall there’s sufficient hose to reach any corner of the shop or reach outside.
Sidebar: Preventative Maintenance
If you purchase a good quality air compressor, you’ll know that it’s likely a good idea to keep the maintenance up – even if it’s because of the simple fact that a quality compressor setup isn’t exactly cheap to replace. The folks from Devair have a maintenance schedule that’s easy to follow, and makes perfect sense. Keep in mind this schedule is based upon the fact a compressor will see an average of 40 hours of use per week. If your usage varies, then adjust your personal schedule accordingly.
Note: When servicing an air compressor, shut the power to the unit Off, and drain the tank of air pressure. Devair points out that if you service belts and pulleys, always reinstall the respective belt guards.
Maintenance
Daily:
Drain moisture from the tank
Weekly:
Check the pump oil level and top off if necessary
Quarterly:
Change Oil and Filter
Check condition and alignment of belt, flywheel and motor pulley
Check operation of the safety valve
Check pressure switch unloader (or CPR unloader) to ensure the compressor unloads whenever the motor shuts down.
Clean and/or blow off pump fins as well as the motor
2000-Hour Maintenance:
(Following 2,000 hours of operation, the compressor should have a more thorough inspection and overhaul as necessary):
Lubricate electric motor
Inspect and replace as necessary, the following components:
Pump Valves
Check Valves
Safety Valves
Pressure Gauge
Belt(s)
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