All too often, the selection of an air compressor is based entirely on the wrong criteria. Even with the best of intentions, the typical purchaser may select a compressor based on price, appearance, tank size or published horsepower ratings alone. As is so often the case, once the compressor is installed and running, the owner may be disappointed in its performance.
What factors must be considered?
1. Pressure requirements (psig).
2. Actual cfm (cubic feet per minute of air volume delivered).
3. Horsepower (directly related to cfm).
4. Size of receiver tank (measured in U.S. gallons).
5. Tank configuration (vertical or horizontal).
6. Compressor features.
7. Type of control system.
8. Available poser (voltage, phase and amps).
9. Air quality needed.
10. Service requirements.
What do the specs really mean?
1. Cfm rating: Cubic feet per minute ratings can be deceptive unless they’re interpreted properly. A compressor rating of say, 31 cfm, may only refer to the piston displacement of the pump. In actuality, output at 100 psi may drop to 27 cfm. So, if you need a compressor that outputs 30 cfm at 100 psi, buying a compressor that’s “rated” at 31 cfm may be marginal at best. Read the fine print, and make sure you understand if the cfm rating is simply piston displacement or acfm (actual cfm) at a specific pressure (psi).
2. Psi range: Pounds per square inch of pressure must exceed the psi requirement for your hungriest air tool. In other words, you need a higher stored pressure than the pressure needed by the tool, so that you don’t work the pump to death as it tries to keep up with the tool use.
3. Horsepower: This will directly influence the cfm output. In broad terms, the higher the horsepower, the more cfm the compressor can produce.
4. Tank size: Measured in terms of liquid gallons, this simply refers to the physical size of the tank. Tanks are vital for storing air so that the compressor can shut off to cool down and save energy. They also provide a cooling zone to remove bulk liquids.
5. Tank configuration: Your choice of a vertical or horizontal tank will be based on available floor space.
6. Stages: Single-stage or two-stage compressors are available. Single-stage compressors use one cylinder to build air pressure, while two-stage compressors use an initial lower-pressure cylinder to begin the process, feeding into a higher pressure cylinder to complete the process.
Selection guide: cfm and psi
1. Determine the maximum air pressure (psi) that you’ll need, simply by identifying the single highest psi requirement-tool among all of the items in your shop. For instance, this may be a tire changer/inflator that demands 150 psi. If only one piece of equipment needs a much higher pressure, you’re better off choosing a compressor for that purpose. Most tools only need 80-90 psig. Running them higher increases tool wear and wastes energy. According to Kaeser Compressor Inc.’s Michael Camber, every 2 psig increase in pressure increases energy requirements by 1%. Also, allow for pressure drop between the compressor and points of use. Narrow air pipes and contaminated filters can easily cause a 10-20 psig drop.
2. Add up the total cfm requirements for the tools that may be used at the same time. For instance, an air drill, an orbital sander, and one impact wrench might be used simultaneously in your particular shop at any given time. Add the cfm ratings of those items. For purposes of this example, a 4 cfm air drill, 10 cfm orbital sander and a 4 cfm 1/2-inch drive would use a total of 18 cfm. Remember that all of your pneumatic tools are not usually operated at the same time. As Camber noted, “If five stations all operate impact wrenches and they each operate only 10% of the time, the chance that they are all in use is 0.001%. In reality, most people don’t know how much air their tools use or what percentage of the time they use them. Check with your compressor sales person for recommendations.”
3. Add 10% to your cfm requirement (this is to ensure efficient operation in case of air leaks or future expansion). If your initial cfm requirement was 18, expand this to 19.8 cfm (18 x 1.10 = 19.8) for your actual cubic feet per minute need.
4. In anticipation of an eight- to 10-hour working day (using a reciprocating type air compressor), increase the acfm by yet another 15% to allow for extended service life. Our final cfm requirement in this case is 19.8 x 1.15, or 22.77 cfm.
NOTE: These estimates are for reciprocating (piston) type compressors. If you plan to purchase a rotary style compressor, you don’t have to add this additional 15%.
In our example then, we need a compressor that offers 150 psi, and is capable of supplying about 23 cfm. If you choose a lower cfm-rated compressor, you will simply be more limited in the number of air tools that can be operated at the same time.
The moral of the story: While horsepower is always important, pay more attention to cfm ratings.