Tech Stuff

Air compressor technology: Match the compressor to your specific needs

Determining compressor cfm

What cfm capacity do you need? Following is a generalized formula for selecting the cfm rating of your new compressor. Add up the cfm requirement of all of the pneumatic tools that you plan to run (air wrenches, grinders, drills, spray guns, etc.). This total cfm should represent the grand total of all of the air tools that you would realistically plan to run at the same time.

Once you have this tool cfm total, add to this a 10% margin to compensate for potential air leaks (leaks at fittings, etc.). Then add another 15% as a safety margin (for piston compressors).

This compensates for tool cfm ratings and the published rating of the compressor (any of which might not be accurate in the real world).

As another example, if your total tool cfm potential use in your shop is, say, 20 cfm, you’ll add 10% (2 cfm), then add another 15% (about 3 cfm), for a total compressor capability of 25 cfm.

While you can get by with less when only running one or two tools at once, you really need to determine a realistic level that will be needed during your maximum workload at any given time.

Total cfm requirements = ________ cfm

(Add up cfm needs of all tools that may be used at the same time)

Cfm x 1.10 = ________

(Increase of 10% to compensate for potential air leaks)

New cfm x 1.15% = ________

(Increase of another 15% to provide margin of safety and longer service life for pump operation)

Maximum psi needed = __________

In summary: Tool total cfm + 10% + 15% = compressor cfm needed. If planning to run a rotary style compressor, you won’t need to add the additional 15%.

Horsepower and motor features

Generally speaking, most true 5 hp compressors can likely provide up to about 18 cfm, ideal for small shop use. A 7.5 hp compressor should be able to provide around 27 cfm at 100 psi.

Cutaway of a typical rotary screw compressor housing. Clearances are extremely tight.
<p>Cutaway of a typical rotary screw compressor housing. Clearances are extremely tight.</p>

While this range of compressor can usually operate on single-phase electric circuits, horsepower ratings above this range may require three-phase wiring in your shop building.

If considering a piston type compressor, look for quality features:

• Cast iron crankcase and cylinders.

• Aluminum connecting rods.

• Tapered roller bearings.

• Stainless steel reed valves.

• Fan-type flywheel (for better cooling and less moisture buildup).

• Motor oil sightglass to monitor oil level.

• Pressurized oiling system.

• Low-oil cut-off switch.

Single-stage or two-stage (reciprocating style)

If you plan to install a piston (reciprocating) style compressor, for shop use, consider a two-stage unit. A two-stage compressor begins to compress air in a low-pressure cylinder. That air is then passed through a cooling coil on its way to a second, high pressure cylinder. In other words, the air is compressed in two progressive stages. As compared to a single stage compressor, higher pressures and motor longevity are the results. Two-stage compressors are normally used for pressure ranges of 125 psi and greater. A two-stage compressor will feature an intercooler system (usually a coil) to reduce moisture buildup between the two cylinders.

Types of air compressors

In total, there are five types of compressors on the market. While this group includes the reciprocating type (piston motor), rotary screw type, rotary sliding vane type, rotary lobe, and the centrifugal type, only three are applicable to automotive repair shop applications: piston type, rotary screw and rotary vane.

• Reciprocating type compressors are the most common and least expensive. These feature an electric-driven motor that features a crankshaft, connecting rods, pistons and valves to pump and compress air (similar to a piston engine). These are available in a wide range of horsepower levels, from fractions of hp to around 25 hp.

• Rotary screw compressors feature a pair of “screws” (like a twin-screw supercharger). As the screws turn, the air is squeezed between the helical “teeth” and is compressed.

An example,of a relatively compact rotary screw compressor for automotive shop applications. The Kaeser Airtower is available in models ranging from 4 to 7.5 HP, 9 to 77 scfm and operating pressures of 125 – 217 psig.
<p>An example,of a relatively compact rotary screw compressor for automotive shop applications. The Kaeser Airtower is available in models ranging from 4 to 7.5 HP, 9 to 77 scfm and operating pressures of 125 – 217 psig.</p>

• Rotary sliding vane compressors feature an eccentrically slotted rotor, stator and a series of blades. As the rotor turns, the vanes are forced outward from inside the rotor slots, and exhaust air is compressed. Reciprocating, rotary screw and rotary sliding vane compressors are positive-displacement compressors (where the air is mechanically squeezed).

• Rotary lobe compressors feature twin shafts that are each equipped with cylindrical lobes (rotating and squeezing air in a similar manner as rotary screw). These are intended for large industrial applications and absolutely not appropriate for automotive repair shop applications. Both rotary lobe and centrifugal compressors are designed for heavy industrial use only.

Why pick one style over another? Let’s face it: One of the primary factors in any equipment purchase involves initial cost. Reciprocating type compressors have been around seemingly forever and are the most economical to purchase (of course, price is always dependent on quality).

Following are a few “tips” when considering compressor type.

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