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.
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.
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.
• 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.
Reciprocating type compressors
• Fairly simple and time-tested design
• Relatively lower purchase price
• Easy installation (but should be secured to the floor due to vibration)
• Two-stage versions are available for higher efficiency
• Wide range of horsepower levels below 25 hp
• With regularly scheduled air filter, oil and drive belt changes, easy and inex-
pensive to maintain
• Reasonably forgiving in dirty shop environments
• Greater number of purchasing locations (more readily available at both pro and
mass-merchant locations; with a wide variety of quality)
• Valves and rings wear and must be replaced routinely
• Limited duty cycle (60%-70%)
• Not designed for continuous operation
• Not as energy-efficient as rotary screw type
• Operates at higher temperature (as compared to rotary screw type)
• More difficult to remove moisture and oil from output air
• Lower air quality
• Easy installation (no need to anchor to floor)
• Much quieter operation (greatly reduces shop noise)
• Designed for continuous operation (unlimited duty cycle: can run at 100%
indefinitely without overheating)
• High air quality
• Lower energy cost (more efficient than reciprocating type)
• All tend to be very high quality
• Higher purchase price as compared to piston type
• Higher maintenance cost (when required)
• Easy to install
• Less noisy than piston type
• Low rotational speeds
• Few moving parts
• More durable in dirty environments
• Oil injected designs have oil carryover
• Generally limited to 125 psig.
• Vanes must be replaced as routine maintenance
Not for repair shop applications. Very expensive and designed for heavy industrial applications such as pneumatic conveying systems, waste water treatment, etc. Volume ratings are up to the 5,650 cfm range.
These function somewhat similar to rotary screws, but instead feature a pair of rotary tri-lobes (each shaft features three cylindrical lobes that mesh together at a tight clearance).
Any compressor needs a control system in order to regulate operation in accordance with the air demand. A “constant-speed control” is required if the compressor will be operated on a continuous basis, where the air demands are steady and do not fluctuate.
Whenever the air requirement is 75% or more of the compressor’s capability, or if the motor start-up will occur more than six to eight times per hour, a constant-speed control is the best choice. If air requirement is less than 75% of the compressor’s capability, a “start-stop” control is the logical choice. If the compressor is rated at 33 cfm, but demand will be less than 24.75 cfm, a start-stop control makes sense.
A “dual control” offers the best of both worlds, providing for either constant or start-stop operation. A manual switch allows the operator to select either format at any given time.
Receiver tank and moisture drain
All air compressors, regardless of style, require receiver tanks. The tank collects moisture created by the compressor heat.
The size of the receiver tank has a direct bearing on compressor motor life. The larger the tank, the more reserve pressurized air is available for output. The smaller the tank, the more the motor has to run to keep up with demand. For most shops, a 50– to 80-gallon tank size is fine.
Some tanks feature no drains, or have poorly designed drains, which are often less than convenient to access. As moisture accumulates in the tank, the available air volume is reduced. This causes the compressor to run more often than needed, wasting energy and increasing compressor component wear.
Automatic drains are available (offered in a variety of styles) that sense moisture levels and drain as needed. Since many people simply forget to manually drain their tanks on a regular basis, an automatic drain is an excellent addition (these can range from about $100 to $500). Certain rotary screw compressor systems feature automatic drains as either standard or optional equipment.
Cost of rotary screw vs. piston type: Myth buster
If you’re like many shop owners, your initial reaction may be, “Gee, I just can’t afford a rotary screw compressor.” Chances are, you’re fooling yourself. Yes, a rotary screw compressor carries a higher initial purchase price tag as compared to a piston type. However, one of the common mistakes that many shops make is to not consider the long-term cost. As but one example, many shops quickly tire of the noise that a piston type compressor generates. In order to cut down on noise, a shop may have a special room built to “hide” the compressor.
When you consider the cost of having a local contractor build a small room addition inside the shop to the tune of, let’s say, $3,000 to $5,000 or more, the total money spent (the piston compressor, filters, separators and the enclosed room) could have easily purchased a rotary screw type compressor that’s plug ‘n’ play ready. Yes, you could perform the construction yourself, but in reality, who has the time? From the outset, you could have a compressor that’s quiet and more efficient for the same or even less money.
Also, consider the wear and tear aspect. A piston compressor eventually requires rebuilding due to long-term wear of piston rings, cylinder walls, pistons and valves. As parts wear, more oil loss (pass-through) occurs, making the compressor less efficient and more prone to contaminate output air. This translates into down-time and more repair expense.
A rotary screw compressor’s screws don’t contact each other (no blade wear) and are able to deliver compressed air on a continuous basis when required (capable of 100% duty cycle with no drop in pressure, as compared to the approximately 60% to 70% duty cycle of a typical piston type compressor).
According to industry experts, a rotary screw compressor will last about 10 times longer as compared to a piston type, before requiring major service.
There’s nothing wrong with a piston type compressor. Either style (piston or screw) will get the job done. If you can talk yourself into the initial higher purchase price of a screw type, you’ll enjoy the lower noise level and the long-term operating benefits that this type offers.
Rotary compressors offer distinct advantages in noise, heat and air quality, but if these factors don’t directly impact productivity or workplace comfort/safety, they shouldn’t drive your buying decision.
If you don’t need much air, or don’t need it often, a piston compressor will be more cost effective. The more air you need, the stronger the argument for a rotary screw or rotary vane compressor. ●
Comparison of applicable compressor types
Piston type Rotary screw Rotary vane
HP range up to 25 hp 3 - 650 hp 2 - 200 hp
Pressure (psi) up to 175 up to 215 up to 145
Volume (cfm) up to 100 9 - 3,000 8 - 1,100
How a typical rotary screw air compressor works
Atmospheric air is drawn through a pre-filter into the dry type air intake filter (1) where it is cleaned prior to compression. The atmospheric air is then compressed with the efficient screw airend (2). The airend is driven by an electric motor (3).
In this example, oil is injected (4) into the rotor housing to lubricate, seal and remove the heat generated by compression. As a result of this internal cooling mechanism, the temperature during compression reaches only 180 degrees Fahrenheit under normal operating conditions. Oil is separated from the compressed air stream by a three-stage oil/air separator (5). The separated oil is cooled in an oil cooler (6), flows through a micro oil filter (7) and returns to the injection point. This oil circulates solely by internal pressure, eliminating the need for an oil pump. The oil temperature is controlled over the entire range from cold start to full load or idle operation by an oil thermostatic valve (8).
The compressed air is separated from the injected oil in an oil separator cartridge (9), and is passed to an aftercooler (10) via the minimum pressure check valve (11). This valve maintains minimum system pressure to ensure continuous oil injection into the airend. The temperature of the compressed air is cooled to within 10 to 15 degrees F of ambient temperature by the air-cooled aftercooler, which removes a large percentage of moisture from the compressed air.