Tech Stuff

Air compressor technology: Match the compressor to your specific needs

Reciprocating type compressors

Pros:

• 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)

Cons:

• 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

• Noisy

• Lower air quality

Rotary screw

Pros:

• 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

Cons:

• Higher purchase price as compared to piston type

• Higher maintenance cost (when required)

Rotary vane

Pros:

• Easy to install

• Less noisy than piston type

• Low rotational speeds

• Few moving parts

• More durable in dirty environments

Cons:

• Oil injected designs have oil carryover

• Generally limited to 125 psig.

• Vanes must be replaced as routine maintenance

Rotary lobe

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).

Control system

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.

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