Choose the Right Air Compressor

Consider centrifugal as well as screw units.

By Amin Almasi, rotating equipment consultant

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For general-purpose air services, a standard turbocompressor (with several successful operating references) from a reputable manufacturer usually is a good choice. Leading-edge designs using direct drive (i.e., no gear box), which have only one moving part spinning at high speeds, may be the best option for many applications.

Relatively large applications (say, above 400 kW) call for use of oil-free magnetic bearings. For medium-size turbocompressors, simple and robust water-lubricated bearings or air-foil bearings suffice. Small-size units can get by with permanently lubricated and sealed bearings in special applications.

Turbocompressors lend themselves to modularization, which can reduce the number of components and consequently cut both capital and operating expenses. A modular design may require change of some turbocompressor parameters (usually minor and module-specific adjustments) for each application. Figure 4 shows modern air-compressor-package modules in a plant.

A fully integrated design with a turbocompressor using a direct-drive high-speed driver, a vertical air tank and an integral refrigerated dryer (probably with filters) can be supplied in a compact one-piece package.

Today's high-speed, variable-speed direct-drive turbocompressors can provide energy savings of up to 30% compared to conventional machines (regular constant-speed turbocompressors or ordinary screw compressors).

Air services by their nature require a capacity control system that can cope with highly variable demand. This is another aspect where turbocompressors have an edge over screw compressors. Variable-speed drives (VSDs) usually are better matched to turbocompressors. For medium-range applications, where a VSD is too expensive to use, a system with variable inlet guide vanes can provide turbocompressor capacity control. Turbocompressors can handle 20–100% turndown, with a higher efficiency and more reliable capacity control system than oil-free screw compressors in air systems.

An inherent characteristic of an air turbocompressor is that as system pressure decreases, capacity increases. This can be properly matched with the requirement of the air consumption patterns in many applications.

The turbocompressor air package produces the right volume of air required by an application. Using a highly responsive turbocompressor with the correct head/flow curve, a smaller air reservoir (usually two-to-three times smaller than that for screw compressor units) can be used at a relatively lower pressure, which could result in much less leakage in the air system.

Always consider local conditions, particularly the site level and ambient temperature, in air compressor design/selection. The dynamic nature of the turbocompressor can result in the head generated dropping as elevation rises because of lower air density. The turbocompressor mass flow and capacity at a given discharge pressure increase as the ambient temperature decreases. So, carefully choose rated point conditions (i.e., the worse-case scenario) when specifying air compressor requirements. Only a correct rated point can result in a sufficient and optimum package. Usually, a good selection for the worst case is a hot humid day and a realistic maximum temperature.

Successful air-system performance depends upon selection of the right dryer, which can be either refrigerated or desiccant.

Refrigerated dryers commonly are used for dry air services. They employ a refrigeration system to lower the compressed air temperature to well below the ambient temperature. This condenses the moisture vapor into liquid that can be drained out of the system, and also decreases the dew point of the compressed air. As long as the compressed air doesn't cool below this new dew point, any remaining moisture will remain in the vapor phase. The dew point (at line pressure) should be at least 10°C below the minimum recorded temperature at the plant site or the consumption points (equipment, actuator, etc.). High-temperature refrigerated dryers precool the air before it enters the dryer.

Desiccant dryers operate by directing the compressed air across a bed of material that adsorbs moisture vapor. These dryers can produce dew points lower than those from refrigerated dryers and so are preferred when required air quality is extremely high.

Air filters come in a variety of types:

• Moisture separator. It mechanically separates water, oil, etc. from the air.
• Particle filter. Designed to capture dirt and dust, it may remove some water and oil mist.

• Coalescing filter. A fine filter for removing oil aerosols/mists and fine particles, it usually is placed after a dryer.
• Vapor adsorber. Such a unit, e.g., an activated-carbon filter, eliminates vapors (e.g., oil or water vapor). It should be installed after all other filters and dryers.

Use of multiple types of filters in the system can enhance effectiveness.

For small- and medium-size applications, many packages now combine a rotary screw air compressor, storage tank and dryer in one durable and compact unit. Turbocompressors can't compete with oil-free screw compressors in small sizes. However, for medium- and large-size packages, compact high-speed turbocompressors using oil-free bearings can provide more and cleaner air with higher reliability and far less noise and vibration.

AMIN ALMASI is a rotating equipment consultant based in Brisbane, Australia. E-mail him at

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