Choose the Right Air Compressor

Consider centrifugal as well as screw units.

By Amin Almasi, rotating equipment consultant

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Compressed air is used extensively in process plants. It's a clean, safe and efficient utility that can be employed where other energy types (such as electricity) might pose a potential hazard. Compressed air is the motive force for many instruments, actuators, control valves and equipment. In addition, some processes require compressed air.

Most air compressors are electrically driven but engine-driven units (Figure 1) also are used.

The "air" is naturally contaminated with solid particles such as dust, soot, salt, etc. It also contains water vapor, which if not removed can cause significant corrosive damage under the compression. Many applications require completely dry and oil-free air, to avoid risk of contaminating sensitive pneumatic equipment.

For a system to operate efficiently, the supply of air from the compressor has to match demand, which will rise and fall over different and complex patterns. In conventional air systems using screw compressors, the compressed air being generated but not used is the single largest cause of energy wastage. Sometimes, because of an inefficient air compression system, poor air-system energy management, improper capacity control and other reasons, more than 60% of the energy used in air compression is wasted. Generally, the issue of the energy efficiency is overlooked in air compressor packages.

Air compressor selection depends upon factors such as capacity, discharge pressure, required air quality and ambient conditions. Air compressor packages usually are tailor-made to customer specifications and local regulations.

Single- or two-stage oil-free screw compressors or single-stage oil-flooded screw compressors commonly are used for air services. Figure 2 shows two examples of screw compressor packages as well as the air receiver (the vertical vessel).

Few manufacturers produce large (say, above 0.8 MW) oil-free (dry-type) screw compressors. For some large sizes, only two or three oil-free screw compressor manufacturers can provide successful references. Other limiting factors in oil-free (dry-type) screw compressors are the discharge temperature and the differential pressure.

In view of those limitations, plants often opt instead for oil-flooded screw compressors for medium and large applications (even for dry air ones such as instrument air or nitrogen packages). However, such compressors frequently suffer relatively high rate of wear and need complex sealing systems (whose seals are prone to abrasion and operational problems). Oil selection also presents a great challenge; highly sophisticated and expensive synthetic oil usually is necessary for successful operation.

In oil-flooded screw compressors, air is mixed with a large quantity of oil; the oil then is extracted at discharge using very sophisticated multistage separation/filtration methods. Even the best oil-flooded screw compressor packages with the most effective and reliable oil separation systems still pass a certain level of oil. This amount of oil in a volume of compressed air may seem insignificant but for a compressor generating large air flow it can add up very quickly. The oil can build up in actuators (and other systems) and lead to sluggish and interrupted response. On the other hand, malfunction of a component in the complex oil-separation system can cause contamination of the air system with a huge amount of oil, which could result in extensive damage and potential for highly dangerous catastrophic failures (explosions, serious injuries to personnel, etc.).

Vendors of oil-flooded screw compressors often offer "oil carryover" guarantees. However, they usually don't cover "oil carryover at any upset condition" and "oil carryover over the service life."

Oil-free turbocompressors generally are the best options for dry air services, e.g., for instrument air, dry process air, etc. They have fewer wear parts than screw compressors and many use advanced oil-free bearings, so their reliability is better. Turbocompressors generally are lighter and smaller than comparable screw compressors. The most common turbocompressor designs have two or three centrifugal stages for pressure in the 7–12 Barg range. Centrifugal turbocompressors with eight-to-ten stages (usually in a form of integrally geared machines) can reach pressures up to 100 Barg (or even more).

Units meeting the standards of the American Petroleum Institute (API), such as API 672 for integrally geared centrifugal turbocompressors, have been used for decades in critical air services. Complying with the standards' engineering, design, manufacturing, inspection and test requirements probably raises the cost of these highly engineered turbocompressors to two-to-four times that of conventional packages. However, they usually can provide three-to-five years' operation without any shutdown. Figure 3 shows an integrally geared API centrifugal compressor in a compact and well-arranged package.

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