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.
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.
DRYERS AND FILTERS
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.
MAKE THE RIGHT CHOICE
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 email@example.com