Modern air compressors for pneumatic systems

Pneumatics is a section of science, engineering and technology that deals with the study and application of pressurised air (or alternative pressurised gas such as pressurised nitrogen) to produce mechanical motion, control action and actuations. In other words, a pneumatic actuator or device converts energy (in the form of compressed air) into mechanical motion. The motion can be rotary or linear, depending on the type of actuator (or device). Advantages of pneumatic systems are:

  • Simplicity of design and control.
  • Reliability, pneumatic systems generally have long operating lives and require little maintenance.
  • Operational benefits, for example, because air is compressible, actuators or equipment is less subject to shock damage. Air absorbs excessive force, whereas fluid in hydraulics more directly transfers force. Compressed gas can be stored, so machines still run for a while if electrical power is lost.
  • Safety – there is a very low chance of fire compared to hydraulic oil or electrical systems. Newer and modern machines and devices are usually overload safe.

The equipment, instruments, actuators, control systems and tools which run on compressed air are extremely critical for modern plants and units. The compressed air could be one of the most expensive utilities considering the amount of valuable energy consumed to compress the air and complex drying and filtration systems used. Sometimes, because of an inefficient air compression system, poor air-system energy management, improper capacity control and other reasons, more than 50 percent of the energy used in air compression is wasted. Generally, the issue of the energy efficiency is overlooked in air compressor packages.

Modern air compressor technologies will be discussed in this article; there is a great market available for turbo-compressors in air services. Integrally-geared centrifugal compressors and direct-driven centrifugal compressors (using oil free bearings such as magnetic bearings) for air compression applications are discussed.

Plant operators strive to improve the cleanness (particularly elimination of the oil carryover) of the air in units and factories since even a small amount of oil, liquid or dirt in many instruments or actuators can lead to serious problems. Considering this fact, it should be point out that all positive displacement compressors are involved (in some extent) mating-part rubbing, sliding, friction or liquid sealing which could result in oil, liquid or particle carryover by the compressed air. The selection of the air compressor depends on factors such as capacity, discharge pressure, required air quality, and ambient conditions.

Screw compressors for air services
Single or two-stage oil-free screw compressors or oil-flooded single stage screw compressors have been used for air services. There are limited manufacturers for large oil-free (dry-type) screw compressors. For some large sizes there are only a few oil-free screw compressor manufacturers with successful references. Other limiting factors in oil-free (dry-type) screw compressors are the discharge temperature and the differential pressure. Because of the shortage in suitable oil-free (dry-type) screw compressors, oil-flooded screw compressors are often employed for the medium and large air compression applications even sometimes for dry air applications such as instrument air or nitrogen packages with many stages of oil separation. This solution is not recommended and in such cases turbo-compressors would be a better option.

Turbo-compressors for Air Services
Oil-free turbo-compressors are generally the best options for dry air services, such as instrument air services, process dry air services, and others. For general purpose air or pneumatic services, a manufacturer-standard turbo-compressor from a reputable manufacturer (selected from a compressor model with several successful operating references) can be an optimum solution. There is a great market available for turbo-compressors for medium and large dry air services. Turbo-compressors are practically more efficient than oil-free screw compressors. They can offer a truly dry (oil-free) air compression. They have less wear parts compared to positive displacement compressors (including screw compressors). Modern turbo-compressors for air services use advanced oil-free bearings (contactless magnetic bearings, and sometimes seal-less hermitically-sealed designs). Generally, because of high wear, high frictions and high leakages, a positive displacement compressor results in less operational efficiencies than their theoretical efficiencies. There would be deterioration of the efficiency as the operation continues. However, the turbo-compressors always offer a high operational efficiency and extremely good reliability.

Air services, by their nature, require a highly variable capacity control system. The variable-speed capacity control as one of the best and efficient capacity control method is usually better match to turbo-compressors. In addition, for some medium range applications, when a VSD (variable speed drive) could not be used due to a relatively high cost, a variable inlet guide vanes (IGV) system can be used in turbo-compressors for the capacity control. A 30–100 percent turndown can usually be achieved using turbo-compressors, with a higher efficiency and more reliable capacity control system compared to positive displacement machines (such as oil-free screw compressors) in air systems.

An inherent characteristic of air turbo-compressor is that as system pressure decreases, capacity increases. This can properly be matched with the requirement of the air consumption patterns in many applications. The steepness of the head-capacity curve is dependent on the impeller and compressor design and can probably be match with the air consumption characteristics. In many air systems using positive displacement compressors, there are complaints about “no enough air”. In most cases, the problem is inadequate air flow due to an undersized compressor or the rise of actual demand compared to the designed air system capacity. Increasing the system pressure in a system using positive displacement compressors to solve this problem could not be effective since a higher pressure will increase the amount of air lost through leakage. A properly selected turbo-compressor can deal better with this issue. Turbo-compressors are generally lighter and smaller than comparable positive displacement compressors.