The demand pattern in compressed air systems are often dynamic and therefore few systems operate at full-load all the time. Part-load performance of a compressed air system is a critical factor affecting the efficiency of the system, and is primarily influenced by compressor type and system controls. When the pressure in the system is higher than what is required, more energy – approximately 7% for every extra bar (DOE, 2003) – and more compressed air is consumed – mainly because air flow in unregulated use point and at the leaks will increase. Typically, 150 kPa (1.5 bar) more pressure will force the unregulated users in the system to consume 20% more air. (Carbon Trust, 2012).
Consequently, maintaining the pressure profile of the system at the optimal levels is essential for efficient operation of the system. This is achieved with the deployment of compressed air system controls. The type of control system that is best suited for a plant greatly depends on the type compressor used and the demand profile in the plant. If a system has a single compressor with a very steady demand, a simple control system may be appropriate. On the other hand, a complex system with multiple compressors, varying demand, and many types of compressors will demand a more complex control system (US DEO, 2003. p.20) Compressed air system controls can be the most important single factor affecting system performance and efficiency. Modern microprocessor based systems provide closer and control and quicker response allow compressed air systems to operate at lower average temperatures without going beyond minimum system requirements. Their impact is depicted in figure below (US DOE, 1998).
Compressed air system controls often include three elements:
Individual control strategies include approaches such as start/stop and load/unload, modulating (throttling), dual-control, variable displacement, and variable speed drives. These have their advantages and disadvantages and thus the selection will depend on the plant-specific factors.
More sophisticated control strategies are used for systems with multiple compressors. The advanced versions of such controls monitor the situation in different parts of the entire system and adjust the number and output of compressors connected to the system.
Pressure/flow controllers are used in conjunction with the previous control strategies and often separate the supply side from demand side. Storage, which is sized to meet anticipated fluctuations in demand, is an essential control strategy. High pressure air enters to the storage from the compressor end and is available to reliably meet fluctuations in demand at a constant lower pressure level.
More information about these control strategies is available in the resources listed below.