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Wednesday, March 16, 2011

Compressed Air System Control Strategies

Improving and maintaining compressed air system performance requires not only addressing individual components, but also analyzing both the supply and demand sides of the system and how they interact, especially during periods of peak demand. This practice is often referred to as taking a systems approach because the focus is shifted away from components to total system performance.

Matching Supply with Demand
With compressed air systems, system dynamics (changes in demand over time) are especially important. Using controls, storage, and demand management to effectively design a system that meets peak requirements but also operates efficiently at part-load is key to a high performance compressed air system. In many systems, compressor controls are not coordinated to meet the demand requirements, which can result in compressors operating in conflict with each other, short-cycling, or blowing off—all signs of inefficient system operation.

Individual Compressor Controls
Over the years, compressor manufacturers have developed a number of different types of control strategies. Controls such as start/stop and load/unload respond to reductions in air demand by turning the compressor off or unloading it so that it does not deliver air for periods of time. Modulating inlet and multi-step controls allow the compressor to operate at part-load and deliver a reduced amount of air during periods of reduced demand. Variable speed controls reduce the speed of the compressor in low demand periods. Compressors running at part-load are generally less efficient than when they are run at full-load.

Multiple Compressor Controls
Systems with multiple compressors should use more sophisticated controls to orchestrate compressor operation and air delivery to the system. Network controls use the on-board compressor controls’ microprocessors linked together to form a chain of communication that makes decisions to stop/start, load/unload, modulate, and vary displacement and speed. Usually, one compressor assumes the lead role with the others being subordinate to the commands from this compressor. System master controls coordinate all of the functions necessary to optimize compressed air as a utility. System master controls have many functional capabilities, including the ability to monitor and control all components in the system, as well as trending data, to enhance maintenance functions and minimize costs of operation. Most multiple compressor controls operate the appropriate number of compressors at full-load and have one compressor trimming (running at part-load) to match supply with demand.

Pressure/Flow Controllers
Pressure/Flow Controllers (P/FC) are system pressure controls that can be used in conjunction with the individual and multiple compressor controls described above. A P/FC does not directly control a compressor and is generally not part of a compressor package. A P/FC is a device that serves to separate the supply side of a compressor system from the demand side, and requires the use of storage. Controlled storage can be used to address intermittent loads, which can affect system pressure and reliability. The goal is to deliver compressed air at the lowest stable pressure to the main plant distribution system and to support transient events as much as possible with stored compressed air. In general, a highly variable demand load will require a more sophisticated control strategy to maintain stable system pressure than a consistent, steady demand load.


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