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Improving the efficiency of compressed air systems

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hes dec 18 15Improving the efficiency of compressed air systems

Compressed air production is troubled by inefficiencies; however, by taking closer control of the process and by monitoring the effectiveness, it is possible to improve compressor efficiency and make substantial savings, as BOGE’s Carl Sharpe, Sales Manager UK & Ireland reports.

Compressed air system controls are designed to match supply with demand and while there are several commonly used methods of achieving this, they may be far from efficient. (Read More) 

At its most basic, a start/stop arrangement may be used to turn the compressor motor on or off in response to the discharge pressure. Marginally more sophisticated is constant speed control (load/unload), which allows the motor to run continuously, but unloads the compressor when the discharge pressure is adequate. Another method includes modulating controls such as throttling the inlet control in order to vary the compressor output to meet flow requirements.

Clearly, the efficiency of compressed air systems is largely determined by the effectiveness of their controls, and manufacturers have sought to improve control methodologies by adopting better motor control techniques, more specifically, by using variable speed drives (VSDs). VSD technology delivers both smoother motor starting (ensuring longer motor life and reduced stress on a facility’s power delivery system) and a more stable air supply pressure by matching the motor speed to instantaneous demand. Energy savings of as much as 50% are possible when the compressor motor is controlled by a VSD.

A word of warning here: a VSD installation cannot compensate for a compressor that is improperly sized for its loads. If the loads are high relative to the output of the compressor, for example, being required to operate at more than 80% of its capacity – then the savings achieved through variable speed operation will be minimal or nonexistent.

Equally, VSD installations will perform poorly if demand is far below the capacity of the compressor for much of the time. Operating a compressor at very low speeds creates a number of potential problems, including motor overheating and excessive internal losses. As a result, compressor manufacturers set a minimum turn-down speed for their variable speed machines, typically 30% of their maximum output. If demand falls below this threshold, the machine should be configured to revert to stop-start operation.

Effective control is one thing; measuring the effectiveness of that control is quite another. There are several ways to measure a compressor’s performance, the most obvious approach being to use a data monitoring device that tracks both the compressor and air delivery system performance. These monitoring systems can also be deployed to flag servicing requirements based on actual usage, providing early warning of potential compressor or air distribution system problems, as well as a 24/7 analysis of the monitored unit’s energy efficiency.

As well as fault and maintenance indication, data monitoring can include remote data polling and server-based, long term data retention for trending, while embedded web browsers can be used to display operating parameters at any location with internet access. BOGE’s ‘airstatus’ system is a good example, retaining 24 month’s worth of data for each installation. For service engineers on the move, this system allows monitoring via a smartphone app that keeps an eye on the status of the system and displays alarm messages via email or SMS. Regular system audits can also be undertaken in accordance with the international standard ISO 11011:2013, which effectively divides a compressed air system into three subsystems: supply, transmission and demand.

Measurement is thus a primary function of good compressed air system management, which, when combined with effective and appropriate compressor control techniques, will ultimately provide a boost, not just to energy efficiency, but to overall reliability as well, resulting in significant operational cost savings.

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