FEATURE COMPRESSORS & COMPRESSED AIR CENTRAL CONTROL HELPS SPREAD THE LOAD


Ken Revell, business line manager, Compressor Technique Service division, Atlas Copco Compressors UK, says optimised central control brings many benefits to multiple compressor installations


C


entral controllers are the backbone of an optimised compressor installation,


managing the whole compressor room, matching the air supply to demand while offering pressure stability, and contributing to energy efficiency. It is a proven fact that a 1bar pressure band reduction can achieve as much as 7% in direct energy cost savings. To take full advantage of optimisation


benefits, it is necessary to look beyond individual machines and consider the compressor installation as a whole. It is essential to be prepared by establishing demand patterns, detecting and rectifying weak points such as air leaks in the supply system, and defining potential areas for improvement. Few air systems operate at full load all


the time. Part-load performance is therefore critical and is primarily influenced by the compressor type, the air demand profile, and the control strategy. A simple control system may be


appropriate to a facility with a single compressor and a very steady air demand. However, a more complex system with multiple compressors and dryers, varying demand, and many types of end users, will require a more sophisticated strategy of compressor control since this function is the important factor affecting system performance and efficiency. Put simply, with a


compressed air system comprising more than one compressor or dryer it may be difficult to determine which unit to run and when. This dilemma is especially true if demand fluctuates daily. A multiple system that isn’t properly controlled risks the production of too much air, and impacts on production costs. Even a compressor running unloaded uses up to 20% of the energy consumed at full load. For example, in a system that relies on


start sequence control the load/unload pressure of each compressor can be set to react to changes in air demand and, if the system pressure drops, an additional compressor will switch to loaded running. However, the sequence will always be the same, and any economic advantages offered by VSD-powered compressors in


10 FEBRUARY 2020 | PROCESS & CONTROL


pressure drops rapidly, it will select the largest capacity compressor(s) in the system to compensate. If pressure drops slowly, a smaller rated unit will be triggered to balance the required pressure level. A multiple compressor installation benefits from optimised central control thanks to: • Flexible compressor sequencing –


Controllers ensure that compressors work in groups and adjust to meet demand • Workload equalisation – By spreading


the workload over multiple compressors, a central controller avoids unnecessary overloading of individual machines • Continuous use of VSD machines – A


the installation will not be fully utilised. It is likely that there will be relatively large steps between different compressors’ loading and off-loading which means the system operates across a broad pressure band. This results in higher pressures than required, too much unloaded running and a punitive energy bill. When it comes to compressor co-


ordination, conventional network controls use the cascade set point method to operate the system as a whole. Although this procedure avoids selecting part-load compressors, it still presents the


problem of approaching a minimum pressure requirement as more and more compressors are added and the range of compressor load and unload set points increases. More sophisticated


network control systems use single set- point logic to make their operational decisions to start/stop, but the ultimate solution is a centralised controller that can supervise and orchestrate a complete compressed air installation irrespective of compressor type, equipment brand, and plant room location. Such an intelligent system will be capable of linking all compressors and dryers, lowering overall pressure band, optimising performance, and boosting energy efficiency. With a sequence controller, if pressure


drops it will select the next compressor in line regardless of capacity. An optimised unit will select the best machine combination to deliver the required air output in the most efficient way. If the


To take full advantage of optimisation benefits, it is necessary to look beyond individual machines and consider the compressor installation as a whole


central controller ensures that in a mixed unit installation VSD compressors are prioritised over baseload or fixed-speed compressors. • Regulation of system pressure – With a


central controller, machines can operate within a pre-defined and narrow pressure band. This allows the system to meet demand while still optimising energy efficiency. • Shutdown scheduling – Central


controllers can help operators schedule maintenance periods for non-working hours to avoid incurring extra costs • Fewer service visits – This is because


premature wear is reduced. • Production security − Without the


Advanced central control optimisers are designed from the outset to meet the smart factory’s future Industry 4.0 connectivity requirements


system pressure control offered by a central control unit, there is the risk that pressure anomaly alerts may be overlooked and application process output affected as a consequence. In recent times there have been


advances in capabilities of centralised controllers. For example, Atlas Copco’s Optimizer 4, the successor to its ES units, offer opportunities to select the most efficient combination of compressor units at defined working pressures while balancing unit running hours for all types of rotary stationary compressors − VSD, fixed-speed, oil-lubricated, oil-free, turbo or low-pressure. Connectivity is not confined to same brand units or to a single location within a production site, enabling remote control via Wi-Fi, Ethernet, communication bridges and repeaters. In support of the users’ ISO 50001


energy management system continuous improvement system targets, the latest optimiser controllers enable LAN and SCADA system connection to any device for data control and extraction, together with local visualisation and analysis of energy consumption and efficiency.


Atlas Copco www.atlascopco.com


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