February 2020

Central 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 significantly to energy efficiency. Ken Revell, business line manager, compressor technique service division, Atlas Copco Compressors UK, says it is a proven fact that a 1bar pressure band reduction can achieve as much as 7% in direct energy cost savings. T o 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, which in turn has a negative impact on production costs. Even a compressor running unloaded uses up to 20% of the energy consumed at full load. Sequence control 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 the installation will not be fully utilised. It is likely that there will be relatively large steps between different 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 pressure drops rapidly, it will select the largest capacity compressor(s) in the system to compensate. If pressure drops slowly, a smaller Process, Controls & Plant Focus on: Compressed Air compressors’ loading and off-loading which means the system operates across a relatively broad pressure band. The net result will be 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. Optimised performance More sophisticated network control systems use single set-point logic to make their Optimising a compressed air system 20 | Plant & Works Engineering www.pwemag.co.uk February 2020