The lower the pressure level of the compressed air generated, the more cost effective the production. However, it is necessary to ensure that all active consumers are supplied with sufficient compressed air at all times. Improved control systems make it possible to reduce peak pressure. In principle, there are several ways to ‘narrow’ the pressure ranges, thus reducing the pressure of the compressed air generated. These possibilities are listed below and illustrated in Figure 3.35:
●direct readjustment via mechanical switches o n t he compressors. The cheapest way t o adjust the pressure range of a compressor is to use mechanical pressure switches. Since the setting sometimes changes by itself, these control switches have to be readjusted from time to time
●intelligent control using a frequency converter compressor or optimal compressor size. The pressure range is readjusted by means of a frequency converter compressor functioning as a peak load compressor and adapting its speed drives to specific compressed air needs, or by means of a master control which switches to a compressor of the most appropriate size
●reduction of the pressure range right to the ‘limit’ (optimised intelligent control). The intelligent control system reduces the pressure range to the point which allows the compressor network to operate just above the limit of under supply. Figure 3.34 shows different efficiencies of those control systems.
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The VSD-based control of a compressor which can be used in intelligent and optimised intelligent control systems usually proves to be cost effective only in the case of a new purchase, because the subsequent installation of a frequency converter in an existing compressor is not recommended by manufacturers.
With optimised intelligent control, the pressure of the compressed air can thus be reduced from an average of 8.2 to 6.9 bar, which corresponds to energy savings of 9.1 %. Optimising the control involves only minor costs and can generate savings in the range of several hundred MWh/yr, that is, tens of thousands of euros (e.g. with an installed compressor performance of 500 k W, savings of about 400 MWh/yr can be achieved and about EUR 20000/yr can be achieved in the case of 8700 operational hours/year).
Energy Efficiency (2009) 3.7.9