Control Systems: Comprehensive Control

This selector equally divides the operating times and starts among the connected compressors. The start sequence can be switched on manually or automatically, following a time schedule. This basic selector uses an on/off pressure transducer, with one transducer per compressor, as a simple and practical solution. The disadvantage is that there are relatively large steps between the different compressor's loading and off-loading levels. This results in relatively broad pressure bands (the span between maximum and minimum levels) for the installation. This type of selector should therefore not be used to control more than 2–3 compressors. A more advanced type of start sequence selector has the same type of sequence control, but with only one, centrally-positioned, analogue pressure transducer. This manages to keep the installation's total pressure band within a few tenths of a bar and, as a result, can control 2–7 machines. A start sequence selector of this type, which selects the machines in fixed sequences, does not take the capacity of the compressors into account. The connected compressors should thus all be approximately the same size.