An automotive parts manufacturer wants to better control the air compressors. They installed a system of permanent monitoring and discovered the following (a quote from the Maintenance Engineer) “You should see what the datalogging revealed … 73% of the time, the system was operating over 30kW/100cfm. For only 5% of the time below 25 kW. In that 73% time, the average was realistically at least 40 kW/100 cfm. Ouch!!!”
This company should be commended for actually measuring their system, very few do this. Once this efficiency problem is discovered, a strategy needs to be created to solve the poor efficiency problem. This system has three fixed speed compressors (1 x 150 hp and 2 x 100 hp) that are locally controlled, with poorly cascaded pressure switches combined with inlet modulation control (the least efficient way to control compressors).
The company would like to add a variable speed drive compressor. However, to avoid control gap, they need a 200-hp compressor (sizing rule states the VSD variable range must be equal to or larger than the capacity of the biggest fixed speed compressor). If they install this large VSD compressor, they have discovered it will run at minimum speed during light loads (evenings and weekends). Operation of VSD compressors at below minimum speed should be avoided.
They looked to a compressor sequencing control to solve their problem. Fortunately, many controllers are now available that have a type of internal control that will detect low system flow and swap in a smaller compressor to match the load should there be one available. Rather than using cascaded controls where the compressors operate in a set sequence order, implementing smart compressor control will provide much better flexibility and maximum savings.