When multiple compressors in a common grid are controlled by their individual pressure switches, all of them can not be set to the same load / unload values, as the compressors would load / unload together. In order to properly control multiple compressors, the individual pressure control bands will have to be cascaded. Hence, each one is set at different load / unload bands. In such an arrangement, the compressor set to unload at the highest pressure works as Base load, whereas the compressor set to unload at the lowest pressure works as Trim load.
This causes a very large as well as varying pressure band depending on the combination of compressors used to meet the air demand.
Cascade control schemes increase power consumption in a system due to the elevated pressure. In the example above, the last compressor to start is set at the plant’s minimum allowable pressure of 85 psig and the first compressor to turn on and consequently the last one to turn off with reducing demand is set to load at 100 psig and to unload at 110 psig. In low demand situations, the system can be running at 20 to 25 psig above the minimum required pressure. This would generate about 15 percent more energy consumption at the compressor than is required for the demand. Additionally, unregulated demand in the plant would now consume more cubic feet per minute (cfm) at the elevated pressure level, increasing waste. Elevated pressure wastes energy and creates artificial demand in the system.