Another potential rule of dumb says that every 1 psig increase in pressure changes the input power by 0.5%.
Therefore, the thinking goes, “If I reduce the pressure by 10 psig at the compressor discharge, I’ll get a 5% decrease in the input power required.” There are some limitations you might not be aware of.
This only applies to positive-displacement units, not dynamic units (Table 1). The net effect on a centrifugal discharge pressure change (not including any unloading controls) is machine-specific and you must refer to its performance curve. However, on many commercial units at higher pressures, the flow (scfm) drops and at lower pressure the flow increases; usually at the same or a similar input power. This effect generally covers a relatively narrow operating band.
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