Why does discharge flow rate not take into account the fluid recycled to the compressor through the bypass system?

Answer 1

We need a compressor to supply volume at pressure. When we are not using all the volume the compressor is supplying, the overpressure valve, the bypass valve, opens and fluid is recycled to the compressor. This is common in things like the pressure washers that are so often seen on car lots. Usually, we are only interested in what a compressor can or is delivering to do work. What volume at what pressure? We need to know the maximum capability of the compressor at its rated pressure, and that will be delivered with a zero bypass. When we are using only a portion of the capacity of the compressor, we just need to know the flow rate for the application (and the pressure it is being delivered at). We don't care about how much fluid is returned to the compressor by the bypass system. We need the rated pressure of the compressor, and the unit will go all out to maintain that. And when it is running at rated pressure and the flow is low, most of the fluid will be returned through the bypass. That part of the compressor's operation could be looked at as "wasted energy" by someone, but if the application has times when the compressor is running flat out with no bypassed fluid, the "waste" part of the cycle has been factored into costs, but cannot be "changed" because, though we might be using low flows at times, we need all the compressor can produce at others.