System Flowrate
Tech Talk 44

When people say that they exchange 100% of the water in a tank every hour, they typically are not doing that at all. If a tank holds 120 gallons, they are probably pumping 2 gpm into the tank. Their math is correct, but their English is not. To "exchange" all of the water suggests that none of the original water is still there after one hour. That would only be true if all the water was drained (dry) then refilled; otherwise, it is constantly being mixed and only about 60% is being exchanged with each equal volume of water.