kdk:
A good many economic studies (such as suggested by Sailoday) have been made over the years, and the 2-3 meters per second has been determined to be the best compromise between pipe capital cost and pump operating costs. Of course, there may be special circumstances in specific cases that might warrant yet another economic study.
Another factor to be aware of is that higher velocities increase pipe erosion problems.
All in all, the 2-3 meters per second rule of thumb has served many of us well for many years.
Milton Beychok
(Visit me at www.air-dispersion.com)
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