It should be not less than 10 mm/ meter.if u use straight pipeline there will be chances of biofilm due to stagnant water leads to microbial contamination so slope is must.
Another important thing is that velocity of water in return loop should not be less than 1.2 m/s.
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Pumps are selected to ensure minimum velocity of 1.2 m/s in the return of the loop at peak load consumption.Loop pumps are provided with VFD and interlocked with flow transmitter in return line for maintaining minimum velocity of 1.2 m/s or desired flow and velocity in the return loop.
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