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Inlet Pipe Calculations:

To determine the diameter of the inlet pipe we used the constraint that the maximum shear due to minor losses had to be less than G cell in the last section of the flocculator (G cell = 24/s). The predominant minor losses from the inlet pipe will occur at the exit point and the elbow. The minor loss coefficient for the elbow is 0.9 and the exit is 1.0 so K was set to be 1.0 for this design. The equation to calculate shear due to minor losses in a pipe is shown below. This equation is solved for D, the diameter of the pipe that would provide shear equal to G cell. The equation used is very similar to the equation used to find the baffle spacing for the flocculator.

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Inlet Channels From the Floc Tank to the sedimentation tank.
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Inlet Channel From the Floc Tank to the sedimentation tank with lamella (aerial view).

Variables:
K = 1.0 (minor loss for an exit)

  • This equation was derived from a series of substitutions that can be seen above.
  • The head loss term that is found in the above derivation is assumed to include only minor losses. The minor head loss for this pipe is assumed to be the dominating factor because the pipe is designed to be relatively short in length and the bends and exit are our major sources of shear, the primary locations where floc break up would occur.
  • The velocity equals Q/A for the pipe.
  • The residence time term is found to be the volume over the flow rate through that volume.
  • The volume used was assumed to be the cross sectional area of the pipe times 2 diameters of the pipe.

Note: Initially during preliminary design with a large tank, we were using 55 L/min as the plant flow rate, and the required pipe diameter was 5.11 to achieve the desired G cell value. Due to the cost of 15.24cm (6") bulkhead fittings (nearly $300), we had to find an alternative design. We decided to lower the flow until a pipe with inlet diameter equal to 10.16cm (4") was achieved. We found that the maximum flow for these conditions is 24 L/min, so we changed the flow rate of the sedimentation tank. The option of multiple inlets was considered but this was discarded because then 2 - 4 bulk head fittings would be needed. Thus, the overall flow of the tank was lowered and a smaller tank was chosen.

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