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After the flow of the raw water and alum is merged, they enter into a spiral "rapid “rapid mix coil " ” to accelerate their blending. The mixture then flows through the laminar tubular flocculator. The tube flocculator is comprised of three spiral tubing units (3/8" 8” ) wrapped in a figure eight around six parallel support cylinders (two cylinders per unit, see figure 3). The tubes are arranged in a helical coil because the flocs would stay at the bottom in a straight tubing system, restricting the growth of the flocs. A calibrated in-line pH probe is inserted at the beginning of the flocculator tube and provides a readout of the mixed suspension's hydrogen ion activity. A pressure sensor is also connected to the tubing to protect the system. Velocity gradients in the tubular flocculator cause particles to collide and form flocs. For coiled tube flocculators, velocity gradient is a function of the flow rate and the inner diameter of the tube. 
Figure 3 Tube flocculator. (Updated from Ian Tse's MS thesis, Aug. 2009)
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