2.5.5 Sizing of Paddle Flocculator

Flocculation Tank shall be provided with a flocculator, which shall comprise of a number of paddles fitted on shafts. These shafts shall hang from the rotating arms of a bridge in to the flocculation tank. The rotation of the paddles and shaft shall carry out the gentle mixing necessary for the formation of settleable flocs. Bridge shall be rotated through a central gear motor drive placed on the center of the bridge. Outer ends of the bridge shall move on the walls of the Flocculation Tank. In this section energy required for the gentle mixing of the alum mixed water and the size of the paddles shall be worked out.


Power Input by Paddle Flocculator

Dynamic Viscosity (µ) =2.050 x 10-5 lb.s/ft2
Volume of Flocculator Tank (V) =14004 Cft
Velocity Gradient (G) =50 1/s

Power of Agitator (P) = G2 µV
=(50)2x2.050x10-5x 14004
= 717.70 ft - lb /s
= 717.70/550
= 1.30 Hp
= 1.0 Kw
Efficiency of Motor Power to Water Power = 0.9
Motor Power of the Mixer = 1.0/0.9
= 1.15 Kw

Check for GT
Velocity Gradient (G) x Detention Time = 50 x 20 x 60
= 60000 (Okay)

Provide one variable speed Paddle Flocculator with motor capacity 1.15 Kw and rotational speed of 2 - 10 Rev per Min.

Size of Paddles

The area of paddles is worked out from the following expression:

P = Cd ρ A (V – v)3/2

ρ = 1000 Kg /M3

V (Velocity of tip of the paddle) = 0.5 M /s
v (Velocity of water at paddle tip) = 25 % of V

V - v = 0.5 - 0.25 x 0.5)
= 0.30 M / s
Cd (Coefficient of Discharge) = 1.8

P is a power input by paddle flocculators calculated above.

1000 x 2 = 1.8 x 1000 x A x 0.303
Area of paddles = 1000 x 2 / 1.8 x 1000 x 0.303
= 21 M2
= 226.7 Sft
Provide 2 shafts for supporting paddles and 4 paddles supported by each shaft

Thus total number of paddles = 8
Area of each paddle = 31.84 /8
= 4 Sft
Length of each paddle = 8 Ft
Width of each paddle = 4/8
= 0.5 Ft
Provide 8 paddles each of 8 Ft x 0.5 Ft size