Design of Rapid Mix Tank
Design a conventional vertical-shaft rapid mix tank unit for uniformly dispersing coagulant in 10 MLD of settled raw water as per design parameters given below:
Detention time (t): 20 – 60 s
Ratio of tank height (H) to diameter (D): (1:1 to 1:3)
Ratio of impeller diameter (DI) to tank diameter (D): (0.2:1 to 0.4:1)
Velocity gradient (G): >300 /s
Gt: 10000 – 20000
Tank diameter (D): < 3 m
Paddle tip speed (vp): 1.75 – 2.0 m/s
Velocity of paddle relative to water (v): 0.75 x paddle tip speed
Paddle area (Ap)/Tank section area (AT): 10:100 – 20:100
Coefficient of drag on impeller blade (CD): 1.8
Maximum length of each impeller blade (L): 0.25 x impeller diameter
Maximum width of impeller blade (B): 0.20 x impeller diameter
Impeller height from bottom (HB): 1.0 x impeller diameter
Kinematic viscosity : 1.003 x 10-6 m2/s
Dynamic viscosity of water : 1.002 x 10-3 N.s/m2
Determine tank dimensions (provide a freeboard of 0.5 m), impeller diameter, paddle dimensions, number of paddles, clearance of the impeller from tank bottom, paddle rotation speed and power input requirement.
