Coagulation and Flocculation-I
SOLUTION
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)
Gt: 10000
– 20000
Tank diameter (D): <
3 m
Paddle tip speed (vp): 1.75
– 2.0 m/s
Paddle area (Ap)/Tank
section area (AT): 10:100
– 20:100
Coefficient of drag on impeller blade (CD): 1.8
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.
Solution:
Let
the detention time (t) be 40 s.
Therefore,
volume of tank (V) =
Let the tank diameter (D) be 2 m
Tank
cross-sectional area (Acs) = 3.14 m2
Tank
height (H) = 1.47 m, provide freeboard of 0.5 m
Total height (HT)= 1.97 m, say 2 m.
Tank
height (H) to tank diameter (D) ratio =
(within the
range of 0.33–1.0, hence okay)
Let
velocity gradient, G = 400 s-1
Therefore,
G.t = (400).(40) = 16000 (within the limit of
10000-20000, hence okay)
Let
the paddle tip speed (vp) be 1.8 m/s
Hence,
velocity of
paddle relative to water (v) = 0.75.(1.8) = 1.35 m/s
, or,
or, Ap = 0.335 m2
Tank sectional area = (D).(H)
= (1.47).(2) = 2.94 m2
, which is around 0.15, hence okay.
Let the impeller diameter be 0.8 m, i.e., , which is okay
Choose length of each impeller blade (L) as
0.20m, i.e., (okay)
Choose breadth of each impeller blade (B) as
0.15m, i.e., (okay)
Area of each blade = (L.B) = (0.2).(0.15) = 0.03 m2
Therefore, number of blades to be provided = , say 12
Clearance of the paddles from the tank
bottom = 0.8 m
Paddle rotation speed (w, radians/s) = radians/s,
i.e., revolutions per
minutes
Power requirement is given by, , or,
or, P = 742 Watts, i.e.,
provide 1 KW motor for driving the impeller at 45 rpm.