Page 258 - Water and Wastewater Engineering Design Principles and Practice

COAGULATION AND FLOCCULATION 6-35

n rotational speed, revolutions/s
D i impeller diameter, m
3
r density of liquid, kg/m
The impeller constant of a specific impeller can be obtained from the manufacturer. For the radial-
flow impeller of Figure 6-18 , the impeller constant is 5.7, and for the axial-flow impeller it is 0.31.

Example 6-7. Design a cylindrical flash mixing basin by determining the basin volume, tank
diameter, dimensions, required input power, impeller diameter from manufacturer’s data pro-
vided below, and its rotational speed using the following parameters:
3 3
Design flow rate 11.5 10 m /d
Rapid mix t 5 s
1
Rapid mix G 600 s
Water temperature 5 C
Place impeller at one-third the water depth
From manufacturer’s data, the following impellers are available:
Impeller type Impeller diameters (m) Power number (N p )
Radial 0.3 0.4 0.6 5.7
Axial 0.8 1.4 2.0 0.31

Solution:
3
3
3
a. Convert 11.5 10 m /d to m /s.
3
3

.
11 5 10 m /d 3
0133 m /s
.
( 86 400 s/d)
,
b. Using Equation 6-13 , determine the volume of the rapid mix basin.
3
V Qt (0133. m /s )(5 s ) 0665. m 3

c. Using the radial impeller guidance from Table 6-5 , assume H / T 2.0, that is H 2 T.
For a round mixing tank
2
()
T
2
V ( T )
4
and
/
⎧ () ( . 3 1 3
40 665 m ) ⎫ ⎪
T ⎨ ⎬ 0751 or. 0 75 m
m
.
⎩ ⎪ 2 ⎭
and
H 2(0.75m) 1.5m
and because the impeller is at 1/3 water depth
B (0 333 )(1 5 m ) 0 5 m
.
.
.