Page 255 - Water and Wastewater Engineering Design Principles and Practice

6-32 WATER AND WASTEWATER ENGINEERING

Example 6-6. Design a static mixer for the following conditions:
3
Design flow rate 150 m /h
Minimum water temperature 5 C
Mixer aspect ratio 1.5
Design COV is 1%

Solution. This problem is solved by iteration; that is, a set of reasonable assumptions is made
and then the calculations are performed to verify or correct the assumptions.

a. Select a pipe diameter of 400 mm for the design flow rate.
b. Select the number of elements to achieve a COV of 1%, that is, six elements.

c. From Figure 6-16 determine the headloss per element is 0.16 kPa per element. The total
headloss through the mixer is then
)
.
(6 elements )(0 16 kPa/element 0 96 kPa
.
d. With six elements that have an aspect ratio of 1.5, the length of the mixer is
L ( no of elements aspect ratio pipe diametter)
)
.
)
(
(
()(61 5 )(040 m) 3 6 m
.
.
.
e. The volume of the mixer is

()(040. m ) 2 3
V (3 6 . m ) 0 45 m
.
4
f. The detention time is
045 m 3 3
.
t 3 0 . 10 hor 10.9 s
3
150 m /h
g. The water power imparted by the mixer is estimated using Equation 6-16 with
1.00:
3
(9 807 kN/m 3 )(150 m /h )
.
QH (096 kPa)(0 102. m/kPa) 0 0400 kW
.
.
P
,
3 600 s/h
where (0.102 m/kPa) is a conversion factor.
h. Estimate the velocity gradient using Equation 6-12. Using the water temperature of 5 C
3
and Appendix A,
1.519 10 Pa · s
⎛ 40.0 W ⎞ 0 5 .
G ⎜ ( ⎝ 1 519 10 3 Pa
s)( 0 45 m )⎠ ⎠ 241 9. s 1
3 ⎟
.
.