Civil Engineering Reference
In-Depth Information
473
Motor horsepower
1.15 hp (0.85 kW )
550
0.75
Use a standard 1.25 hp (1.0 kW) motor. Now check the rotational speed and
G
-
values.
k
P
ND
g
35
Assume
D
2 ft (0.61 m); then each blade is 1 ft (0.305 m) long.
(1.25
550
0.75)
1.0
1.94
N
3
2
5
N
2.0 revolutions / sec
120 rpm
Use a 4-to-1 variable-speed drive, and check the value for
G
at the lowest speed
(120 / 4
30 rpm).
3
30
P
1.0
1.94
2
5
60
7.76 ft-lb / sec (10 W)
and:
1/2
7.76
G
11.5 sec
1
2.896
10
(12
12
14)
5
The range in
G
-values is satisfactory. For standardization, put the same unit in all four
compartments. It may be possible to reduce the horsepower in the third and fourth
compartments because of the lower
G
-values required.
Example 2 (Paddle-Type Unit)
Assume the plant flow rate is 20 mgd (75. ML / d) and the lowest temperature is 53.6
F
(12
C). The alum doses, based on jar tests, ranged from 20 to 30 mg / L. From Equation
10-26, determine the optimum
G
-value.
G
24 sec
1
and
20.8 sec
1
respectively
for
T
30 minutes.
Use a
G
-value of 24 sec
1
and a detention time of 30 minutes. The product
Gt
is 4.32
10
4
, which is within the recommended range.
Assume a three-compartment system with the optimum
G
-value in the middle. The
G
-values on either side would be 30 sec
1
and 15 sec
1
.
Assume a rectangular-shaped flocculator and a 12-foot sidewater depth:
