Civil Engineering Reference
In-Depth Information
P
1.356
q
(
H
)
g
(10-17)
P
Q
(
H
)
g
(metric)
where:
P
power input, ft-lb / sec (W)
q
flow rate from orifice hole—jet discharge, cu ft / sec (m
3
/s)
C
d
a
H
total jet energy loss, feet (m)
/2g
density of water, lb / cu ft (kg / m
3
)
g
gravitational acceleration 32.2 ft / sec
2
2
(9.81 m / s
2
)
a
area of orifice, sq ft (m
2
)
jet velocity, ft / sec (m / s)
C
d
discharge coefficient (
0.75)
Substituting known values in Equation 10-17 yields:
P
0.97
Ca
d
3
(10-18)
P
500
Ca
(metric)
3
d
One important advantage of jet mixing is that untreated raw water or partially
treated water (e.g., the treated backwash water from the filters) can be used for chem-
ical injection. A valve on the chemical pump discharge line allows flexibility in chang-
ing the power input. This system can be adjusted to accommodate changing raw-water
conditions.
Hydraulic Mixing
Hydraulic jumps have been used for mixing chemicals. Frequently, plant flows are
measured by a Parshall flume or other similar device that incorporates a hydraulic
jump downstream by including an abrupt drop in the channel. The coagulants are
introduced immediately upstream of the flume. Typical residence times are about 2
sec with a
G
-value of about 800 sec
1
.
Chow presents the mathematical equations required to compute the
G
-values.
46
The headloss through the flume varies with the flow rate and can be computed or
obtained from discharge tables. The principal advantages of this unit are:
No mechanical equipment to operate and maintain
•
Lower cost because there is no separate rapid mix unit
•
DESIGN EXAMPLES FOR RAPID-MIX SYSTEMS
In-Line Blender
If jar tests indicate that adsorption-destabilization is the preferred coagulation mech-
anism, an in-line blender is the appropriate mixer. Assume the following plant sizing
and criteria:
