Page 314 - Fundamentals of Water Treatment Unit Processes : Physical, Chemical, and Biological

Mixing 269

ratio is also affected by the system constants, i.e., the ‘‘power- n 3500 rpm. This may be determined best by
number,’’ P, and the ‘‘flow-number,’’ Q. trial, i.e., mounting two or more candidate motors.
It may be that n 3500 rpm is an overkill, but the
rationale is that for such a large capital expenditure
10.4.2.3.7 P=V Ratio
as a water treatment plant, testing is warranted in
In the final analysis, the empirical ‘‘P=V’’ is the only quanti- order to reduce the uncertainty of the design. Also,
tative guideline available; typically, the power dissipated per note that a high shear-to-flow ratio is desired, which
3
unit volume varies 0.2 P=V 2kW=m water (0.0076 requires a high n=D ratio.
3
P=V 0.076 hp=ft ) (Myers et al., 1999, p. 35); these values 4. Alum distribution
1
correspond to 200 G 2000 s at 208C. As indicated, the Letthealumbedistributedbyastainlesssteeltubering,
values cover a wide range, albeit the upper limits may be about 400 mm diameter with tube about 50 mm diam-
eter. The ring should be secured by cross beams such
much higher; there are no definitive numerical values.
that the vibration of the ring is not an issue. The tube
should be located about two impeller diameters dis-
Example 10.8 In-Line Pipe Mixing
tance upstream from the center of the impeller array.
The alum should be emitted downstream through six
Given orifices, with the orifices located just opposite the
3
Let Q(raw water) ¼ 1.0 m =s (22.8 mgd); let d(pipe) ¼ respective impellers. The pressure should be sufficient
300 mm as an initial trial.
such that the alum flow is the same through each
Required orifice. A pressure sensor should be located near the
Provide an impeller and alum injection design for an intaketothering-manifold;the alum feedshouldbe by
in-line mixing system. positive displacement pump with an air chamber or
other device to reduce pressure pulses.
Solution
5. Repair and maintenance
The challenge for in-line mixing is to disperse the coagu-
lant uniformly within the raw-water flow so that a large The distribution ring and impellers assembly should
fraction of this ‘‘sub-mixture’’ is exposed to high turbu- be located in a flanged section so that removal can be
lence and thus approaches complete mixing, i.e., blend accomplished with replacement by a straight pipe
fraction 0.99. section. The distribution tube and orifices should be
connected to a hot-water=chemical cleaning solu-
tion for routine maintenance. The pipe section with
1. Selection of pipe diameter alum distribution and impellers should have an
First trial: Assume the pipe diameter is 300 mm; observation window with internal lighting. After
3
v(pipe) Q(raw water)=A(pipe) ¼ (1.0 m =s)=[p cleaning, a dye (e.g., Rhodamine-B) may be injected
2
0.300 =4] ¼ 14 m=s. Let d(impeller) 0.1 m; assume to visually confirm that the orifices are functioning.
the turbulence zone is x(turbulence) 0.5 m. Thus, 6. Redundancy
q(turbulence) x(turbulence)=v(pipe) ¼ (0.5 m)= The alum feed and mixer assembly should be located
(14 m=s) ¼ 0.04 s. About 0.5 s would be better (for with an identical assembly located in another section
the adsorption–destabilization zone of coagulation).
of pipe such that the plant can continue operation
Second trial: Assume the pipe diameter is 1000 mm; without disruption while one assembly is being main-
3
v(pipe) Q(raw water)=A(pipe) ¼ (1.0 m =s)=[p tained or repaired.
2
1.000 =4] ¼ 1.3 m=s. Let d(impeller) 0.1 m; 7. Testing
The alum distribution and mixing setup should be
again, assume the turbulence zone is x(turbulence)
tested with a brine solution or Rhodamine-B dye solu-
0.5 m. Thus, q(turbulence) x(turbulence)=v(pipe) ¼
(0.5 m)=(1.3 m=s)¼ 0.4 s, which is close enough tion to determine the impeller mixing speed and
to 0.5 s. whether the configurations should be changed. The
2. Impeller design tracer concentration may be measured on the effluent
Select d(impeller) 0.1 m, which is arbitrary but side of the mixer system after a ‘‘step-input’’ of tracer.
within the guidelines suggested by Myers et al.
(1999, p. 36). Also assume that the turbulence Discussion
zone x(turbulence) 0.5 m as in the previous para- The design procedure suggested indicates the uncertainty
graphs. The turbulence should extend throughout of the mixing state of the art. A testing procedure should
the pipe section; therefore, select about six impellers produce greater certainty of mixing outcome, but with a
with two vertical shafts. Locate the shafts each higher-than-usual budget required. The same consider-
about 200 m from the centerline of the pipe. Locate ations should be given to any mixing system, e.g., a
the impellers, for each shaft, one in the center of the jet-mixer system or an in-line static-mixer system.
pipe and one each 150 mm above and below the
horizontal centerline, respectively. The intent of 10.4.2.4 Tanks
the design is to fill the pipe section with turbulence
to the extent feasible. The tank and impeller are a ‘‘system’’; therefore the
3. Motors for impellers geometry of each and other characteristics must be specified.
The motors should be direct current with variable The ensuing pressure and velocity fields are unique to a par-
speed control, with power sufficient to give ticular system.