flocculator design
DESCRIPTION
Flocculator Design. Overview. CFD analysis of hydraulic flocculators Ratio of maximum to average energy dissipation rate Inefficiency of energy use due to nonuniformity of energy dissipation rate The great transition at H/S=5 Flocculator Design - PowerPoint PPT PresentationTRANSCRIPT
Flocculator Design
Overview• Analysis of hydraulic flocculators• Ratio of maximum to average energy
dissipation rate• Inefficiency of energy use due to
nonuniformity of energy dissipation rate
• The great transition at H/S=5• Flocculator Design• Head loss, collision potential,
residence time• Geometry of a baffle space to obtain
desired energy dissipation rate
Top View
W = Width of the flocculator channelS = Space between bafflesL = Length of a flocculator channel
W
S
L
Side ViewH = Water depthL = Length of the flocculator channelS = Space between bafflesT = Thickness of the bafflesB =Perpendicular center to center distance between baffles
S T B
H
L
S
1.5S
1.5 S Upper baffle
Lower baffle
Minimum water level
Port from previous channel
Exit to the sedimentation tank entrance channel
Design Considerations• The length of the flocculator channels
matches the length of the sedimentation tank
• Width of the flocculation channel?• Minimum? ________________• Material limitations (polycarbonate or
concrete)• Vary to optimize flocculation
efficiency (function of geometry)• Need to determine• Head loss• Collision potential• Residence time• Baffle spacing• Number of baffles
Human width
More Design Considerations
• Even number of channels for AguaClara design (to keep chemical dose controller near stock tanks)• Even or odd number of baffles
depending on channel inlet and outlet conditions• Begin with the energy source for the
turbulence that creates collisions: head loss for a baffle• Minor losses come from expansions• Expansions come from contractions…
Vena Contracta around a bend?
• Sluice gate (almost closed)*• 0.59
• Small hole in a tank• 0.62
• Exit from a pipe• No Vena Contracta
By Lindsay Lally, Lee Hixon (Own work) [CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0) or GFDL (http://www.gnu.org/copyleft/fdl.html)], via Wikimedia Commons
* Roberson, JA; Cassidy, JJ; Chaudhry, MH. Hydraulic Engineering. John Wiley. (1995) page 217. Original reference is Henry, H.R. “Diffusion of Submerged Jets.” Discussion by M.L. Alberston, Y.B. Dai, R.A. Jensen, and Hunger Rouse, Trans. ASCE, 115, (1950)
Vena Contracta (PVC ) Conclusions
• Draw the most extreme streamline through the transition and determine the total change in direction• If the change in direction for most
of the fluid is 90°, then the PVC is approximately 0.62• If the change in direction for most
of the fluid is 180°, then the PVC is approximately 0.622=0.384
Head Loss coefficient for a Baffle
22
12
out oute
in
V Ahg A
2
1outB
in
AKA
PVCBaffle is the contraction coefficient for a sharp 180º bend (0.622)
Head loss in an expansion
2
1 1 2.56VCBaffle
BK
P
We need to measure this in one of the new AguaClara plants!
B - Baffle
The Energy Dissipation Rate isn’t uniform
• Ideally (for optimal efficiency) the flocculator would have a completely uniform energy dissipation rate• We can evaluate the efficiency of
various designs as a function of H/S
Estimating the ratio of
3JetPlane Jet
MaxJet
VS
P
JetVCBaffle
VV P
Jet VCBaffleS S P
QVSW
31 JetPlane
MaxVCBaffle VCBaffle
VS
P
P P
32 12 2
BB
B
K VVKH
Max
Max
Definition of PJetPlane
True for large H/S where the jet fully expands before the next turn
BHV
Energy lost
time3
4
2JetPlane
VCBaffle B
HK S
P
P
Estimating PJetPlane • The transition value for H/S is at 5
(from CFD analysis) (this is our weakest assumption)• has a value of 2 for H/S<5 (CFD
analysis and Haarhoff, 2001)• has a value of
for H/S > 5• Solve for PJetPlane when H/S = 5• PJetPlane = 0.225
3
4
2JetPlane
VCBaffle B
HK S P
P
134
2B
JetPlane VCBaffleK S
H P P
Results of the CFD analysis and our model equations
Max
is more uniform when H/S is between 3 and 5.
0 10 20 30 400
5
10
15
CFD resultsCurve fit
Ratio of baffle length to baffle spacing
4
3VCBaffle B
JetPlane
K P P
3
4
2Max JetPlane
VCBaffle B
HK S
P
P
2 For H/S<5
Prior to 2015 all AguaClara designs had an H/S >5.Future designs will have a 3<H/S<6
Why a transition at H/S of 5?
• Jets expand in width at the rate of approximately 1 unit in width per 10 units forward• Expansion length is 10(0.6S)• Expansion requires a distance of
approximately 6S• The H/S transition is related to the
distance required for the jet to fully expand
0.6S 0.4S
Flocculator Efficiency
H/S = 4H/S = 10
Which space between baffles is better, considering the uniformity of the energy dissipation rate?
This space with very low energy dissipation rate doesn’t contribute much
How can we optimize the design of the flocculator?• Efficiency will be a function of
some dimensionless grouping ()
• We used computational fluid dynamics (CFD) and the software FLUENT
• We also used flow expansion analysis (total energy loss) and jet flow analysis (maximum energy dissipation rate)
H
S
2 3 4 5 6 7 81 10 1000.6
0.7
0.8
0.9
1
H/S
Flocculation Efficiency
is the flocculation efficiency given the non uniform energy dissipation rate, a function of
13
12 3
V
V
dV
V dV
4
3VCBaffle B
JetPlane
K P P
CFD results
161
Collision Potential13
B B
16
1
(dimensional analysis and CFD)
The CFD results shown in the graph over predict the efficiency for the case of =1. The equation correctly predicts an efficiency of 1 for uniform energy dissipation rate.
Prior to 2015 AguaClara used designs that were far from
the optimum• A compact plant layout was possible for
small flows by using a vertical flow flocculator with a high H/S ratio
• For small plants the width of the channel was determined by the need to construct the channel using humans (45 cm or more)
• The space between baffles was very narrow and thus H/S was very high (for low flow plants)
• Small plants needed longer residence time and more baffles to achieve adequate flocculation because efficiency, , was somewhat reduced.
13
B B
New Approach: Always efficient
• Add obstacles to have a maximum H/S ratio of between 3 and 6.• Flocculation
efficiency, , and ratio of max to average energy dissipation rate, , can be considered constant
30 L/s 10 L/s
Collision Potential per Flow Expansion
13
2
16
12
33
16
21
12
e e
ee
ee
ee e
HV
V VKH
K H
No velocity dependence!
Collision potential for one flow expansion
Energy dissipation rate is energy loss per time
Flocculation efficiency is a function of the uniformity of the energy dissipation rate (CFD analysis gives this exponent)
12 4 6
4e e
eK H
Hydraulic residence time for one expansion zoneThese are the average velocities through the expanded flow area
Height of one expansion zone (in a vertical flow flocculator)
A Simple Result
• The collision potential depends primarily on the flocculator geometry• It is possible that the collision
potential will be somewhat affected by the Reynolds number, but this effect, if any, will probably be small• We can design a flocculator for the
maximum flow rate and be confident that the flocculator will also work for lower flow rates (Flow doesn’t affect the collision potential).
12 4 6
4e e
eK H
Example (no extra obstacles)• Ife had a value of 1 m2/3 in the first
channel, what value would the sum of e be for the first channel?
Number of baffles= ______Careful…Number of chambers = ___Channel = ________
17
18 m2/3
18
12 4 6
4e e
eK H
Example:• How many spaces between baffles, (N)
would be necessary if the flocculator has a depth of 2 m and if the space between baffles is 0.5 m? Assume a required collision potential of 75 m2/3.H = _______
S = _______= ______B = _______ = _______N = _______
4HS
75 m2/3
1.54 m2/3
0.5 m2 m
49
23
23
75m 491.54mExp
N
12 4 6
4B B
BK H
2 2
1 1 2.56VCBaffle
BK
P B
KB2 HFloc
4
4 2
1
6
1.54m
2
3
Complaints? What is missing?
• What was the flow rate for the previous example?• What was the width of the channels?• It seems that we are missing
something• We only have been using the
constraint of the target collision potential• We also need to use a constraint
related to the amount of energy we are using for flocculation
Energy use in Flocculation
• Head loss • High head loss results in a tall
building for the water treatment plant
• High head loss means higher velocities and that reduces settling of flocs in the flocculator
• Some gravity flow water supplies don’t have much elevation difference between source and storage tank
• Energy dissipation rate • Higher allows lower residence time• Higher results in smaller flocs
13
Floc Floc
FlocFloch
g
2
2e eVh K
g
Turbulence or viscous shear limits the size of flocs
• A floc in turbulent flow experiences a shear force due to the drag of the fluid on the floc caused by the velocity gradient
• The floc will also rotate and this will reduce the difference in velocity between the floc and the fluid
The Influence of or Max
• The value of determines the maximum size of the flocs and the head loss through the flocculator
• Max of 10 mW/kg was the AguaClara standard (2011-2015)
• Summer 2015 new designs have head loss of approximately 40 cm• Expect smaller flocs (but still captured by plate
settlers)• Less sedimentation of flocs in flocculator• Smaller flocculator
4 Flocculator unknowns2 equations – We pick 2!• Collision potential (sets settled
water turbidity)• We can either set the energy
dissipation rate (max and average) or the head loss• Either parameter could work as input for
design• Select head loss because it is the most
intuitive for designers as it determines water levels in the plant
13
Floc Floc Floc
Floc gh
Two Design ApproachesGiven energy ( or ) and
• Start big and then design the details• Calculate volume of flocculator• Split it into channels• Then design baffles, and obstacles to fill
the channels• Start small and then work up to the
big details• Design baffle spacing and obstacles• Then design channels and flocculator
(AguaClara approach as of summer 2015)
(AguaClara approach prior to summer 2015)
Given , and , Find and
• Calculate energy dissipation rate
• Hydraulic residence time
32
16
Floc
Floc
gh
13
F oc ol Fl c
lFloc
F oc
gh
16
13
FlocFloc
11 2
32Floc
FlocFlocgh
Now we have equations starting from our input parameters
1
2 3Floc Floc Flocgh
Relationship between geometry and energy
dissipation rate
WS A
QVWS
2
2ee
V VKH
13
2e
e
K QSH W
3
2e
e
K QH WS
Kinetic energy dissipated per residence time
Continuity
AguaClara calculates W based on reactor volume and baffle width limitations
Capital H is a height, Small h is head loss.
Minimum number of expansions per depth of
flocculator
Solve for maximum distance between expansions, , using = 6
eHS
HS
P Max
Max
e
HS
HS
P
13 4
2Max
Max
HSee
QKH
W
P
3
2e
e
K QH WS
Min
Max
Floce
e
HN
H Round up to get the minimum number of
expansions per depth of the flocculator
Eliminate S
1 10 1000.1
1
10
100MaximumAverage
Headloss (cm)
Ener
gy d
issip
atio
n ra
te (m
W/k
g)
Energy Dissipation Rate, Head Loss and
Residence Time Floc
Flochg
2 13 6
FlocFloch
g
32
16
10Floc
Floc
gh mWkg
Floc
Floc
gh
16
13
FlocFloc
This graph assumes an efficient flocculator and
Proposed design summer 2015
2375Floc m
11 2
32
390 6.5minFlocFloc
Floc
sgh
Design Algorithm (new in 2015)
1. Flocculator volume given head loss and collision potential
2. Energy dissipation rate3. Minimum channel width required to achieve
H/S>3 and required for constructability4. Number of channels by taking the total width
and dividing by the minimum channel width (floor)
5. Channel width (total width over number of channels)
6. Maximum distance between expansions7. Minimum number of expansions per baffle
space8. Actual distance between expansions9. Baffle spacing10.Calculate the obstacle width to obtain the
same jet expansion conditions as produced by the 180 degree bend
Min
Max
Floce
e
HN
H
11 2
32Floc
FlocFlocgh
13 4
2Max
Max
HSee
QKH
W
P
13
2Min
BMin HS
K QWH H
P
32
16
Floc
Floc
gh
13
2e
e
K QSH W
Almost Real Designs(Flocculator exit depth of
2 m)• What sets maximum channel width?• What sets minimum channel width?• Why this cycle of channel widths?
0 50 100 150 2000.4
0.6
0.8
1
1.2
MaximumDesignMinimum
Flow rate (L/s)
Cha
nnel
wid
th (m
)
Velocity guidelines?• Why does
V increase with flow rate?• Why does
V increase in steps?• Why does
V remain constant above 70 L/s?
0 50 100 150 2000.1
0.2
0.3
0.4
0.5
Max (10 State)Max (Schulz)DesignMin (10 State)Min (Schulz)
Flow rate (L/s)
Vel
ocity
(m/s)
132 e
e
HVK
Design Scaling (Design Engine version
7099)50 L/s0.56 m wide channels6.68 m long
10 L/s0.53 m wide channels4.33 m long
70 L/s0.72 m wide channels7.27 m long
20 L/s0.55 m wide channels5.90 m long
More details• The ports between channels should
have the same cross sectional area as WS
• The number of chambers per canal (except in the last canal) is even – the number of baffles is odd
• The number of chambers in the last canal is odd – the number of baffles is even
• Why?
An interesting designNo this wasn’t AguaClara…
CEPIS Horizontal Flow Flocculator
A few Reflections• Floc size doesn’t seem to be a significant
constraint for flocculator design• We may increase energy dissipation rate
significantly as we experiment with maintaining small flocs that primary particles can attach to
• Our broad goal is to maximize performance at minimum cost. Thus cost minimization may be an important constraint for setting the target energy dissipation rate.
• Maintaining the flocs in suspension is another important constraint
max
Reflection Questions
• How does the collision potential in a flocculator change with flow rate?• What is the average
energy dissipation rate in a flocculator?• What is the maximum
energy dissipation rate for small H/S?
12 4 6
4e
eK H
Reflection Questions
• What are some alternate geometries?• How else could
you generate head loss to create collisions?
0.6S 0.4S
Reflection Questions• What is the relationship
between potential energy loss and the average energy dissipation rate in a flocculator?• How did AguaClara get
around the 45 cm limitation?
Review
• What is the symbol for collision potential?• Approximately how much
collision potential is required for a flocculator?• What is ?• How does the non uniformity
of influence efficiency of energy use?
Flocculation Equation Summary
3JetPlane Jet
MaxJet
VS
P
332 12 2 2
e ee
e e e
K V KV QKH H WS
Max
13
Floc Floc
16
1
12 4 6
4e e
eK H
Floc
Floc
gh
Definition of
Two equations for average energy dissipation rate
13
2e
e
K QSH W
Baffle spacing
Flocculation efficiency
Collision potential
Collision potential for one expansion
Non uniformity of energy dissipation rate
13 4
2Max
Max
HSee
QKH
W
P
Maximum distance between expansions
13
2Min
BMin HS
K QWH H
P
Minimum channel width
Conclusions• Energy dissipation rate determines
the spacing of the baffles.• Energy is used most efficiently to
create collisions when the energy dissipation rate is uniform. Therefore H/S between 3 and 5 is best.• Collision potential is a function of
geometry and not a function of flow rate