Flocculator Design Overview - Cornell University
1/10/2017OverviewFlocculator Design Analysis of hydraulic flocculators Ratio of maximum to average energydissipation rate Inefficiency of energy use due tononuniformity of energy dissipation rate 5 The great transition at Flocculator Design Head loss, collision potential, residence time Geometry of a baffle space to obtain desiredenergy dissipation rateTop ViewSide ViewH Water depthL Length of the flocculator channelS Space between bafflesT Thickness of the bafflesS T BB Perpendicular center to center distance between bafflesW Width of the flocculator channelS Space between bafflesL Length of a flocculator channelExit to the sedimentation tank entrance channelMinimum water levelSW1.5 SUpper baffleSLower baffleHPort fromprevious channelL1.5SLDesign Considerations The length of the flocculator channels matches thelength of the sedimentation tank Width of the flocculation channel? Minimum?Human width Material limitations (polycarbonate or concrete) Vary to optimize flocculationefficiency (function of geometry) Need to determine Head loss Residence time Baffle spacing Number of bafflesMore Design Considerations Even number of channels for AguaClaradesign (to keep chemical dose controllernear stock tanks), but this may change ifflocculators get smaller Even or odd number of baffles dependingon channel inlet and outlet conditions Begin with the energy source for theturbulence that creates shear that createscollisions: head loss for a baffleCEE 4540: Sustainable Municipal Drinking Water TreatmentMonroe Weber-Shirk1
1/10/2017Vena Contracta ( VC ) ConclusionsVena Contracta around a bend? Sluice gate (almost closed)* Draw the most extreme streamline 0.59through the transition and determinethe total change in direction If the change in direction for most ofthe fluid is 90 , then the VC isapproximately 0.62 If the change in direction for most ofthe fluid is 180 , then the VC isapproximately 0.622 0.384 Small hole in a tank 0.62 Exit from a pipeBy 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 No Vena Contracta* Roberson, JA; Cassidy, JJ; Chaudhry, MH. Hydraulic Engineering. John Wiley. (1995) page217. 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)Head Loss coefficient for a BaffleWhich space between baffles isbetter, considering the uniformityof the energy dissipation rate?Head loss in an expansione - expansionthe contractioncoefficient for a sharp180º bend (0.622)This space with very low energydissipation rate doesn’t contribute muchWe need to measure this in one of the new AguaClara plants! of 5? Jets expand in width at the rate ofapproximately 1 unit in width per 10 unitsforward Expansion length is 10(0.6S) Expansion requires a distance ofapproximately 6S The transition is related to thedistance required for the jet to fullyexpand0.6S0.4SWhy a transition atFlocculator Efficiency 4 10Simplify flocculator design bydesigning for high efficiency Efficiency will be a function of thevariability of the energy dissipationrate We expect a relation of the formsuch that efficiency is 1 when 1 and efficiency is less than 1for higher values of We “solve” this unknown by alwaysdesigning efficient flocculatorswith 3 H/S 6CEE 4540: Sustainable Municipal Drinking Water TreatmentMonroe Weber-ShirkS2
1/10/2017Prior to 2015 AguaClara used designsthat were far from the optimum A compact plant layout was possible for smallflows by using a vertical flow flocculator with ahigh ratio For small plants the width of the channel wasdetermined by the need to construct the channelusing humans (45 cm or more) The space between baffles was very narrow andthus was very high (for low flow plants) Small plants needed longer residence time andmore baffles to achieve adequate flocculationbecause efficiency was reduced.Viscous collisions or inertialcollisions Prior to 2016 I had assumed that theappropriate length scale comparison wasparticle separation distance and Kolmogorovlength scale – thus concluded inertia wasimportant2016 Particle separation distances are smallerthan inner viscous length scale Collisions in turbulent flocculators aredominated by viscosity (fluid shear, notturbulent eddies)** Edge of knowledgeNew Approach: Always efficient Add obstacles to have High head loss results in a tallerbuilding for the water treatment plant High head loss means higher velocitiesand that reduces settling of flocs in theflocculator Some gravity flow water supplies don’thave much elevation differencebetween source and storage tank Velocity gradient (G) Higher Higherallows lower residence time̅ results in smaller flocs10 L/sa maximum ratioof between 3 and 6. Flocculation efficiencycan be consideredconstant (and close to1)Collision Potential The target collision potential used for thedesign of AguaClara plants since about 2013has been 37,000 The actual collision potential in operatingAguaClara plants may be lower because thehead loss per baffle may be lower than weassumedEnergy use (head loss) in flocculationcontrols velocity gradient Head loss30 L/sThe Influence of ̅ or GMax̅ or ̅ determines the head loss through theflocculator Maximum size of the flocs is controlled by The value of ̅ or ̅ (assuming shear limits attachment) GMax or Max (assuming floc break up controls max size)Not yetknown Max 10 mW/kg (GMax 100 Hz) was the AguaClara standard(2011‐2015) Summer 2015 new designs have head loss of approximately40 cm Expect smaller flocs (but still captured by plate settlers) Less sedimentation of flocs in flocculator Smaller flocculator Casey Garland has testedCEE 4540: Sustainable Municipal Drinking Water TreatmentMonroe Weber-Shirk̅ values as high as 340 Hz3
1/10/2017The design inputs for flocculation We need collisions and thus G is a logical design specification We need to specify energy use Velocity gradient ‐ ̅ Energy dissipation rate ‐ Total head loss ‐ Or t ( )̅More time helpsdiffusion of coagulantnanoparticles to claysurfacesOur current choice of parameterthat sets energy input is head loss Head loss is independent of temperature Velocity gradient is f(temperature) Start with ( ̅ , ̅ ) and coldest temperatureOption 1Higher G meanssmaller flocs and moreelevation drop (headloss) throughflocculatorCurrent approach Calculate Calculate hFloc Start with (hFloc,Option 2Currentapproach̅ ) and coldest temperature Calculate ̅ Calculate ̅ (and hence ̅ ) will increase when theflocculator is operated at warmertemperatures due to decrease in viscosity Solve for channel width to setconstraints on viable solutionsDesign the reactor geometry toget the target velocity gradientKinetic energy dissipatedper residence timeContinuityRectangulargeometrySWAThis is the minimum channel width ifwe set 3 and set the expansionheight to equal water depthis height of one expansion zone.Could be the depth of water if the onlyexpansion is from the 180 degree bendThis is our general equation relating velocitygradient to reactor geometryMinimum number of expansionsper depth of flocculator (given W)Eliminate SSolve for maximum distancebetween expansions, , 6usingRound up to get the minimum number ofexpansions per depth of the flocculatorElevation viewAs channel gets narrower the spacing between bafflesgets larger.Channels narrower than this would have barely any ornegative baffle overlap!Our Design ApproachGiven energy (or ) and G Start big and then design the details(AguaClara Calculate volume of flocculatorapproach as ofsummer 2015) Split it into channels Then design baffles, and obstacles to fill thechannels to get target We can use this design approach becausewe are assuming that we will design for highefficiency (3 6) and thus we don’thave to add extra volume to account forinefficiencies. (Don’t forget this requirement!)CEE 4540: Sustainable Municipal Drinking Water TreatmentMonroe Weber-Shirk4
1/10/2017Design Algorithm (as of 2016)and G Start with2.3.4.5.6.7.8.9.Velocity gradient and flocculator volume givenhead loss and collision potentialMinimum channel width required to achieve 3 and required for constructabilityNumber of channels by taking the total width anddividing by the minimum channel width (floor)Channel width (total width over number ofchannels)Maximum distance between expansionsMinimum number of expansions per baffle spaceActual distance between expansionsBaffle spacingCalculate the obstacle width to obtain the same jetexpansion conditions as produced by the 180degree bendCollision potential for one flow expansionHeight of one expansion zone (in a vertical flow flocculator)Hydraulic residence time for one expansion zoneThese are the average velocities through the expanded flow areaEnergy dissipation rate is energy loss per timeCollision potential is a function of velocity.This suggests that a flocculator would performpoorly if the flow rate were decreases. I don’tknow if anyone has ever demonstrated that!Almost Real Designs(Flocculator exit depth of 2 m)Velocity guidelines?1Number of channels2345Human hipChannel width (m)1.2Sheet width10.8MaximumDesignMinimum0.60.4050100150Flow rate (L/s)Design Scaling(Design Engine version 7099)10 L/s0.53 m widechannels4.33 m long20 L/s0.55 m widechannels5.90 m long0.5 Why does V What sets maximum channel width? What sets minimum channel width? Why this cycle of channel widths?50 L/s0.56 m widechannels6.68 m long70 L/s0.72 m widechannels7.27 m long200increase withflow rate? Why does Vincrease insteps? Why does Vremainconstantabove 70 L/s?0.4Velocity (m/s)1.Viscous Collision Potential per FlowExpansion (the detailed perspective)0.30.20.1050100150200Flow rate (L/s)Max (10 State)Max (Schulz)DesignMin (10 State)Min (Schulz)More details The ports between channels should have thesame cross sectional area as WS The number of chambers per canal (except inthe last canal) is even – the number of baffles isodd The number of chambers in the last canal is odd– the number of baffles is even Why?CEE 4540: Sustainable Municipal Drinking Water TreatmentMonroe Weber-Shirk5
1/10/2017extraextraUse a Pipe with orifices to make aflocculator for small flows (S D) G2 K eV 32HeV 4Q DPipe 2 ContinuityH e HS DPipeEstimate the orifice diameter2 DPipe K eorifice 1 D2 vc Orifice 2DOrifice Here we assume that S is like D 4Q Ke G2 2 HS DPipe DPipe 2 Ke 4Q 2 G 2 HS3DPipeclosely may be less than what we calculate Vena contracta may not be as severe for orificesthat are close to the inner diameter of the pipeRound to nearest inner pipediameter? Or round down toget higher velocities toprevent sedimentation?17H e HS DPipe Insufficient length for full expansion beforenext orificehe K eV22gNeed to find actual Ke given pipe diameter to develop target GG he K eReplace residence time with volume/Q 4 gheQ2 H e DPipehe 2G 2 H e DPipeDOrifice 22 vc DPipeextraUse a Pipe with orifices to make aflocculator for small flows (H D) G2 2Ke K eV 32He4QV DPipe 2 Continuity G2 K e 4Q 2 DPipe DPipe 2 37 3 DPipeG 2 HSMax132Q 3 K eorifice 1An interesting designNo this wasn’t AguaClara H e DPipeHere we assume that S is like D4 gQG H e DPipeV16Q Ke 42g2 g 2 DPipe4 gQ2 K eorifice 1We need to estimate Ke!extraghe The head loss for these orifices spaced so3Estimate the orifice diameter usingthe correct value of KeG vcDPipeDPipe K 4Q 3 7 e2 2 G Round to nearest inner pipediameter? Or round down toget higher velocities toprevent sedimentation?CEPIS Horizontal Flow FlocculatorCEE 4540: Sustainable Municipal Drinking Water TreatmentMonroe Weber-Shirk6
1/10/2017A few Reflections Floc size doesn’t seem to be a significantconstraint for flocculator design We may increase energy dissipation ratesignificantly as we experiment with maintainingsmall flocs that primary particles can attach to Our broad goal is to maximize performance atminimum cost. Thus cost minimization may be animportant constraint for setting the targetvelocity gradient. Maintaining the flocs in suspension is anotherimportant constraintReflection Questions max How does the collisionpotential in a flocculatorchange with flow rate? What is the ratio ofto ̅for well designed hydraulicflocculators? Why might mechanicalflocculators break more flocsthan hydraulic flocculators?0.6S0.4SReflection Questions What are somealternategeometries? How else could yougenerate head lossto create collisions?Reflection Questions What is the relationshipbetween potential energyloss and the average velocitygradient in a flocculator? How did AguaClara getaround the 45 cm limitation? How does the nonuniformity of (or G)influence efficiency ofenergy use?Conclusions Energy dissipation rate determines thespacing of the baffles. Energy is used most efficiently to createcollisions when the energy dissipation rate isuniform. Therefore H/S between 3 and 6 isbest. Collision potential is a function of geometryand a function of flow rateCEE 4540: Sustainable Municipal Drinking Water TreatmentMonroe Weber-Shirk7
Design Min (10 State) Min (Schulz) Flow rate (L/s) Velocity (m/s) Design Scaling (Design Engine version 7099) 50 L/s 0.56 m wide channels 6.68 m long 10 L/s 0.53 m wide channels 4.33 m long 70 L/s 0.72 m wide channels 7.27 m long 20 L/s 0.55 m wide channels 5.90 m long More details The ports between channels should have the
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