influence of air knife wiping on coating thickness in

8/12/2019 Influence of Air Knife Wiping on Coating Thickness In

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Issue 6 Influence of Air-Knife Wiping on Coating Thickness in Hot-Dip Galvanizing 71

effects of t he outlet pr essu re, the nozzle to stri p dis-tance, t he slot opening, t he edge baffle plate, aswell as th e tilting angle of air knife were studied. Itshow s tha t uniformity of coating thickness can beimproved by optimizing important parameters andth e design of nozzle. Fur the rmo re, the practicalproducti on data of t he contin uous galvanizing line(CGL) are analyzed with regression method. Th e sam-ples of th e galvanizing steel ar e selected to measurethe thickness of zinc deposition on th e steel surfaceby metallographic method. All the results made itpossible to establish four first-order polynomialmodels that gave coating thickness as a function ofall those parameters. Using these models, the mostoptimal operating conditions can be predicted.1 Experimental Procedure

The study object and all of data and sampleswere from No. 4 CGL at Angang Steel. Th e prehea-ted strip comes from the annealing furnace and zincbath with molten zinc of 99. 77% is heated at 460 C.Th e jet pressure is controlled by a digital pressureregulator. Stepping motors control both horizontaland vertical distance between the strip and nozzle.As illustrated in Fig. 1 , air jet injected normally ontothe surface of moving stri p is divided into two differ-ent wall jets. In the upper region y>O) , since thestrip moves upward, the molten zinc moves in thesame direction at both wall boundary and freeboundary. In the lower region y


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72 Journal of Iron and Steel Research, International VOl. 1 9

slot opening D2 ) Run numerical simulation of an impinging jet

using the FLUENT and correlate the obtained pres-sure and shear stress profile on the strip surface.

3) Compare the predicted coating thickness throughthe developed model, with the actual measured coat-ing thickness in the plant.2 Mathematical Modeling

The mathematical model is developed based onthe following assumptions :

1) Th e fluid flow on the liquid coating layer canbe described by the steady state, two dimensional equa-tions of incompressible, constant viscosity creep flow.

2 ) Th e pres sure across th e relatively thin incom-pressible coating layer is assumed to be constant.

3) There is no slip between the liquid zinc layerand the strip on the strip-fluid interface.The coating thickness, based on the assumptions

made, is determined by th e following equation.Motion equation:

(1)where, u is the velocity in the zinc film; P is thepressure in the liquid zinc due to impinging air jet; uis viscosity of liquid zinc at 460 C ; p is density ofzinc; and g is gravitational constant.

Conservation of mass: th e zinc withdrawal rateq can be expressed as,

Boundary conditions :u=V, at x=O and u -=r at x=h, 3 )

'where, V, is the strip velocity; r is the shear stress.Th e non-dimensional coating thickness T non-

dimensional shear stres s S and t he effective gravita-tional acceleration G are given as,

dudx

Introdu ce the non-dimensional withdrawa l rateflux Q in Eqn. 2 ) as

dQd TAnd by setting 0, the non-dimensional

coating thickness is obtained asSt

2G= ( 6 )From Eqn. ( 4 ) and Eqn. (6) , it is clear that if

the pressure and shear stress profile on the strip su r-

face are known , the coating thickne ss can easily becalculated.3 Numerical Analysis Model

Numerical simulations were carried out usingFLUENT, a finite element analysis software, togenerate pressure and shear stres s profile on thestrip surface. In numerical analysis , the equations ofcont inui ty, two-dimensional time depend ent Navier-Stoke s, en ergy, and state equation were used asgoverning equations. When Reynolds number isgreater than 4 00, realizable K turbulent modelsare used to solve the turbulent stress. Th e schemat-ic computational domains and each boundary condi-tion are shown in Fig. 3. The grid in the regionshaving more possibility of changing in pressure, ve-locity and others, makes the exit grid of air knifedenser. The numbers of mesh and mesh type are about2. 6X l o 5 and hexahe dral, respectivelyC8'. T he inletpressure and outlet pressure boundary are located a tthe entr y and outside of air knife. A non-equilibriumwall function on wall boundary of air knife and stripsurface was used. Strip velocity was set at 1.67 m/s.and the media for gas wiping is compressed air. Indiscrimination function , pressure-velocity couplingused SIMPLEC algorithm, and pressure correctioninferior-relaxation factor is 1. 0. Th e pressure, mo-mentum equation, and K equation are second-orderupwind format.

ship\

V

'*

Pressure-outlet

Air knife outlet

1 ressure-outletGrid system and boundary conditionig. 3

4 Results and DiscussionIn the air jet wiping process , in order to opti-

mize the pressure and shear stress, it is necessary to


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Issue 6 Influence of Air-Knife Wiping on Coating Thickness in Hot-Dip Galvanizing

study the influences of the importan t parameters andnozzle geometry. Th e regression analysis is carriedout to develop the correlations which relate to thepressure and shear stress profile on a moving stripsurface. Th e numerical analys is model is used tostudy the effect of different operating parameterssuch as nozzle pres sure, nozzle to strip distance,nozzle slot openi ng, edge baffle plate , and tiltingangle of air knife.4. 1jet pressure is gauss distribution.

Pressure distribution of strip surfaceBased on the jet wiping force the ory , impinging

( 7 )Th e correlation for the shear stress on the verti-

cal moving strip due to the impinging air jet is given byr=r,.,[erf(O. 8337) -0. Z7e-0.6g3;] (8)

where, P is maximum value of impinging pres-sure; v= z / b s strip coordinate; b is the half-val-ue width of pressure distribution; rmaxs maximumvalue of shear stress; erf p ) = e dy is errorf u n ~ t i o n [ ~ l .

Th e distributions of impinging jet pressure andshear stress are shown in Fig. 4. As long as it iswithin the range of the process parameters impin-ging pres sure maximum value half-value widt hof pressure b and she ar stres s maximum value rmaxfstr ip surface can be achieved. According to Eqn. ( 7 )and Eqn. ( 8 ) , strip surface pressure distribution canbe got. T h e three parameters quantify the ability ofair knife impinging jet. Th e greater the peak valueof impinging pressure is, the stronger ability of airknife impinging jet is; The smaller pressure half-value width of pressure is, the more air flow concen-trated and stronger impinging ability is. The greater

p =p x -O. 693;max

2 - 2

7 3

-0.02 0.01 0 0 01 0.02Length of striplmFig. 4 Impinging jet pressure and shear stress distribution

the maximum value of shear stress is, the strongerthe ability of blowing is. A s a result, the maximumvalues of impinging pre ssur e and shear str ess reflectthe magnitude of s trip surface pressur e, and half-value width determines the pressure distribution.Reaching . 5 mm distance to the entrance ofstrip, jet pressure attenuates to 0 and shear stressincreases to maximum value.4 . 2 Influence of air knife outlet pressure

Th e influence of the outlet pressure on maxpress ure and shea r stre ss is shown in Fig. 5. The out-let pressure of ai r knife ranges f rom 40 to 80 kPa, at5 kPa interval. With the same nozzle slot , influenceof the outlet pressure on the strip surface is linearwith peak value of pre ssure and shear stress. Th epeak value of pressure and shear stress increase withthe increase in outlet pressure, and under the sameoutlet pressure condition they decrease with the in-crease in the nozzle to st rip distance. But it is non-linear for pressure distribution and mostly decidedby the distance between nozzle and strip. When th edistance is more than 1 6 mm from strip surface, t hehalf-value press ure width is smaller and the p ressure

40 50 60 7 80 40 50 60 70 80Air-knife outlet pressure/kPaInfluence of outlet pressure on a) impinging jet pressure and b) shear stressig. 5


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74 Journal of Iron and Steel Research, International Vol. 19on the strip is more concentrated by increasing thepressure. When the distance is less than 1 6 mm, in-fluence of th e outlet pressure on pressure dis tribu-tion becomes less important.

4 .3 Influence of nozzle to strip distanceThe influence of t he nozzle to strip distance on

maximum pressure and shear stress is shown in Fig. 6.The nozzle to strip distance is set from 8 to 21 mm, in-flection point obtained for 11 .4 m m Th e peak value ofpressure decreased with th e distance increasing, butthe rules are different. When the distance is less

than 11.4 mm, peak values of pressure and shearstress are more stable. A s the distance exceeds acritical value 11 .4 mm , peak value of pres sure andshear stress attenuations rapidly, but half-valuepressure width increases. Th e numerical analysis re-sult s show th at t he influence of nozzle to stri p dis-tance on pressure field is segmenta tion differences.The reason is that when the distance is less than11. 4 mm , the impact of high-speed air flow to liquidzinc has not been fully developed. Wit h the distanceincreasing, air flow develops entirely and impingingpressure and shear stres s begins to attenuate .

P=80 Pa70A 6 0

8 10 14 18 22

I50 b)

2001 8 10 14 18 22Nozzle to strip distancehnmInfluence of nozzle to strip distance on a) impinging jet pressure and b ) shear stressig. 6

4 . 4The influence of th e nozzle slot opening on max

pressure and shear stress is shown in Fig. 7. Thenozzle slot width is taken from 1.0 to 2. 0 mm, at0. 2 mm interval. This is an important process pa-rameter. Th e slot opening directly affects ability of cop-ing with liquid zinc. It can be clearly seen that when theslot opening is less than 1.2 mm it would lead to pres-sure and shear stress attenuate rather largely. But whenthe slot opening is more than 1. 3 mm, it will result in

Influence of nozzle slot opening divergence, which reduces air jet wiping to scrapeliquid zinc. The slo pe of t he curv e for peak value ofpressure increasing is less than ste ep slope of th ecurve for shear stress decreasing when slot openingis more than 1. 3 mm. So reasonable slot openingshould be less than 1. 3 mm. Th e pressure half-valuewidth has relation with nozzle slot o pening . When thedistance is closer, and slot opening increases, theimpinging pressure has higher degree of convergence.When t he distance is further away, and slot opening

1 0 1.2 1.4 1 6 1.8 2.0 1.0 1.2 1.4 1.6 1.8 2.0Nozzleslot widthlm

Fig. 7 Influence of nozzle slot width on a) impinging jet pressure and b ) shear stress


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increases, the impinging pressure is more divergent[ '.It is easy to see the edge effect in Fig. 8 , be-

cause of easine ss of jet flowing to the surrounding.A s a result , the impinging pressure at th e neigh-bourhood of th e stri p edge becomes small to comparewith that in the strip centre area.

T o obtain good uniformity of coating thickness,

it is necessary to design air knife with the variablenozzle slo t opening. Based on numerical ana lysis ofnozzle slot, the current designed wedge nozzle slot isplaced 6 centre parallel gaps at distance of 110 mm,having an opening of 1. 1 mm. And nozzle edges ex-tend 5 gaps at distance of 110 mm which increase0 . 025 mm every slot shown a s Fig. 9.

8.0~10' 7.00~10'

5.00~10'6 00x104

2.00x10

3.00x10'4.00~10'gl.OOXlO

n00 0.1 0.2 0.3 0.4 0.5 0.6 0 0.1 0.2 0.3 0.4 0.5 0Length of strip/m

a) Wedge nozzle slot; ( b ) Constant nozzle slot.Fig. 8 Influence of nozzle slot on edge impinging pressure

1.225mm 1 l O m m l.10mm 1.225mm

Fig. 9 Wedge nozzle slot design

4.5 Influence of edge baffle plate of air knifeThe nozzle slot's impact of s trip transverse pressureshows that increasing slot opening in the strip edge re-

gion can restrain over-galvanized. Because different stripwidth will lead to different nozzle positions, over-galva-nized usually occurred in the area of 40 mm awayfrom the edge. Nozzle slot design can not avoid over-

will cause liquid zinc splashing . The control shoul dbe based on the reasonable nozzle slot, and emphasison optimization and design of edge baffle plate.Shown as Fig. 10, edge pressure distribution has lit-tle attenuation with baffle plate, but it can be clearlyseen pressure decreasing at distance of 1 2 mm awayfrom the edge without baffle plate. T he simulationshows that the wider the baffle and the more narrowthe gap between strip and baffle is, the less outwarddeflection flow of s tri p edge affected region is. Theregional pr essu re of str ip edge can be enhanced withthe baffle, so it can effectively control over-galvani-zing at th e edge by adjusting the gap abo ut 2 mm and

galvanized at the edge of different widths. Over opening by increasing baffle dimension properly.

.00~10~l .00~104-1.00~1040 0.05 0.10 0.15 0.20 0.25 0.30 0 :

I .

0 0.05 0.10 0.15 0.20 0.25 0.30 0 :Lengthof shiplma) With baffle plate; ( b ) Without baffle plate.

Fig. 10 Influence of edge baffle plate

4 6 Influence of tilting angle of air knifeFig. 11 shows the distributions of impinging

pres sure and she ar stres s with the variation deflec-tion angle +O , 5 , l o , 15 used in the current nu-


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Table 1 Experimental parameters matrix of coating thickness regressionTest CW.i,/(g * m - Z ) CW,,.,/(g * m - z ) V , / ( m * s-l) H / m m Z/mm P,,.,lkPa D /m m CW,.i

1 40 49 . 82 110.00 500 11 39 . 513 1. 20 53 . 382 40 47 . 52 115 . 02 300 9 3 2 . 0 1 6 1 . 2 0 47.543 40 46 . 35 100 . 02 300 9 27 . 008 1. 20 4 5 . 8 34 40 43. 14 79.9 8 550 10 26.005 1. 20 51 . 735 40 44 . 35 100 . 02 500 1 0 3 2 . 5 1 5 1. 20 45 . 616 50 56 . 04 70 . 02 350 . 00 8 13.016 1. 1 5 57.097 50 58 . 11 70. 98 500 11 21 . 020 1. 1 5 60. 618 50 51. 58 130.0 2 400 8 30.008 1. 15 56 . 679 50 53. 23 103 .02 400 10 27 . 008 1. 1 5 55 . 2610 50 52 . 42 61 . 02 400 9 11 . 618 1. 1 5 57 . 2511 60 62.60 139. 98 500 1 0 32 . 608 1. 10 64.5012 60 59 . 99 79 . 98 550 10 1 7 . 0 2 6 1. 10 66.571 3 60 62.09 115.02 600 10. 5 2 7 . 0 2 3 1. 1 0 64. 5714 60 61 . 06 94 . 98 450 10 20.530 1. 1 0 63 . 5215 60 71. 95 64.98 650 11 11. 519 1. 10 68. 5516 90 97.5 5 97. 98 550 10 14.01 8 1. 1 0 9 5 . 0 317 90 93 . 20 120 . 00 400 10 18 . 017 1. 10 96 . 131 8 90 94.97 120.0 0 280 9 16. 516 1. 10 93 . 6819 90 95 . 38 90.00 500 12 16 . 013 1. 1 0 97.6420 90 95 . 97 111 . 00 500 10 16 . 315 1. 10 97 . 87

E1 044 . 8 Measuredcurve- argetmass

I I5 10 15 20

( a ) CW.,,=40 g/ m Z; b) CW,, ,=50 g/m2; ( c ) CWa, , =60 g /m2; d) CWa, , =90 g /mZ .Fig. 12 Result of regression model fitting curve

target coating mass is 50 g/m2. This value correspond-ing to coating thickness is approximately 53 pm. Con-sidering the difficulties to change the parameters in anindustrial environment, especially the regulation of

the impinging jet pressure, the optimu m thickness isexpected to remain in the same range. When str ipvelocity changed from 70 to 1 2 0 m/min , the modelspredicted coating mass variation of less than f 1 2 g/m


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* 78 Journa l of I ron and Steel Research, International Vol. 19~

that translated the coating thickness variation shouldbe less than 1. 68 pm.

Conclusions1) The mathematical models were developed to

predict the coating mass. Th e models consist of pa-rameters of strip velocity, t he jet nozzle press ure,the nozzle to strip distance, nozzle slot opening anddistance between the knives and bath. Th e coatingmass predictions which have multiple correlation co-efficients R2more than o. 9483 are within f 1 4 ofthe measured coating mass.

2 ) The pressure and shear stress profile re-quired in model were calculated through regressionanalysis, carried out using numerical simulation.The effect of the outlet pressure on the strip surfaceis linear with peak value of pressure and shearstress. But it is nonlinear with pressure distributionand mostly decided by the nozzle to strip distance.

3 ) The numerical simulation using FLUENTresults show that nozzle to strip distance should becontrolled under a critical value 11. 4 mm in order toimprove the stability of coating thickness and uni-f ormi y.

4 ) A new air knife system with variable nozzleslot opening and edge baffle plate are recommendedto deal with th e edge over-galvanized problem. Un-der th e same nozzle condition, slight tilting around6= 5 of air knife is the most effective to controlcoating thickness and reduce possibility of splashingat No. 4 CGL of Angang Steel.References:[ l ] Tae-Seok CHO , Young-Doo KWON, Soon-Bum KWON. A

Stud y of t he Influence of Air-Knife Tilting on Coating Thick-

111

1 2 1

ness in Hot-Dip Galvanizing [J]. Jou rnal of The rmal Science,2009, 18(3): 262.Myrillas K, Gosset A, Rambaud P, et al. Technique for Dela-ying Splashing in Jet Wiping Process [J]. Chem Eng Process,2010, 9(11): 5899.Ki Jang AH N, Myung Kyoon CHUNG. A Noble Gas WipingSystem to Prevent the Edge Overcoating in Continuous Hot-DipGalvanizing [J]. ISIJ International, 2006 , 46(4) : 573.Delphine LACANETTE, Stephane VINCENT, Eric AR-QUIS, et al. Numerical Simulation of Gas-Jet Wiping in SteelStr ip Galvanizing Process [J]. ISIJ International, 2005, 45( 2 ) : 214.ZHAN G Hui. Numerical Simulation of Hot-Dip Metallic Coat-ing Process [J]. Int J Heat Mass Transfer, 1995, 38(2): 241.Yang Hong-xia, Yang Hong-yu, Cui Li-li. How to Controland Apply CLECIM DAK Dynamic Air Knives on GalvanizingLine [C] // 2009 CSM Annual Meeting Proceedings, 2009, 2 :143 (in Chinese).Pravin H , Ananya M , Shantanu C. Mathematical Modeling ofJet Finishing Process for Hot-Dip Zinc Coating on Steel Strip[J]. ISIJ International, 2005, 45(2): 209.YUAN Yin-mei, LI Chao-xiang. Numerical Simulation of theHot Galvanizing Line [J]. Journal of Anhui University of Tech-nology and Science, 2006, 21(4): 27 (in Chinese).CHENG Ji-min, YAN Hong-kai, NIE Xing-li, et al. Numeri-cal Simulation of the Air Knife in Hot-Dip Galvanizing [J].Heavy-Duty Machinery, 2009, 2 : 37 (in Chinese).Deng Chun-rui , Fang Yi-ming, Liu Wen-shu. Th e DynamicAir Knife Application in Hot Dip Galvanizing Line [C]//2009CSM Annual Meeting Proceedings, 2009, 8 : 789 (in Chi-nese).CHANG Tie-zhu, ZHANG Qing-dong, JIANG Zheng-lian.FEM Simulation of Strip Edge Over Coating During Hot-DipGalvanizing [J]. Iron and Steel, 2009, 1 44( 1): 51 (in Chi-nese).Soon-Bum KWON, Dong-Won LEE, Young-Doo KWON.Experimental and Computational Studies on Coanda NozzleFlow for the Air Knife Application [J]. Journal of ThermalScience, 2007, 16(2): 164.Jalel Ben nasr , Ali Snoussi, C hedly Bradai. Optimization ofHot-Dip Galvanizing Process of R eactive Steels: MinimizingZinc Consumption Without Alloy Additions [J]. MaterialsLetters, 2008, 2(67): 3328.

influence of air knife wiping on coating thickness in

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