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mechanically suffi-ciently working stripline, the customerusually requires anincreased throughputtogether with a high-er maximum opera-tion temperature anda higher upper limitof strip thickness.

Furthermore the downtime due tofurnace problems should be reducedand the maintenance simplified.This article reports about someimportant developments and innova-tions of WSP GmbH which increasethe performance and the efficiency ofsuch plants considerably.

The new WSP-floatationnozzle system

Conventional floatation nozzle sys-tems consist of nozzle arms or ribswith constant width across the stripwidth, Figure 1 left. The nozzle ribitself has slot nozzles or rows ofclosely spaced nozzle holes which,

Kramer, C. (1), Kramer, T. (2)

together with nozzle opening in thenozzle rib bottom between the slotjets care for building up a pressurecushion between strip and nozzlebottom. This pressure cushion givesthe floatation force.The WSP-floatation nozzles [1], Fig-ure 1 right, have nozzle bottoms likea longitudinal cross section of a bar-rel with a width decreasing from thecentre to both edges. This new nozzlebottom shape has essential advan-tages:1. The slot jets enclosing the nozzle

bottom at its longitudinal sides areinclined against each other in theplane view. Due to this inclinationand the decreasing distancebetween the slot jets the gas flowat the edges out of the pressurecushion is hindered what increas-es the floatation force andimproves the lateral strip stabilisa-tion.

2. Because of the larger width in thecentre the nozzle bottom area isincreased which again improvesthe floatation force.

3. Due to the larger nozzle bottomarea in comparison to convention-al rectangular nozzle ribs, moreround jets can be placed betweenthe slot jets thus increasing theheat transfer for at least 15 % atthe same volume flow.

Figure 2 shows the comparisonbetween the floatation pressure coef-ficient plotted vs. the floatationheight for a conventional and for thenew WSP-floatation nozzle system.The open area for the compared noz-zles is the same. Not only the levelbut also the progression of the forceis increased. This allows the floata-tion of heavier strip and improves thestrip stabilisation. Therefore scratcheson the strip surface due to contactwith the nozzles are avoided, even for

New developments for continuous heat treatmentof floatingly and touchless guided metal strips ofcopper alloys

Fig. 1:

The continuous heat treatmentof high quality copper alloystrips is executed in plants with

high protective gas circulation andtouchless guided strip. The core pieceof such plants is the nozzle systemwhich provides the floatingly suspen-sion and the stabilisation of the stripand, in addition, cares for the con-vective heat transfer. The demand forhigher annealing capacity, a widerrange of strip thickness, and anincrease of the maximum possiblematerial temperature are a particularchallenge for the thermoprocessingengineer. Especially for the replacement of anold worn out furnace in a still

Recent developments which improve the technol-ogy of strip floatation furnaces for copper alloystrip are described. The new nozzle system al-lows higher floatation force, higher heat transfer,and improved strip stabilisation. New designsmake a considerable increase of the maximumoperation temperature possible. The reportedresults are especially important for furnaceexchange and plant upgrading projects.

allows a much faster achieving of thesteady state condition and thusreduces the transient length of stripwhich usually has to be scrapped.

Novel furnace design

Modern floatation furnaces for cop-per alloy strips shall be designed forcontinuous operation temperatures of850 °C and more. Therefore, a designfor the metallic internal furnace partsis required which is resistant againstcreep deformation. Furthermore dif-ferent thermal elongation of differentcomponents should not lead to defor-

mations of the internal casing. Thenew design is [3] shown in Figure 3.The upper nozzles and the lower noz-zles of the floatation nozzle systemare supplied by separate fans. Thisallows an easy adaptation of thefloatation force by a change of thefan speed. The fans can be positionedin the roof, Figure 3a), or in one sidewall, Figure 3b). The later design is ofadvantage if the furnace is positionedon the shop floor or should replacean old furnace with the positioning ofthe fans in one side wall, too. In bothcases the flow duct has the shape of ahorizontally positioned U. For thebadly rolled strips. The new nozzle

system shows furthermore a smallerdecrease of the floatation force withdecreasing strip width than conven-tional nozzle systems.Because of the essentially increasedfloatation force which allows thefloatation of copper and brass stripsup to 3 mm thickness with protectivegas (about 96 % N2, rest H2) of tem-peratures of 750 °C to 800 °C, it ispossible to decrease the convectiveheat transfer of the nozzle system tothe strip without decreasing the dis-tance between strip and lower noz-zles essentially. This is especially ofadvantage if in an existing plant theproduction speed can be increaseddue to the new furnace technology.So, with the existing strip accumula-tors higher productions are possiblebecause the time necessary for thecoil change can be increased byreducing the strip speed during thisprocedure. The final material temper-ature and the material propertiesremain unchanged by this noveldynamic operation of the floatationfurnace. For a new plant with thistechnique even the exit strip accumu-lator can be omitted which reducesthe number of rolls touching thereadily annealed and clean stripwhich improves the surface quality.The reliable reduction of the convec-tive heat transfer with still sufficientfloatation force is possible by a newdeveloped control concept for thestrip heating [2]. This new technolo-gy is furthermore of advantage whenthe strip cross section, together withthe strip surface, is changing fromcoil to coil. The new technology

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Fig.3a:

Fig. 2:

Fig. 3b:

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design according to Figure 3a) theentire flow circuit is a stiff weldedconstruction of high temperaturealloy. In case of the design Figure 3b)a specially designed connection pieceallows different elongation of the fanbox and the nozzle plenum duct. Dueto the design of the connection pieceno bending moments and no longitu-dinal forces can act on the fan box oron the nozzle plenum box. Thereforeno deformation due to different ther-mal elongation can occur. The gapsbetween the different flow duct com-ponents are small and designed forminimised gas flow leakage.Especially in the first heating zonesof a floatation furnace, where a highheat input into the strip is required,the temperature distribution on theflow duct walls is very non-uniform.An investigation based on a numeri-cal modelling [4] shows temperaturedifferences between the hot areasclose to the radiant tubes and thenozzle system up to 300 K. The noz-zle ribs are the coldest part becausethe back flow of gas from the stripwhere by heating the strip the gastemperature is reduced, will "cool"the nozzle rib surface. The differencein thermal elongation due to thishuge temperature differences is enor-mous. Therefore, in the new furnacedesign, the radiant tubes do not pen-etrate the flow ducts, but are posi-tioned on the suction side of the fan.

Thus the problems of sealing the flowduct in spite of thermal elongation -the radiant tubes are fixed and theflow duct moves - cannot occur. Fur-thermore straight radiant tubes areused which can be sealed 100 % gastight in a simple manner with con-ventional flanges at the outer furnaceshell. So the exchange is easier and aleakage through which oxygen couldenter the furnace is absolutelyexcluded. A further advantageagainst conventional P-shaped or U-shaped radiant tubes is that at astraight single radiant tube no elon-gation differences which create ther-mal stresses can occur.Inside the flow ducts of WSP furnacesthe convective heat transfer from theflow duct wall to the gas is increasedby special means. By this design thetemperature of the sheet metal areasfacing the hotter radiant tubes isreduced. Furthermore the materialsare selected according to the expect-ed maximum material temperatures.When selecting the material it mustbe considered that a furnace with anominal maximum "operation tem-perature" of 850 °C, which corre-sponds to the gas temperature at thenozzle exit, can easily have parts inthe flow duct which obtain tempera-tures of 1000 °C and more.Special care is taken on the design ofthe fans, Figure 4. The fans developedand manufactured by WSP have

round plugs and are sealed againstthe external housing by roundflanges with double seals. The shaft issealed by a gas chamber seal which ispressurised by protective atmosphereand the pressure in this chamber ismonitored. The fan motor togetherwith the fan shaft bearing are fixed tothe plug. So different displacementsbetween the fans, standing on a plat-form or fixed to the shop floor, andthe furnace casing cannot occur. WSPhot fans can be equipped with anoscillation monitoring system andwith a special lifetime survey soft-ware which calculates the stillremaining lifetime depending on thegas temperature and the fan speed. Soa fan exchange, which may be neces-sary if the fan operating underextreme conditions, reaches theallowed limit of creep deformation,can be planned in advance and anon-expected breakdown of the plantdue to a fan problem is completelyexcluded.

New strip turn with inte-grated cooling

Especially for an exchange of a heattreatment plant in an existing line, incombination with an increase of pro-duction an augmentation of thecooling capacity is of importance.Therefore, a strip turn was developed[5] at the exit of the plant which doesnot only care for the strip centringcontrol but is also equipped with ahighly efficient cooling, Figure 5.The new strip turn consists of a mul-titude of rollers of small diameterwhich are arranged in the shape sim-ilar to a quarter of a circle. Therollers are covered by sleeves of spe-cial blend Aramid fibres of increasedtemperature resistance. Because ofthe effective cooling by cooling gasnozzles arranged between the rollers,the cooling effect is increased andthe rollers cooled by the gas actadditionally as "cooling rollers". Soin spite of an increase of strip speedand throughput the temperature forthe immersion of the strip into thewater closure at the end of the linecan be kept constant or even bereduced.

Fig. 4:

Process Technologies for Non FerrousMetals, Conference papers published byMesse Düsseldorf GmbH

(1)Prof. Dr.-Ing. Carl Kramer, con-sulting engineer, chairman of thescientific board of WSP GmbH

(2)Dr.-Ing. Thomas Kramer, generalmanager of WSP GmbH

constant "by design" and may notexceed values of about 1 N/mm stripcross section.

Literature[1] C. Kramer and E. Fiedler (Inventors):

Bed of air cushion nozzles to guide websin a floating manner, EP 0 864 518 B,granted 31.10.2001

[2] C. Kramer (inventor): Verfahren zum Be-trieb einer Durchlauf-Wärmebehand-lungsanlage mit über-wiegend konvek-tiver Wärmeübertragung, German patentapplication, 103 37 502.3, 14.08.2003

[3] C. Kramer and E. Fiedler (Inventors): Ap-paratus to guide webs in a floating man-ner, EP 0 864 518 B, granted 25.06.2003

[4] K. Hornig, R. Bölling, H.-J. Odenthal, H.Pfeifer, K. Berns, K. Schmitz, Strö-mungssimulation eines Schwebeband-ofens für NE-Metallbänder, GAS-WÄRME International (52) Nr. 5/2003,S. 256 - 262

[5] C. Kramer (Inventor): Umlenkvorrich-tung für bewegte Bänder, German patentapplication 103 26 071.4, 10.06.2003

[6] C. Kramer: A new Technology for theHeat Treatment of Floatingly GuidedMetal Strips of Copper and AluminiumAlloy - Heating, Soaking, Fast Cooling;1st International Conference on Plant and

Continuous strip furnacesfor extreme temperaturesand 100% hydrogen as protective atmosphere

For extreme material temperatures upto 1000 °C a continuous strip furnacewas developed with high convectiveheat transfer by gas circulation andup to 100 % hydrogen as protectiveatmosphere [6]. The strip can be guid-ed in a catenarian curve or verticallywith a 180° free strip turn at the low-er end. In this turn the strip is touch-less stabilised by floatation systems.These floatation systems also care byperfect lateral strip stabilisation forthe strip centring without any addi-tional means. The advantage of thisdesign in comparison to vertical fur-nace according to the state of the art,is that the strip, in case of a suddenstrip stop, may freely shrink withoutbuilding up excessive tensions whichare the usual reason of a strip ruptureat such occasions. The strip tension is

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Fig. 5: