Annealing The benefits of annealing lehrs · major importance to avoid breakage in the lehr and to...

The purpose of annealing lehrs isto cool the glass gradually to nearroom temperature, giving it the

desired properties. The process involves travelling

through the viscous-elastic region of theglass – which is still a matter offundamental research – prevailing inmodern annealing lehrs.

The fundamentals are critical forcontinuous glass ribbon annealing ofarchitectural, automotive, electronicsand foamed applications.

In the case of foamed glass, the role ofthe annealing lehr is to cool thecontinuous foam in a single piece toroom temperature. After cooling, thefoam must be easy to cut and beresistant to thermal shock on one sidewhen submerged in a bitumen bath at200°C. For patterned glass, the glassmust be easy to cut during productionand at the glass processing plant. In thecase of float glass, the glass must also bescratch free up to a certain level.

BreakageThe breakage behaviour of foamed glassand flat glass is fundamentally different.In the case of flat glass, a singlemicroscopic crack can growcontinuously, as long as a certain stresslevel is present.

For foamed glass, the growth of amicroscopic crack is the result of acombination of individual microscopiccracks, which can be identified by thecracking of individual cells[I].

For this reason, a certain compressivemembrane stress is put on the glassribbon edge by installing a controlledtemperature gradient, while for foamedglass a temperature gradient over theribbon is not desired.

Good control of the temperaturegradient over the full ribbon width is ofmajor importance to avoid breakage inthe lehr and to obtain a well controlledresidual stress during cutting of theribbon; both during production and atthe glass processing plant. For this

reason, more or fewer control zones areused, depending on the required ribbonwidth. Good control of the temperatureunder and above the glass is important,to attain the required flatness. Thereforeseparate cooling beneath and above theglass is introduced.

Annealing temperatureThe temperature profile needed toanneal the glass depends entirely on thevelocity of the glass forming, thicknessand nature of the glass. For 6mm thickglass, the maximum tensile residualstress due to the thickness gradient isrecommended to be between 9kg/cm²and 12kg/cm². For foamed glass, amaximum tensile (residual) stress of2.3kg/cm² is recommended for110kg/m³ density foam[II].

SizeTypical flat glass annealing lehrs arebetween 25m (electronic) and 200m(architectural) in length and depend onthe required load, ribbon width andresidual stress value specified. Similarlengths are possible for foamed glasslehrs[III], allowing a velocity between 3.6and 40cm/min for 12cm foam.

As a consequence, annealing lehrsoccupy a large part of the productionline and for this reason, accurate

calculations will drastically reducecapital expenditure (Capex). Above acertain lehr length, the glass qualitycannot be improved further, so the extraCapex, incurred due to safety factors onestimates instead of accurate calculation,has no added value.

ProcessBelgium-based Cnud-Efco offerstechnical solutions in terms ofengineering and equipment for formingand annealing glass. The companyworks with soda lime glass which has anannealing point at 546°C and strainpoint at 515°C.

The points for other glass can becalculated by scaling to the respectiveviscosity points. Fig 1 (overleaf) showsthe actual annealing profile.

After glass forming, fast, indirectcooling from 600°C to 540°C isperformed in zone A. By using fastcooling, the glass surface hardensrapidly and scratches by rollers are lesslikely. Below 540°C, there is slowercooling in zone B, down to 480°C, whenstress relaxation at this stage becomesnegligible. However, structuralrelaxation is still present at 450°C,

Hans Strauven* explains how annealing lehrs are a critical step in theproduction of patterned, flat, electronic and foamed glass.

The benefits of annealing lehrs

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Annealing

� View of a float glass lehr.

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which builds up additional residualstress. In the case of electronic glass,improved glass stabilisation can be usedto avoid shrinkage, at electronicprocessing temperatures (400°C), bylengthening the first lehr zones.

The residual stress is calculated withthe R. Gordon - O. S. Narayanaswamymodel[IV]. This model gives an accuratedescription of the stress and structuralrelaxation, making up the residual stress.The phenomenon of structuralrelaxation, responsible for 50% of theresidual stress of flat glass was firstreported elsewhere[V]. In the case ofthick-foamed glass, annealing is slowand the glass can be considered asstabilised or without structural stress.

Below 480°C, the cooling rate is 400°Cin zone C (structural relaxation is ratherslow). It is general practice to useindirect cooling down to 400°C andeven lower. Below this temperature,cooling by radiation, assisted by naturalconvection becomes too slow.

Zones A, B and C consist of aninsulated metallic tunnel with air-cooledheat exchangers above and below theglass, which are organised in differentcontrol zones covering the ribbonwidth. In zones A and B, it is still above450°C and more or less SO2 is injected tocover the glass and rollers with aprotective layer. Zones A and B are builtwith AISI304 while standard ST37 isused in zone C.

It makes sense to increase theefficiency of the heat exchangers in zoneA by using a different geometry ormaterial. Underneath the ribbon, theheat exchangers are susceptible to thepossibility of broken glass during startup or production changes.

The exchangers are easy to clean usingdoors in the sidewalls. In addition,peepholes are foreseen above the glasslevel. The typical pitch of the rollers forarchitectural glass is 500mm in zone A, B

and 600mm in zone C. The diameter,material type and surface of the rollers[VI]

depend on the glass produced. It isimportant rollers are installed in such away that cleaning of the rollers duringproduction can be done efficiently[VII] toensure a perfect glass surface.

Sidewall adjustable heaters areinstalled to heat up the lehr and toadjust the temperature of the ribbonedge. Heating drawers over the fullribbon width are typical with electronicglass, on line coated float glass andfoamed glass.

Below 400°C for standard architecturalglass, forced convection can be used tospeed up the cooling of the glass. Forelectronic glass, it is necessary to startthis type of cooling at a lowertemperature, especially when glass witha high expansion coefficient is used, aswith chemical strengthening. In the caseof foamed glass, natural convection andradiation allow a cooling rate where themaximum temporary stresses(2.3kg/cm²) are reached because glassfoams become highly insulating at lowertemperatures[VIII]. For this reason, anadapted insulated tunnel can be usedover the full length and forcedconvection is not advised.

The forced convection is performedwith heated air (RET zones[IX]) down to200°C. The air is heated by the glass(architectural glass) or an extra heaterfor ultra-thin glass and is blown throughnozzles organised in different controlzones above and beneath the glass, tocontrol the temperature across the ribbon.

Below 200°C in the F zones, air atambient temperature is used to cool theglass to 70°C. The geometry andlocalisation of the nozzles is the key togetting maximum cooling withminimum energy consumption and stillallow cleaning of the glass cullet duringa production change. Today, RET and Fzones are designed with the state of the

art knowledge of turbulence to reducethe pressure drop[X].

ConclusionAlthough annealing lehrs are inproduction for long periods, there arestill improvements to be made. Today atleast 10kWh/tonne or about 1.5% of thetotal primary energy (operatingexpenditures = Opex) for float glass isused to cool the glass to 70°C, while theextracted heat can be converted inenough mechanical energy to drive allthe fans of the lehr. In the case ofelectronic and foamed glass, the load(and so heat input) of the lehr is ratherlow and extra heating is necessary. Thisis always done electrically, but the use ofwaste heat from other parts of theproduction line could also be consideredto keep the lehr at the desiredtemperatures. Electronic glass always hasedges, which are more difficult to cool.This was accomplished by lengtheningthe lehr, but could also be done today byimproved heat exchangers.

Annealing lehrs have to be built withthe smallest environmental footprintpossible. This has to be achieved byaccurate calculations and innovativeengineering, using new technology andmaterials. On-line simulation can be alsobe organised to improve productionchanges and operator training. �

References[I] C. Maes, A. Van Moffaert, H. Frederix and

H. Strauven, Phys. Rev. B, 57(9), p4987 (1998)

[II] Foamglass Industrial Insulation Handbook,

p91 as found by Google

[III] Patent WO 2009/141456 A1 : H. Strauven,

Pittsburgh Corning Europe NV

[IV] O. S. Narayanaswamy, Stress and Structural

Relaxation in Tempering Glass, J. Amer. Ceramic

Soc., vol. 61, no. 3-4, pp. 146-152, 1978.

[V] P. Gérard and L.R. Troussart,

Rev.Univers.Mines, Swies 91,7 [I II 396-409

(1951)]

[VI] http://www.newhudson.com/glass_

industry.html

[VII] http://www.newhudson.com/

whats_new.html on CNUD-EFCO lehrs

[VIII] Foamglass Industrial Insulation Handbook,

p92 as found by Google

RET is the abbreviation of the French

‘Recirculation en temperature’ as put by CNUD-

EFCO Int.

I.E. Idelchik, Handbook of Hydraulic Resistance,

978-8179921180 (2005).

*Hans Strauven PhD, R&D Manager,Cnud-Efco International, Belgium.Website: www.cnudefco.com

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