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GLASS INTERNATIONAL NOVEMBER/DECEMBER 2000 21

High quality mixed batch is essential for producing quality glass andmaximising production yields. Here D D Burgoon* of Toledo EngineeringCompany points out that the glass making process starts at the batch plant

and describes the critical features of good batch plant design.

In Mr Burgoon’s article in GlassInternational, September 19991, thepoint was made that the glass man-ufacturing process is only as goodas the weakest element in the

process, that is, batching, melting/condi-tioning, forming, annealing, cold endtreatment, or packaging (fig 1). Similarly,batch plant performance is only as goodas its weakest element, whether it be,materials handling, weighing, mixing orthe electronic controls (fig 2). This articleis written to highlight the essential ele-ments of good batch plant design.

Materials handlingDuring unloading of the various raw mate-rials, care must be taken to avoid bothcontamination and segregation.Contamination shows up in the glass asoff-chemical composition, or as stone typedefects. Segregation will show up as stria-tion (cord or ream).

Contamination can usually be avoidedwith good operational practices, such asmaking sure the unloading hopper isclean before use, using a non-residualor non-contaminating bucket elevatorboot, and using a distribution systemthat cannot leak between adjacent silopositions. For critical applications, dedi-cated unloading facilities are used foreach material.

Each raw material is subject to segrega-tion during the silo filling and withdrawaloperations. Silos should be kept as full aspossible to minimise segregation createdby the impact of the material striking thepile during the fill operation. In someapplications, diffusers are installed at the

silo inlet to minimisesegregation duringfilling. In addition,particle size for allraw materials needsto be in the samerange and this is controlled during theprocurement phase.

The final control stage in segregationtakes place at the silo bottom, where massflow devices such as bin activators, specialflow control inserts and special bin bottomshapes are used to minimise or eliminatefunnel flow discharge. Ideal mass flow in asilo would be for all material in the cylin-drical section to draw down at the samerate, thereby keeping the surface profileunchanged. Achieving mass flow at dis-charge helps to compensate for segrega-tion created during the filling operation.Furthermore, mass flow assures a uniformsource of material at the weigh feederinlet, which is essential for accurate,repeatable ingredient weighments.

Batch weighingBefore designing the batch weighing sys-tem, the glass technologist establishes thebatch composition required to obtain thedesired glass characteristics. The technol-ogist also establishes the statistical toler-

ance limits to which each rawmaterial must be weighed in orderto stay within the suitable range ofproperties. One common propertyused for statistical process control isglass density. Typical glass densitylimits for container glass are in the0.0020 gm/cc range, whereas thelimits for float glass are in the

0.0002 gm/cc range. Increasingly, directglass analysis, such as x-ray defraction,allows direct tracking of the key oxidepercentages. With the desired glass char-acteristics established, the design of the

batch weighing sys-tem can proceed.

Feeding devices(not conveyingdevices) are used tofeed raw materialsto the scale(s). The

feeder type, for example, vibratory,screw or gate is dependent on the rawmaterial characteristics. Free flowing/lowpermeability materials are usually han-dled by vibratory feeders (fig 3), whereaserratic flowing/high permeability (easilyaerated) materials are handled by specialscrew feeders, with a flush control device(fig 4).

All scale feeding devices must be sizedfor both feed rate and necessary weigh-ment resolution (fig 3). The design para-meter for necessary resolution (variationin angle of repose at cut-off, plus feedercoast) are usually more than sufficient tosatisfy the feeder capacity requirements.

Many years ago, TECO developed theSuperfine feeder for handling finemesh/high permeability materials (fig 4).A rotary vane feeder serves as an air-lockto guard against material flushing. Therotary vane feeds a small surge hopperand when full, the material overflows andprovides a fast-feed rate for filling thescale. Once near the desired weight, therotary vane feeder stops and a specialscrew feeder continues to feed materialfrom the surge hopper, until the batchweight is satisfied. A safety butterfly valvecloses to assure that no additional materialwill flow to the scale. The Superfine feedercan deliver almost any feed rate capacitydesired. This feeder type has been used inconjunction with minor ingredient scales,as small in capacity as 20lb. with scale res-olution of 0.02lb.

A well designed batch plantcan improve profitability

� Fig 1. Basic process steps of a glass manufacturing operation.

� Fig 2. Basic process steps of a glass batchingoperation.

� Fig 3. Vibratory feeder with cut-off resolution shown.

The glass compo-sition and statisticallimits set by theglass technologistprovide the basisfor determining scale resolution andaccuracy, to which each ingredient isweighed. Scale resolutions of 1:5000 arecommon and in some cases, 1:10,000 ormore are used. The weigh hopper mustalso be designed to assure cleanout at dis-charge. The weighing system must beenclosed to contain dust, thus the scalesystem must be designed to be immunefrom internal pressure effects, which canbe created by the operation of a dust col-lector, bucket elevator, and/or mixer.Pressure effects on an improperlydesigned weighing system can createerrors that are not immediately apparent,since the scale would be a combinationgravimetric plus pressure force detector.One method of obtaining pressure immu-nity is to have the weigh feeders enter thehopper side (not through the top) (fig 5),or by using a feeder ring, which is a pres-sure counter-balancing scheme.

A check scale is a vital element of anybatch processing plant and it is a simpletask if a separate batch surge hopper isused between the weigh gallery and the

mixer. In recent years, however, batchplant structural height (and cost) are

being reduced, by eliminating thebatch surge hopper and mounting

load cells under the mixer orpneumatic blender/trans-

porter. This requires spe-cial know-how to negateerroneous forces createdby mixer motion, con-duits and piping, whichcan affect the scale accu-racy and repeatability.

MixingThe design of the mixing

system is critical since it is the final stagein developing necessary mixed batchhomogeneity. Today, pneumatic blendingis typically used for fine mesh ingredientapplications, like E-glassfibre batch, andthe turbine-type mixer for the coarsemixes, such as soda-lime batch. Bothmethods fluidise the batch and introducea turbulent force to create the mixingaction. The mixing time to achieve thedesired degree of mixed batch homogene-ity is established at start-up.

Cullet is generally introduced into theprocess after the mixing operation, toreduce wear and maintenance of themixer, and to avoid cullet/mixed batchsegregation. TECO prefers to batch-weigh cullet and blend it with the mixedbatch as it is conveyed to the storage binat the furnace.

Mixed batch, regardless of howit is mixed, is subject to segrega-tion during conveying to storageat the furnace, and subsequentdelivery to the furnace charger.Proper design of the conveyingsystem and transfer points willminimise segregation. Most soda-lime batch is wetted with water inthe mixer, as a final step to pro-vide cohesion, minimise particlesegregation, and dusting.

Typically, mass flowbins are used forbatch/cullet storageat the furnace. Thisis very important forbatches that cannotbe wetted and mustbe conveyed dry.

ElectroniccontrolsThe last process ele-ment requiring

attention at the time of design is the elec-tronic control system. Strain gauge loadcells and digital scales are universally usedfor weight detection. PLCs, with updatetime in the 25 to 50 millisec range, arecommonly used to obtain adequate con-trol resolution for the weighing process.Each ingredient is weighed under thesupervision of real-time statistical control,which monitors each weighment and auto-matically adjusts the ingredient cut-off, inline with the trend of previous batches,thereby minimising process upsets.

The furnace operator is commonlyresponsible for the batch plant opera-tion. Information and data from thebatch plant is transmitted electronicallyto the furnace control room, as well as tothe plant and company-wide informa-tion systems.

ResultsTo test how well a new batch plant (ormodernisation) is performing, one cancompare glass density variation with acounterpart, that is, the obsolete batchplant or a similar facility within theorganisation. TECO recently provided,on a design/build basis, a new batchplant for an existing three-furnace glasscontainer plant. 30 days beforeswitchover to the new batch plant, theglass density data was noted for all threefurnaces. After switchover, the densitywas noted for an additional 30 days. Theresults of the ‘before and after’ glass den-sity conditions are shown in fig 6.Obviously, the -30 day is glass densitybefore switchover and +30 day are theresults with the new batch plant. Anapproximate ten-fold improvement inglass density variation was obtained withthe new batch plant.

Who said well performing batch plantsaren’t beneficial? In the above example,the ten-fold improvement in glass densitystability increased the pack rate a few per-centage points, which had a positive anddirect impact on profitability.

References1 Better plant performance begins at thebatch house, D D Burgoon, GlassInternational September 1999, pg 9.

22 GLASS INTERNATIONAL NOVEMBER/DECEMBER 2000

* D D BurgoonToledo Engineering Co Inc, Toledo, Ohio,USA. Fax+1 419 537 1369.

� Modern batch plant for the production of ‘E’ glass(continuous) glassfibre.

� Fig 4. Superfine feeder withrotary vane feeder, surge w/overflow, special screwfeeder and butterfly shut-off valve.

� Fig 6. Glass density his-tory 30 days before/afternew batch plant.

� Fig 5. Pressure balanced vs pressure sensitive weighhopper/feeder arrangement.

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