How to Fill a Tank
Filling furnace’s with cullet is a vestigial process, an artifact from the past. Back in the day furnaces were filled cold with cullet and then heated-up. This worked well enough when throats were not submerged and bottoms were not made of fused cast. In fact I found that when a fiberglass furnace was cooled down for lack of business and wasn’t drained, it took 72 hours to thaw the glass and restore flow. This was the same amount of time it took to fill a furnace after a standard heat-up. I’ll mention now that E-glass reinforcement fiberglass doesn’t use cullet and filling is always done with 100% batch.
So why do people in the 21st century think that they should fill with cullet? There’s one definite reason which is that blanket chargers don’t tolerate pushing batch into an empty furnace. On large sideport furnaces with suspended walls there’s no better alternative than blowing cullet. I learned that in 1992 when we tried to slide batch down a chute. Needless to say it was slow, difficult and inferior to blowing cullet. This however is the only time I’d advoate 100% cullet filling. The other reason that people fill with cullet is that the industry is conservative and often keeps doing things the way they have always.
I can say that having filled more than 30 furnaces, I have often had problems filling with 100% cullet and never had issues when filling with nominal 20% cullet in standard batch. Here’s a comparison between the two:
The beauty of filling with low cullet is that the batch will flow out of the doghouse(s). It makes a small pile and doesn’t fuse immediately.
The chargers run slowly at 20% of production speed until there is enough glass to float, about 12 hours.
Once the batch is floating the chargers can be ramped to full production levels and the rest of fill goes quickly. A quick fill is good because it means the glass pushes through the throat and helps prevent freezing. It also makes it relatively simple to calcualte when the furnace will be full because we charge at a steady and measurable rate.
Another successful fill. Glass was flowing during daylight hours and the forming department was available to control the drains.
