Glass Melting Technology: A Technical and Economic ... - OSTI

A.2. Manufacturing flexibility
Placed within a manufacturing facility, a glass melting system is connected to downstream product
forming, finishing, selecting, and packaging equipment. A costly capital investment, the furnace
refractory lining can operate continuously for five to 14 years while other structural components may
serve for decades. Within the operating lifetime of a furnace, a variety of business changes can challenge
the flexibility of a melting unit. A number of operating variables must be met within the lifetime of a
furnace.
• Color changes
The color of glass being produced might be changed for containers, float glass or tableware. Whether
using either a drain and fill or on-the-fly procedure, non-productive periods result. Size and
configurations of the furnace can influence productivity during color changes.
• Glass Chemistry
Glass properties are dictated by the glass chemistry, and the raw material sources of oxides can impact
furnace performance. Selection of refractories to optimize melting economics occurs at the time of
original construction, but changes in glass chemistry can affect furnace life or glass quality variables.
•Recycled cullet
Container and fiberglass insulation industries have increasingly used recycled glass cullet. Handling and
melting of cullet require changes in equipment and operating techniques. Large inventories of cullet are
maintained on-site so small changes in recycling content in existing melter configurations do not disrupt
glass quality or melting stability efficiencies.
• Changes in pull rate
Product size, forming techniques, and business (sales) conditions often require that a furnace expand the
range of glass outputs. Many furnaces have multiple forming lines that accept the glass. They may be
varied or idled for a variety of reasons. Furnace wear is dependent on time. When a furnace is in soak
(zero pull) for any period of time, the wear on the refractory lining is noticeably nearly equal to high pull
conditions. The glass moves and circulates both ways through the throat and wear is continuous, even
when the production machine is pulled. No pull other than bottom taps, and maybe a small stream at the
orifice occurs.
With regard to energy consumption, glass furnaces use an enormous amount of energy just to stay hot. A
color TV furnace squanders half the total energy in keeping the crown, flux and regenerators hot. A
massive amount of the refractories are at elevated temperatures. The heat required to reach those
temperatures can be calculated as mass times heat capacity and temperature difference. In addition, the
heat loss of, at best, 200 Btu/sq.ft. to over 1000 Btu per sq. ft. for crowns, up to 3000 sq.ft. crowns for
large TV and float furnaces, and higher for bottom and flux and all the breast walls, port walls and
regenerator walls, results in substantial heat loss. The water cooled throats or air cooling on walls and
throats and heat loss through electrodes (at least 10 times the walls with water cooling possibly 100 times
the walls, must also be factored in.
Furthermore, if product demand is elastic, rather than all you make can be sold, there is no optimum pull
rate. As pull rate increases, the life of the furnace decreases and down time for repair must be considered,
as well as cost of repair.
• Product quality
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