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

Thermal momentum
Thermal momentum is one of the most difficult aspects of furnace operation. The total mass of molten glass
and hot refractories contributes energy to the melting process. This energy must be replenished by heat from
flame combustion and electric boosting sources. When tonnage changes, the equilibrium of the melt must be
re-established. For even modest pull rate changes (5 to 10 percent), these phenomena can occur over a
number of days. Actual versus expected energy input and stabilization of the glass bath temperature during
transition is understood by viewing operating conditions over a previous period of days. Instrumentation
using data storage and graphic presentation techniques helps the furnace operator better understand the
condition of the melter.
Product measurement
Seed count usually triggers the first concern in melting of flat, container and textile fiberglass. Batch or
refractory stone counts, which are reported as production loss percentage, or defects per 100 lb. of glass,
require investigation of the melting operation. Taken on a shift basis, specific tests or observations provide
feedback for controlling the melting parameters in the production area to measure quality of the final glass
product.
Thermal heat transfer
To begin thermal heat transfer in the melt, the combustion process generates a flame from which radiation is
directed to the melting batch and molten glass bath. Re-radiation from the refractory structure into the
melting system is acceptable if refractory service limits are not exceeded. Convection heat transfer by
physical contact with batch or molten glass can be effective, but higher gas product velocities lead to
entrainment of fine batch ingredients to the exhaust gases, which can cause refractory deterioration and air
pollution. To establish a low-velocity, high-luminosity flame away from the refractory structure but in
proximity to the melting process, furnace design must be integrated with operation procedure. Location of
the flame envelope controls heat transfer to the melting batch, provides the required thermal profile by
locating along the tank side wall and provides the heat necessary to refine the freshly formed glass.
Most operations rely on temperature profiles obtained by sensors within the furnace at the crown, in the glass
batch, through the bottom or sidewall flux or by surface readings with portable optical pyrometers of the
melter superstructure. These reference readings help to establish control parameters for consistent melting
and refining results, i.e., final glass quality.
Furnace hot spot
To establish and maintain a hot spot during furnace operation, most glass manufacturers universally accept
the system of furnace temperature gradient (or profile). In this system, hotter glass expands and rises to a
higher physical position in the melt and flows on the surface toward colder areas at lower elevations. The
batch piles are contained behind the hot spot, preventing partially melted glass from leaving the melting zone
by this surface flow. The furnace operator reads the temperatures along the length of the melter in the tuck
stones or breast wall blocks immediately after the firing is interrupted for a reversal. During fire offs of a
regenerative furnace, as temperature drops rapidly, the precise time and window of time allowed for thermal
averaging must be replicated precisely on each side for each reversal.
Electric boosting
With electric boosting systems, the furnace can provide energy directly to the molten glass batch, and the
evolution of gases, principally carbon dioxide from lime and soda ash, enhances heat transfer through the
batch and accelerates melting. Use of electric boost or bubblers increase both thermal and compositional
homogenization.
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