Published on Feb 21, 2020
In simplest terms, making glass consists of mixing a batch of dry ingredients, heating the batch until molten and uniform in composition, forming the molten glass into the desired shape, and annealing, (i.e., reducing temperature of the product in a controlled manner to minimize residual stresses).
A representative composition for soda-lime glass (90 percent of all glass produced) is 56 percent sand (silica), 18 percent each of soda ash (sodium carbonate) and limestone (calcium carbonate), 7 percent feldspar, and 1 percent additives. In addition, anywhere from 10 to 50 percent, but typically 15 to 20 percent, of the starting material is recycled glass, depending upon the availability of material of suitable composition.
Additives such as boric acid, potassium carbonate, and alumina aid in making a uniform product without defects and in imparting specific properties or color to the glass.
Glass types other than soda-lime glass include lead-alkali silicate glass (used for TV picture tubes and lead crystal) and boro-silicateglass (used for ovenware and laboratory glassware).
The equipment used for melting may be as simple as a ceramic lined pot filled with a batch in the evening, melted overnight in a gas-fired furnace, and drawn upon during the day as the source of glass gobs that are blown and shaped into desired specialty products.
Or, at the other extreme, the equipment may consist of a large, multi-story structure with a glass melting area of a thousand square feet. These furnaces are continuously fed with material delivering a stream of up to several tons per hour of flowing glass.
These large operations use automated machinery for mixing and delivering the raw materials and for drawing, forming, annealing, and handling the glass product taken from it. A melting operation may continue steadily for a few years until the condition of the furnace indicates its refractory lining is no longer serviceable.
The production rate, furnace size and "pull rate" are related considerations in the physical sizing of an electric glass melting furnace. The pull rate is the melting surface area in the furnace per daily ton of production. A furnace is sized by the required production rate and an estimated pull rate. Pull rates for electric melters are lower than those for fuel-fired furnaces. Thus for the same production rate, electric glass melting furnaces are smaller than fuel-fired furnaces. Power requirements for an electric glass-melting furnace can be calculated using the energy required to melt and refine the specific type of glass being produced and appropriately accounting for heat losses due to furnace losses. Glass composition can influence the shape of the furnace because various molten glass viscosities require different furnace designs to optimize heat transfer within the molten glass bath.
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