This invention relates generally to glass processing equipment and more particularly pertains to a forehearth furnace for use in glass processing for the purpose of delivering a flow of molten glass from a melting furnace to a spout and thence to subsequent forming equipment.
The typical forehearth furnace comprises an elongated trough for supporting a bed of molten glass. Above the trough is an elaborate roof structure which, in combination with the trough, defines a longitudinal tunnel for processing the molten glass through the forehearth. Such a forehearth includes means for generating heat energy to the molten glass which may comprise a series of gas burners and/or electrodes along the length of the tunnel to directly heat the molten glass in the trough. Such a forehearth further includes a means for removing heat from the molten glass. It is through the appropriate disposition and control of the heating and cooling means that temperature gradients in the molten glass are reduced whereby the desired viscosity and other desired physical and chemical properties of the glass are maintained.
The existence of an extreme temperature gradient between the molten glass along the center line of the forehearth and its outer edges has long been recognized. For well over fifty years the glass industry has been aware that the capability to induce heat energy along the opposite sides and to remove heat energy from the longitudinal center portion of the molten glass in a forehearth is critical in obtaining improved temperature uniformity. U.S. Pat. No. 1,893,061, issued in 1933, teaches the provision of improved means for cooling glass in the longitudinal median portion of a forehearth while applying heat along the side portions of the glass. The foregoing concept was expanded upon in U.S. Pat. No. 3,582,310, issued in 1971, wherein a heating means is arranged longitudinally along the two side portions of the glass, and the glass along the center line of the forehearth is cooled by removing heat radiated upwardly to a roof structure component which is cooled from above by a transverse pressurized air flow. This patent also teaches the concept of heat removal on the underside of the trough by means of a longitudinally-extending duct supporting a flow of pressurized cooling air therethrough.
An improved forehearth construction was thereafter developed, particularly as disclosed in U.S. Pat. No. 3,999,972, issued in 1976, wherein the general tunnel area above the molten glass in the forehearth was separated into a substantially large cooling area directly above the longitudinal center of the glass and oppositely-disposed longitudinally-extending firing chambers or heating areas directly above the side portions of the glass adjacent the longitudinally-extending trough edges. Representative of more recent attempts to further improve the efficiency of heat removal from the longitudinal central portion of the molten glass in a forehearth is the structure disclosed in U.S. Pat. No. 4,552,579, issued in 1985. In this patent, the desirability of eliminating direct contact of the overhead cooling air with the glass stream therebelow is recognized, and provision is made for improved heat-conductive structure between the central cooling area directly above the glass and the longitudinal cooling channel in the roof structure which carries a continuous flow of cooling pressurized air for removing the heat energy and exhausting it to the atmosphere.
Despite the significant advancements introduced by developments in the structure of forehearths, such as those to be found in the aforementioned patents, there remains a need for improving both the efficiency and control of heat energy generation into, and heat energy removal from, the molten glass in a forehearth.
In the current state of the art of glass processing forehearths, operational efficiency has been enhanced by structural compartmentalization which prevents heat-removing air flow from intermixing with the by-products of combustion in the area immediately above the molten glass, and significant efforts have been made to improve the heat transfer from the cooling area immediately above the longitudinal central portion of the glass to an overhead longitudinal air flow cavity.
Insufficient attention, however, has thus far been given to the tendency of known forehearth structures to inherently preheat the air flow which moves through roof cavities to perform its cooling function. Whether the flow of air is longitudinal or transverse, the various components of the entire roof structure are so hot during operation of the forehearth that the incoming pressurized air, meant to pick up heat primarily from the central overhead partition above the molten glass, is preheated from roof component contact. The ability of any given volume of air to accept the transfer of heat energy from the central area is greatly diminished by the already increased temperature of the air volume arriving at the central area.
A collateral heat-conducting problem also exists in current forehearth constructions with regard to the means of generating heat energy into the molten glass at the oppositely-disposed longitudinal trough sides or edges. As with the roof structure, the entire trough and its support structure, regardless of composition, acts as a gigantic heat sink, and much of the heat which passes from the heating means to the molten glass, immediately downwardly adjacent the oppositely-disposed longitudinally-extending firing chambers is immediately dissipated into the trough sidewalls. This is particularly disadvantageous since the goal is to increase, not decrease, the temperature of the side portions of the glass. Improved homogeneity of the temperature of the molten glass can only be attained by cooling the molten glass along the hearth center line and increasing the temperature of the side portions.