The spokespersons of the different furnace manufactures have various of beliefs and opinions about the convection capability in the furnace models they represent. Forced convection is used together with radiation to intensify heat transfer even for clear glass. Forced convection is arranged with air jets, which are focused to impinge on the glass surface. With forced convection, heating of the coated top side of the glass can be intensified to keep the thickness-wise temperature profile symmetrical, i.e. Low-e coated glass surface can reflect as much as 96% of the thermal radiation emitted by a furnace’s hot surfaces. Such furnaces are becoming very rare on the market due to demands related to tempering of low-e coated glass. In the simplest tempering furnaces, the heating of glass is based on thermal radiation exchange between resistors and glass, and in addition to radiation also natural convection and contact heat transfer from rollers to glass occurs. The capability of a furnace to approach such an ideal heating process clearly depends on the furnace model. just before quenching, the glass temperature should be the same throughout the whole glass surface area, and the same in all glass sheets in a glass loading in a furnace. In an ideal heating process, the heat transfer rate to glass is the same via the top and bottom sides, so that the thicknesswise temperature profile generated to the glass during heating is symmetrical. During the tempering process, glass plates are heated to about 640☌ and then cooled by air jets at a cooling rate depending on glass thickness. In a tempering furnace, glass is placed on top of rotating ceramic rollers. Glass tempering is a process in which the strength of float glass is improved with heat treatment. In this paper, the basics of the heat flux modes are given, a solution method for the glass energy equation is introduced, and some tempering furnaces are theoretically studied to show the portions of the heat transfer phenomena inside furnaces during heating. There is quite a wide range of different tempering furnace models on the market with differing devices for implementing radiation and convection heat transfer. On the lower surface, also contact heat transfer between the glass and rollers takes place. Convection heat transfer between hot air jets and the glass affects both glass surfaces. In a tempering furnace, glass is placed on top of rotating ceramic rollers, where it is exposed to radiation emitted by radiant heaters, rollers and other surfaces inside the furnace. This paper was first presented at GPD 2019 by Dr.
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