Why do we sometimes use forced convection for the cooling of soda lime glass and in other cases not? Some operators and managers would answer that it has always been like that. Nonetheless physics can explain why and how the industrial evolution brought us to this practice.
Large cellular glass ribbons (up to 4m) are annealed by radiation cooling. Cellular blocks, foamed in a mold like a cake, on the other hand, are cooled by forced convection. Why do we use a different cooling process although both glass types have almost the same density and soda lime glass composition? Hollow glass ware is also annealed by forced convection, while flat/float glass needs only radiation cooling down to about 400°C. And yet again the soda lime glass composition is similar. In case of thermal tempering, extreme forced convection is used on these articles.
The geometrical shape of the glass determines the cooling process
In case of annealing, radiation to a cold surface extracts more than enough heat to cool off a surface off 10 kg/cm². Therefore, flat/float glass does not ask for forced convection. This explains why the best annealing furnaces for flat/float glass are functioning with radiation cooling down to at least 400°C. In case of hollow glass, it is geometrically not possible for the complete surface to radiate to a cold surface because annealing happens vertically. As a consequence, forced convection is the only solution to homogeneously cool a hollow glass article. In case of a cellular glass block, annealed vertically, we also need to cool the surface fast to the annealing point. This avoids that the glass block, which is still hot inside, bends due to gravity. In case of continuous foaming, horizontal annealing with only radiation cooling suffices because gravity effects are eliminated.
Lehrs with several control zones guarantee high glass quality
The CNUD EFCO flat/float glass lehrs are equipped with several control zones across the ribbon, under and above the glass plate. That makes it possible to install a small temperature gradient over the ribbon in order to secure a small compressive stress at the edge of the glass and to control bow and counter bow. This compressive stress prevents a (longitudinal) crack to occur. With radiation cooling to a cold surface it is, in opposition to forced convection, easy to install a small temperature gradient. In a CNUD EFCO lehr, this cold surface is created by transporting cold air through tubes and boxes. Below 400°C, the effect of the radiation cooling becomes too small and forced convection is needed to reduce the length of the lehr. In case of tempering, we need an extreme cooling rate starting above the annealing point. In this case forced convection is the only suitable solution.
Read more about our float glass lehrs.