In general all electric furnaces with a cold top are used typically for a production range of 5 – 80 t/d. It is possible to increase the melting capacity up to 200 t/d. More capacity is possible with sufficient developing time. For this furnace type energy is not supplied by means of fossil fuels, but exclusively by means of electric energy supplied by molybdenum electrodes.
Additional heating by means of oxyfuel firing can be installed in most cases, in order to create a hybrid furnace with increased flexibility and pull.
It is possible to use rod electrodes with specially developed, water-cooled electrode holders or block electrodes. The electrodes can be installed in the furnace bottom or in the lateral walls of the tank or inserted from the top.
Appropriate positioning and wiring of the electrodes lead to reduced corrosion of the refractories and thus increase the furnace life. The most efficient concept is usually the usage of top electrodes. In case of a cold top furnace the batch is supplied by an area batch charger at an open side wall of the top. Alternatively a rotary roof can be used, which produces a homogenous and even batch layer. This layer insulates the glass bath from the environment, making additional insulation unnecessary. The melting, refining and homogenising processes in all electric furnaces are effected vertically.
All electric furnaces are not controlled by temperature but rather by electric resistance and batch thickness. There is a very delicate balance between these two measures making the furnace less flexible in respect to pull changes. The flexibility can be improved by means of furnace design.
The specific pull is higher and not comparable with fossil fuel fired furnaces due to vertical melting process. The glass quality can be drastically higher than in a fossil fuel fired furnace. For this reason, high quality specialty glass, e.g. for optical application, is molten with all electric furnaces.
The energy consumption of all electric furnaces is lower than the one of fossil fired furnaces. This is especially true for small furnaces, making small furnaces even more cost efficient. Increasing costs for CO2 allowances can be a deciding factor for all electric furnaces despite higher costs for electric energy in the near future.