Using electricity is the most direct way to insert energy into glass. The HORN® electrical forehearth system works with direct or indirect heating and represents today’s most advanced technology in the areas of forehearth design for special glass applications. The refractory structure design is an up to date modern construction employing all today’s techniques to achieve optimum thermal homogeneity combined with minimum energy requirement.
High quality refractory material is used for channel blocks. All channel block joints are backed up with zircon mullite split tiles and are surrounded with suitably graded insulation material. The roof blocks of the superstructure sections are specially adapted to the heating and glass requirements. High alumina material is used for the superstructure.
The heating with electricity can be made directly into the glass with electrodes or indirectly by installed heating elements above the glass bath.
Direct electrical heating refers to the use of electricity and immersed electrodes in forehearth channels. The glass bath is used as a resistance and the energy is inserted directly into the glass bath.
The nominal specific current loading of the electrodes is very low, so that there is almost no wear of the electrodes. It is not therefore necessary to advance the electrodes during operation and no cooling of the electrode is required.
The most common electrode material is molybdenum. Molybdenum electrodes are suitable for coloured soda lime glasses, and for soft borosilicate C glass. These electrodes are installed horizontally through the channel side walls. Tin oxide electrodes are used for some applications, especially for glass containing lead.
Installations for containers and similar applications utilize short electrodes located at short intervals along the complete length of the channel, with the electrical current passing across the channel. Channels for glass fibre manufacturing operations use a smaller number of longer electrodes installed at greater intervals, with the current flow along the channel.
This heating system involves the use of radiant electrical heating elements installed in the channel superstructure.
There are two types of element which are commonly used. Silicon carbide elements are generally used in the form of rods, which can be installed horizontally across the channel, with the electrical connections on both sides. These elements can be used at temperatures up to about 1300 °C, and can be expected to operate for about 18 – 24 months.
Molybdenum Disilicide (MoSi2) elements (more commonly known as Kanthal Super) are produced as U-shaped elements, which are installed vertically through the superstructure roof. This material can be used at extremely high temperatures in excess of 1700 °C, and the elements can achieve a life of 4 – 5 years.
This technology is particularly useful for high quality production when no suitable gas supply is available.
Combined direct and indirect heating
Under some circumstances it is advantageous to combine direct and indirect electrical heating systems. Immersed electrodes are installed to provide direct electrical heating of the glass bath. Additional energy is supplied by radiant electrical heating elements installed in the superstructure. In the case of glasses containing volatile components the channel can be enclosed by a cover tile, and the radiant heating elements are installed above the cover tile.
The use of indirect electrical heating above the glass surface in addition to the electrodes in the glass increases the flexibility of the installation. If the power supplied by the heating elements is more than the heat losses of the glass bath to the superstructure then there is a net addition of energy to the glass. However, if the power applied is lower than the heat losses there is a net energy loss from the glass bath and a cooling effect is applied.