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Electrical Forhearths with direct heating
 

Direct electrical heating refers to the use of electricity and immersed electrodes in forehearth channels. The glass bath is used an a resistance and the energy is developed directly in 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 lead-containing glasses.
Installations for containers and similar applications utilise 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.
 

 

Advantages

Direct all electric forehearth heating with immersed electrodes offers a number of advantages, including :

- reduced surface losses from glasses with volatile components
- lower energy consumption than equivalent gas heated  forehearths
- larger energy turn-down ratio than gas heated installations
- better thermal homogeneity in coloured glasses
 

 
Application

This technology is used for all electric forehearths for the following types of production :

- soda lime containers, in coloured glasses
- fluoride opal containers
- C glass for insulating fibre
- Pyrex® borosilicate glass.
 

Electrical Forhearth with indirect heating
 

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 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.
 

 
Advantages

Indirect all electric forehearth heating utilising radiant heating elements offers a number of advantages , including:

- Higher energy efficiency than gas heated installations
- no negative influence on glass quality, irrespective of glass composition
 

 
Application

This technology is used for all electric forehearths for the following types of production :

- containers, in all colours
- lighting ware
 

Electrical forhearth combined with 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.
 

 
Advantages

Combined direct and indirect electric forehearth heating systems offer more flexibility than just electrode systems, especially in the case of enclosed channels.
 

 
Applications

The combination of direct and indirect electrical heating is typically used for :

- lead crystal glass
- Pyrex® type borosilicate glass