Making Glass

GLASS MELTING FURNACES

There are two types of glass melting furnaces.

1. Pot Furnaces are structures built of refractory materials in which there is no contact between the furnace and the glass. Glass is melted in several pots made of refractory materials which are resistant to glass attack at high temperatures. The pots are charged with a batch, which is melted over a number of hours and worked on a 24 or 18 hour cycle. An average pot can hold 600-700 Kg of glass. Pot furnaces are used where the glass is formed by hand and mouth blowing. One of the main advantages of this system is that several types of glasses can be melted at the same time. A pot can be used for about 30 melting cycles and thus produce between 18 and 21 tonnes of glass.

Fuel economy is normally achieved by recuperation, i.e., the pre-heating of combustion air by waste heat from the furnace exhaust gases. In this system the pre-heating of the combustion air is done by passing the air through metal tubes on the outside of which the exhaust gases flow towards the chimney. Thus the heat exchange is continuous. Electricity can also be used for melting.

2. Tank Furnaces are used where continuous flow of glass is needed to feed automatic glass forming machines. They are more economical in their use of fuel and are used mainly for the large scale production of containers, flat glass, electric bulbs, tubing and domestic machine made tableware. A large float glass furnace can have a capacity of 2,000 tonnes.

A tank furnace consists of a bath, built of a very special high refractory material, which can resist chemical attack of molten glass at temperatures in excess of 1500^oC and a superstructure where combustion takes place. The quality of refractory materials, used for building the bath, has improved to such an extent that whereas some 30 years ago, the life of a furnace was well below 2 years, it is now over 9 years.

In order to achieve high melting temperatures and fuel economy, a regenerative or recuperative system is used. Both these systems utilise the waste heat of combustion for pre-heating the incoming combustion air.

While in the recuperative system the heat exchange between the combustion air and waste gases is continuous, in the regenerative system the waste gases are passed through a large chamber packed with refractory bricks arranged in a pattern which permits free flow of the gases. The brickwork is heated by the waste gases and after having been heated for some 20 minutes, the direction of firing is reversed. Combustion air is passed through the chamber and the heat thus collected in the brickwork is used for pre-heating the combustion air. The firing is thus from right to left, normally for 20 minutes, during which time the right hand generator is heated and so there is a reversal of firing every 20 minutes. The cycle time can be changed for best heat exchange results and modern furnaces have computer managed control systems, which adjust the time of firing in each direction to achieve the best heat exchange conditions.

Heavy fuel oil or natural gas is normally used for firing tank furnaces. Glass, being an electrical conductor at high temperature, can also be melted by electricity. However, electricity is far too expensive in the UK and is normally used to boost the output from a gas or oil fired furnace. Nevertheless, technological progress in electric melting has enabled the use of all electric glass melting furnaces even at the high cost of electricity.