Fig. 4.—Sectional diagram of a regenerative pot furnace working with covered pots.
As has already been indicated, glass is melted either in pots or crucibles of various shapes and sizes, or in open tank furnaces. The general arrangement of a pot furnace working with closed or “covered” crucibles is shown in [Fig. 4]. In this particular furnace, the “ports” or apertures by which the gas and air enter the furnace chamber, are placed in the floor of the chamber, but these apertures are often placed in the side or end walls, or even in a central column, the object being in all cases to heat all the pots as uniformly as possible and to avoid any intense local heating, which would merely endanger the particular crucible exposed to it, without greatly aiding the real work of the furnace. In pot furnaces, however, in which the more refractory kinds of glass are to be melted, it is generally considered desirable that the flame should be made to play about the pots in such a way as to heat the lower parts of the pots most strongly. In connection with the question of the uniformity of heat distribution in a gas-fired furnace it must further be borne in mind that in the case of regenerative furnaces the direction of the flame is reversed every time the valves are thrown over, and in practice this is done about once every half-hour; this proceeding, of course, tends very much to equalise the temperature of the two sides of the furnace. In recuperative furnaces, on the other hand, the direction of the flame is not changed, and for that reason a flame returning upon itself, usually called a horse-shoe flame, is often employed; this is obtained by placing the entry and exit ports side by side at one end of the furnace; the impetus of the flame gases and their rapid expansion during combustion carry the flame out across the furnace, while the chimney draught ultimately sucks it back to the exit ports, the shape of the flame being shown in [Fig. 5].
Fig. 5.—Diagram of a furnace with “horse-shoe” flame.
In general arrangement, a tank furnace for glass melting resembles an open-hearth steel furnace. The tank or basin, as already indicated, is built up of a number of large fire-clay blocks, forming a bath varying in depth from 20 in. to 42 in. according to the design of the furnace and the kind of glass to be melted in it. The ports for entry of gas and air and for exit of the products of combustion are in most modern furnaces placed in the side walls of the furnace just above the level of the glass, the whole being covered by a vault built of silica brick. [Figs. 6] and [7] show the general arrangement of a simple form of tank-furnace such as that used in the manufacture of rolled plate glass. The furnace indicated in the diagram is intended for regenerative working with alternating directions of flame; in recuperative furnaces the horse-shoe flame is always used in tanks, while this arrangement of ports is sometimes adopted for regenerative tanks also, particularly in the manufacture of bottles. For the production of sheet glass, tank furnaces are generally sub-divided into two compartments and are also provided with various constrictions intended to arrest impurities and to allow only clear glass to pass, but as regards the arrangement of flues and ports there is a very general similarity between various furnaces of this type.
Fig. 6.—Longitudinal sectional diagram of tank furnace.