4. The Cupola, or small blast furnace, is the form of apparatus which is in use in the vast majority of foundries throughout the world, and which is, on the whole, the cheapest and the most convenient arrangement for remelting cast iron for foundry purposes.

Cupolas are generally circular in section; they are lined with ganister, fire-brick or other non-conducting refractory material; and are driven with low pressure blast at or about the atmospheric temperature. Hard coke is generally used for fuel, though occasionally charcoal or gaseous fuel is employed. A small quantity of limestone is usually added, as it fluxes off the sand adhering to the pigs, or the silica produced by oxidation of the silicon in the iron. It also combines with the ash of the coke, and tends to diminish the amount of sulphur which enters into the iron. The blast is driven either by a fan, or more generally by a Roots or other positive blower, the pressure being measured in ounces per square inch. The blast is generally introduced through twyers situated round the zone of fusion.

Varieties of Cupolas.

Cupolas vary from an old boiler tube, lined with a ramming of sand, and fitted with but one twyer (an arrangement which answers sufficiently well for small temporary purposes), to larger and more complex structures capable of melting up to 500 tons per day. Much ingenuity has been expended in the construction of various modified forms of cupolas, the object generally being the reduction of fuel consumption, and the production of a more uniform iron.

The different kinds of cupolas in actual use, or which have been proposed from time to time, are legion. In 1887 M. A. Gouvy, in a paper on cupolas, described no less than thirty-three different kinds, most of which were German. This list did not include the majority of types used in the United Kingdom.

Cupolas may be conveniently classified according to internal shape and arrangement of the hearth; the method of introduction of the air supply; and the kind of fuel employed. Taking these characters in order and dealing first with the internal shape, cupolas may be divided into three kinds.

(1) The old solid bottomed type, which is represented in fig. 21, and which is in pretty general use in small foundries in the United Kingdom. It has the advantage of being cheap and easy to work, but is only to be recommended for small

outputs. The charging door, charging platform, tapping
hole, blast main, blast pipes, and
twyers are sufficiently indicated
on the diagram.

(2) Drop bottom cupolas are generally
employed with large outputs in
the United Kingdom and are in
special favour in America. The
advantage of this system is, that
when melting is finished the
bottom can be readily removed
and the cupola cleaned and re-
paired. A cupola of this type is
shown in fig. 22.

(3) Cupolas with a receiver, of which the Stewart's "Rapid" is a well

known type, are in use in many foundries in the United Kingdom. Such a cupola is illustrated in fig. 23. These

furnaces are driven with a forced draught in the ordinary way, and the special point is that they are provided with a separate receiver in which metal can be allowed to accumulate so as to obtain an iron of special uniformity in such quantity as may be required. In cupolas which are not provided with a separate receiver the amount of metal which can be accumulated at one time is determined by the vertical space between the bottom of the hearth and the point at which the air enters. By a suitable size and shape of hearth many of the advantages of a cupola with a receiver can be obtained without having a separate outside vessel. When regarded from the point of view of the method whereby the air is supplied, cupolas may also be divided into three kinds. (1) Cupolas with one row of twyers. In ordinary practice the air enters under pressure, through openings placed in a horizontal plane around the sides just above the hearth. Various attempts have been made to heat the air, or to inject fuel with the air, but these have not been successful. The reason no doubt is, that the combustion which takes place in a cupola is essentially different in character from that occurring in a modern blast furnace, where, it will be remembered, the air is almost instantaneously converted into carbon monoxide on entering the hearth. In the cupola, on the other hand, carbon dioxide is first produced, and this is converted into monoxide higher up in the cupola on coming in contact with heated coke. As the carbon monoxide is not wanted for any reducing action, any excess burns to waste at the throat of the cupola.

(2) Cupolas with more than one row of twyers.—In Ireland's cupola,

which was introduced about 1860,1 two rows of twyers were employed, and the cupola was provided with boshes like a blast furnace. Such a form is found to give very satisfactory results with large outputs, the introduction of air higher up in the cupola being advantageous, as it leads to the more complete combustion of the carbon monoxide produced as above explained. The Greiner and Erpf cupola is of this type, and has met with much favour in Germany; it is provided with a circle of five or six twyers around the melting zone, while about 12 or 18 small twyers, each about 1 For illustrated description, see F. Kohn, Iron and Steel Manufacture, p. 53.

one inch in diameter, are placed around the furnace higher up in a spiral form.

(3) Suction Cupolas.-In other cases the air for combustion, instead of being forced in under pressure, is aspirated by means of a steam jet arranged in a tube or chimney connected with the cupola. The Woodward cupola which was introduced in 18601 is of this type; and more recently the Herbertz cupola, which is of the same class, has met with considerable favour in Germany. In cupolas of this construction the air is admitted through a circular slit or ring around the melting zone of the cupola.

Fuel Used in Remelting.

It is unnecessary to deal in detail with the classification of cupolas according to the kind of fuel they employ, since, as already stated, coke is almost universally adopted, though exceptionally charcoal, gaseous fuel, or even oil may be employed.

It

The coke most generally used is oven coke of good quality, what is known as 72-hour coke being preferred in the United States. should be bright and hard, low in sulphur, and should contain as little ash as possible. Retort oven coke, i.e. coke made in byeproduct ovens, gives good results in the cupola, if it is free from excess of ash and sulphur. The question of strength is not so important in cupola practice as in blast furnace work. The consumption of fuel varies according to the efficiency of the cupola, the care of the melter, and more particularly with the quantity of iron which is melted per day. With good average work in the United Kingdom about 2 cwts. of coke are required per ton of pig iron. This may be increased to more than twice as much with small outputs, while with especial care and large yields as low a fuel consumption as 14 cwts. per ton of pig iron is sometimes recorded. In the United States it is usual to return the fuel consumption or 66 melting ratio in lbs. of iron melted per 1 lb. of coke, and in practice it is found that 1 lb. of coke should carry from 8 to 16 lbs. of pig iron. In some foundries considerable preventable waste of fuel occurs. One instance was brought under my notice some years ago where the coke consumption was over 12 cwts. per ton of pig iron melted. The gentleman who owned the foundry was attending 1 For illustrated description, see F. Kohn, Iron and Steel Manufacture, p. 53.