The cupola consists of a circular shell of boiler plate, lined with a double thickness of fire brick and resting on a square bedplate, with a central opening the size of the inside of the lining. This bottom is supported some 3 1/2 feet above a solid foundation, on four cast-iron legs. The bottom opening may be closed by cast-iron doors, which swing up into position, and are held so by an upright iron bar placed centrally under them. These doors, protected by a sand bed, support the charge during the heat, and drop it out of the furnace when all the iron has been melted. The legs curve outward and the doors are hinged as far back as possible to protect them as much as can be from the heat of this "drop".
Fig. 124. Section Through Modern Cupola Furnace.
Breast At one side, level with the bottom, is the breast opening, at which place the fire is lighted, and in which the tap hole is formed for drawing off the melted metal. The spout, protected by a fire-sand mixture, projects in front of the breast and guides the metal into the ladles.
On cupolas over 36 inches inside of the lining, a slag hole is provided, which is similar to the tap hole, and is placed opposite the spout and about 2 inches lower than the main tuyeres. Fig. 124 shows a section through a modern cupola furnace, and needs but little further explanation.
In lining the stack, the layer next the shell is usually made of boiler-arch brick about the size of regular fire brick. These are set on end, and should be fitted as tightly together as possible, and laid in a thin fire cement, made of very refractory fire clay and fine sharp silica sand. The object is to fill every crevice with a highly refractory material. Specially made curved fire brick can be purchased for the inside lining, although some foundrymen use the arch brick for this lining as well. The lining over the tuyeres is shaped to overhang them slightly, to prevent melted slag dropping into them during the heat. The lining burns out quickest about 22 inches above the tuyeres, at what is practically the melting zone. The angle shelves riveted to the shell, as seen in the illustration, allow this section of the lining to be renewed without disturbing the rest of the stack.
The oblong air inlets, called tuyeres, are placed about 12 inches above the bed, and connect with an air-tight wind box which surrounds the outside of the stack near the base. The tuyeres direct the blast into the fuel, increasing the heat sufficiently to melt the charge. In the wind box, opposite each tuyere, is an air-tight sliding gate with a peephole, which allows the melter to look directly into the furnace.
In the larger cupolas a second set of tuy&res is arranged about 10 inches above the main ones. They are used, when long heats are run off, to make up for loss of wind caused by the main tuyeres becoming partially choked by slag.
Per Hour (tons)
6 to 7
15 by 18
1/2 to 3/4
1 to 2
7 to 8
18 by 20
1/3 to 1
8 to 9
20 by 24
1 to 2
3 to 5
9 to 12
24 by 24
4 to 10
12 to 15
30 by 36
4 to 8
8 to 20
15 to 18
30 by 40
6 to 14
15 to 40
16 to 20
30 by 45
8 to 16
25 to 60
The height of the tuyeres above the bed varies with the class of work to be poured. Where the metal is tapped and kept running continuously and is taken away by hand ladles, as in stove-plate work, the tuyeres are as low as 8 inches or 10 inches above the bed; while in shops where several tons of metal may be required to fill one mold, the tuyeres are as high as 18 inches above the bed. The height of the spout above the molding floor also varies in the same way; for hand-ladle work it may be but 18 inches above the floor, while a height of 5 or 6 feet may be required to serve the largest crane ladles.
Several feet above the bottom, there is a door in the side of the stack, through which the stock is charged into the furnace. A platform or scaffold is constructed at a convenient level below the charging door, and all stock is charged into the cupola from this platform. It should be at least large enough to store the stock for the first two charges of fuel and iron.
Table III, prepared by Dr. Edwin Kirk, gives the approximate height and size of charging door and the practical melting capacity of cupolas of different diameters.
Blast for the cupola is furnished by either a fan blower or a pressure blower. Fig. 125 shows a modern fan blower, of which the blast wheel is detailed at A. The high speed of the blades forces the air, by centrifugal action, away from the center of the shaft. The casing is so designed that the blades cut off, as it were, at the top of the main outlet, the air being thus forced through the blast pipe. The current of air is continually being drawn into the fan through the central opening around the shaft.
Fan Diameter (inches)
Speed (revolutions per minute)
Wind Pressure (ounces per square inch)
Since air is very elastic, and the pressure in this case depends entirely upon the centrifugal action of the blades, should the tuyeres become clogged, the amount of air forced into the furnace will be reduced proportionately. On the other hand, it requires less power to operate the fan with reduced area of outlet than it does when the discharge is open free.
Fig. 125. Typical Fan Blower.
An idea of the speeds at which blowers should run may be obtained from Table IV.
In the pressure blower shown in Fig. 126, the action is positive, as will be seen from the sectional view A, Fig. 126. The wipers mesh into each other in such a way that they entrap a quantity of air and force it out of the opening.
The full quantity of air is therefore forced through the tuyeres at all times. In such case, the power necessary to operate the blower increases as the tuyeres become choked, and the excessive force of the blast, due to choked tuyeres, is hard on the lining of the cupola.