The cupola should have a blast gage attached to the wind box to measure the pressure of air which enters the tuyeres. The pressure should be sufficient to force the air into the middle of the cupola to insure complete combustion. The unit of air pressure is 1 ounce. From 8 to 16 ounces is approximately the range usual in cupolas of from 48 inches to 70 inches diameter, inside lining.
This pressure is measured by the displacement of water or mercury in a U-shaped tube. With both legs of the tube the same size, as in A, Fig. 127, the graduations represent the pressure of double that height of liquid. Such graduations would be as follows:
With a water gage, a difference in levels of 1.73 inches corresponds to 1 ounce wind pressure, so that the scale graduations per ounce would be spaced 1.735 55 7 1 2 = .865 = 64 = 8in. - 64 in.
With mercury, a difference in levels of 0.127 inch corresponds to a pressure of 1 ounce so that the scale graduations would be spaced 0.127 = .0635 in. = 1 2 16 in.
As this last spacing would be too small for practical use, mercury gages, as at B, Fig. 127, are made with an increased area exposed to the blast pressure, and are graduated accordingly.
Combustion cannot take place without oxygen, of which the air is the most abundant source of supply. For example, in the incandescent electric light, a strip of carbon is heated to a white heat, but it does not consume, or burn up, because all air has been exhausted from within the globe.
In the cupola furnace, both coal and coke are used as fuel. They consist largely of carbon, and, after being lighted by the kindlings, are kept at a glowing red heat by the natural draft through the open tuyeres. The blast supplies the oxygen necessary for a melting heat. The quantity of air forced in by the blast cannot be entirely taken up by the layers of fuel immediately above the tuyeres; thus, complete combustion does not take place until a distance of 18 to 23 inches above the tuyeres is reached. This is termed the melting zone. It is the aim of the melter to keep the top of his bed as nearly as possible at this level, so that the iron resting on it shall be exposed to this intense heat and melt rapidly. As the fuel of the bed burns away, this level tends to be lowered. But the iron on top of it melts and drops to the bottom of the cupola; and the subsequent charge of coke restores the level of the bed for the next charge of iron; and so on.
Fig. 127. Wind Gages.
Running a Heat The following routine must be pursued each time a heat is run off in the cupola:
(1) Clear away the dump from the former heat.
(2) Chip out the inside of the furnace with a special hand pick, removing the lumps of slag which collect about the lower part of the cupola walls, especially above the tuyeres. Where the slag coating is comparatively smooth, do not touch it, as that is the best coating possible for the lining.
(3) Daub up with a mixture of fire sand, held together with about 1:4 fire clay, and, wet with clay, wash to a consistency of thick mortar. Smear the surface to be repaired with clay wash; then, using the hands, plaster the daubing mixture into the broken spots in the lining, being careful to rub it in well, especially about the tuyeres. The top of the tuyeres should be kept slightly overhanging.
The greater part of the daubing will be required from the bottom to the level of melting zone, about 22 inches above tuyeres.
(4) Swing up the bottom doors, and support them by a prop of gas pipe.
(5) Build the bottom; first cover the doors with a 1-inch layer of gangway sand or fine cinders; then ram in burnt sand tempered about the same as for molds. This must be rammed evenly all over the bottom, and especially firm around the edges. The bottom should be made flat and level from side to side, with only a slight rise around the lining which should not extend more than 1 or 2 inches from the lining. The pitch varies with size of cupola; 1 inch to the foot will answer for cupolas of 24 inches to 30 inches diameter inside lining, while one-half that pitch will do for the larger furnaces.
The cupola bottom should be able to vent so that it will dry out quickly, and not cause the metal to boil before the furnace is tapped. It should be strong enough to hold its surface during the heat, but to break and drop at once when the bottom is dropped. Too much pitch causes excess of pressure on the bott, making trouble in botting up; with too little pitch the metal will not drain well, causing a tendency to chill at the tap hole. A little daubing mixture should be worked into the sand bottom just inside the tap hole, to prevent breaking at this point when the tapping bar is forced through.
(6) Lay the fire with shavings first, just inside the breast; then with fine kindling; then with enough large kindling to make sure of lighting a layer of coke sufficient to form the bed. When the rases from the lower part of the bed burn up through, showing that the fuel is well lighted, level up the bed with the addition of a little more coke.
(7) The first charge of iron should he put on now. Follow this with alternate charges of fuel and iron, to the level of charging door.
(8) Form the tap hole; lay a bar of iron about $ inch round in the spout, projecting in through the breast opening; fill in the breast around the bar with a strong loamy molding sand rammed hard. Recess this in well to leave the actual tap hole as short as possible.