Colliau Cupola
The lower portion of the "Colliau" Cupola is composed of two sheet-iron shells, the inner shells being made very heavy and of the same size as the stack proper; the outer shell encircles the inner one and is made air tight, forming the wind-chest, which varies in size according to the size of furnace. In the outer shell are arranged two doors or shutters held in position by tap-bolts, also made air tight, which may be removed and again replaced after cleaning, should any dirt or slag accumulate in the wind-chest. Opposite each tuyere also is a sliding air-tight gate with peep-hole, and in the bevel top is furnished a brass nipple to connect hose from blast-meter into the wind-chest or chamber. The blast is introduced through two flanged openings, one on either side as shown, and reaches the melting point through two flanged openings, one on either side as shown, and reaches the melting point through two sets of six tuyeres each, arranged to concentrate the blast. The tuyeres are so constructed that the melted iron in its downward course cannot pass through them into the air-chamber.
The furnace, as a whole, is simple in its construction. There is no complicated machinery or parts to get out of order, and consequently does not require any more attention or repairs than a common cupola. Combustion means the process of burning, and usually consists in the union of the oxygen of the atmosphere with the constituents of the combustible substances. The combustion of coal is caused by this oxygen passing into a state of chemical union with the carbon and hydrogen contained in the coal; carbonic acid and water-vapor being formed thereby. In all ordinary cases of combustion the amount of heat set free depends upon the amount of oxygen brought into action, rather than on that of the body burned. Hence, the combustible which united with the most oxygen while burning gives off the most heat. Thus, hydrogen in burning, takes up weight for weight, three times as much oxygen as carbon does, and gives off three times as much heat as a consequence. The complete burning of a combustible body requires the consumption of the same quantity of oxygen whether the process be rapid or slow, the amount of heat being the same in both cases. From this it is seen that the intensity of the heat is governed by the rapidity of the combustion. Heat would be liberated from the burning of a pound of coal in ten minutes--six times as fast as if its combustion occupied an hour. This explains why the blacksmith blows his fires, and also why blowing-engines are employed at the cupola, blast-furnace, etc.; the time of combustion is shortened by their use, with a corresponding increase in the degree of heat obtained; besides, this extreme blast serves to expel from the fire all products of combustion which would retard it if allowed to gather therein. However, it may be borne in mind that too much air is detrimental to the burning process, as it serves to convey away the heat, thus cooling the fuel and hindering the rate of combustion.
It is thought by many, that the more air is forced into the cupola, the more rapid will be the melting. From the above it will be seen that this is true only to a certain extent. Time is required to elevate the temperature of the air supplied to the point that it will enter into combustion; if more than this is supplied it absorbs heat rapidly, reduces the temperature, and, as we have seen, retards combustion.
The importance of supplying the exact quantity of air, neither too much or too little, cannot be overestimated. See CHARGING THE COMMON CUPOLA; CUPOLA; BLAST-GAUGE; BLOWERS.