Hot and Cold Blast
When the stream of air forced through a furnace is drawn direct from the atmosphere, it is called cold-blast; when it is heated to 500 deg. before it enters the furnace, it is called hot-blast. The combustible gases which come from the stack are invariably used to heat the air in a kind of oven built near the top of the stack, and surmounted by a chimney which draws off some portion. In this oven a series of pipes are built, around which the fire plays whilst the air is being forced through them before it enters the furnace. A considerable saving of heat is effected by this method, the reduction of the most refractory ores being accomplished in less time and with a less expenditure for fuel than the cold blast. As the melting metal necessarily comes in contact with less fuel, and as a less quantity of air enters the furnace, the chemical reactions are somewhat modified, but there does not seem to be any appreciable difference in the quality of the product. See BLAST-FURNACE; BLAST.