A hot blast refers to the stream of preheated air blown into a blast furnace to promote the combustion of coke and the reduction of iron ore. This technological innovation significantly increased the efficiency and productivity of ironmaking.
Purpose and Function The primary purpose of using a hot blast is to reduce the amount of fuel (coke) required to smelt iron ore and to increase the rate of production. By introducing preheated air, the energy needed to raise the temperature of the incoming air to the reaction temperature within the furnace is already supplied, leading to:
- Lower Fuel Consumption: Less coke is needed to achieve the high temperatures necessary for the chemical reactions.
- Increased Productivity: Higher temperatures lead to faster reaction rates, allowing more iron to be produced in a given time.
- Higher Furnace Temperatures: Facilitates more efficient reduction of iron oxides.
Production of Hot Blast The air used for the hot blast is heated in devices known as hot blast stoves, also called Cowper stoves (after Edward Alfred Cowper, who improved them). These are large, regenerative heat exchangers typically operating in a cycle:
- Heating Cycle: Combustion gases from the blast furnace (or other fuels) are burned inside the stove, heating a large checkerwork of refractory bricks.
- Blowing Cycle: Once the bricks are sufficiently hot, the combustion gas flow is stopped, and cold atmospheric air is blown through the heated checkerwork. The air absorbs heat from the bricks, becoming the hot blast, which is then directed into the blast furnace through a large pipe called the hot blast main and a series of smaller pipes called tuyeres. Multiple hot blast stoves are usually used in rotation, ensuring a continuous supply of hot air to the furnace.
Historical Significance The hot blast technology was invented by James Beaumont Neilson in 1828 in Scotland. Prior to this, blast furnaces used cold air, which was far less efficient. Neilson's invention dramatically reduced the cost of iron production, allowing for the use of raw coal (which could be coked more easily with the increased heat) and leading to a significant expansion of the iron and steel industry during the Industrial Revolution.