Hot blast is a technique used in metallurgical operations to increase the efficiency of the refining process by recycling exhaust gases to preheat air blown into a blast furnace. This reduces the amount of fuel needed and speeds up production, resulting in cost savings. The hot air is injected into the furnace through nozzles called tuyeres.
A hot blast refers to preheated air being blown into a blast furnace. The technique is used in metallurgical operations such as steelmaking to increase the efficiency of the refining process. It works by recycling exhaust gases that would otherwise have to be discharged. Combustion of these flue gases to preheat the air reduces the amount of fuel needed in the blast furnace and reduces the cost of operations.
Metalworking typically uses a blast furnace to convert raw solids such as ore into purified liquid metals. High carbon fuel, known as coke, is used to heat the ore in order to reduce it to its constituent components so that the purified metal can be separated. Hot gases are produced as a byproduct of this process.
There are two methods of dealing with exhaust gases. One option is to clean them of any environmentally hazardous substances and get them out of the system. However, this results in a considerable waste of energy. Instead, the flue gases can be piped to a special stove where they are burned to pre-heat the air which is subsequently fed to the main blast furnace.
Three or four hot air stoves are typically lined up along the vent to the main blast furnace. At any given time, one stove is generally running while the others are being brought up to temperature. The unheated air, known as cold air, is blown with a turbofan through the stove, where it is reheated.
Once heated, this air leaves the stove as a hot jet and is driven into the main line of the blast furnace. If necessary for temperature regulation, cold air can be added via a mains-connected mixing line. A valve controls the release of the cold jet.
Before entering the main chamber of the blast furnace, the hot blast enters a tube that surrounds the structure. This tube has several equally spaced nozzles from which the hot jet is injected into the furnace. These nozzles are known as tuyeres and there may be 15 to 40 of them around the circumference of the blast furnace, depending on its size.
The hot air in a blast furnace, sometimes supplemented with natural gas, burns together with the coke to reach the temperature needed to process the ore. Preheating this air improves combustion efficiency and reduces the amount of fuel required in the main blast furnace. It can also speed up the production of the final machined metal. This produces significant cost savings by recycling energy that would otherwise be wasted.
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