Hot blast

Blast furnace (left), and three Cowper stoves (right) used to preheat the air blown into the furnace

Hot blast furnace: note the flow of air from the two stoves in the background to the blast furnace, and hot air from the furnace being drawn off to heat the stoves

Hot blast refers to the preheating of air blown into a blast furnace or other metallurgical process. As first developed it worked by alternately storing heat from the furnace flue gas in a firebrick lined vessel with multiple chambers, then blowing combustion air through the hot chamber. This is known as regenerative heating. This has the result of considerably reducing the fuel consumed in the process. Hot blast was invented and patented for iron furnaces by James Beaumont Neilson in 1828 at Wilsontown Ironworks in Scotland, but was later applied in other contexts, including late bloomeries. Later the carbon monoxide in the flue gas was burned to provide additional heat.

Hot blast was the single most important advance in fuel efficiency of the blast furnace and was one of the most important technologies developed during the Industrial Revolution. Hot blast also allowed higher furnace temperatures, which increased the capacity of furnaces.

History

Invention and spread

James Beaumont Neilson, previously foreman at Glasgow gas works invented the system of preheating the blast for a furnace. He found that by increasing the temperature of the incoming air to 300 degrees Fahrenheit, he could reduce the fuel consumption from 8.06 tons to 5.16 tons with further reductions at even higher temperatures. He with partners including Charles Macintosh patented this in 1828. Initially the heating vessel was made of wrought iron plates, but these oxidized, and he substituted a cast iron vessel.

The spread of this technology to other parts of Britain was relatively slow. However by 1840, 58 ironmasters had taken out licenses, yielding a royalty income of £30,000 per year for Neilson and his partners. However they had to engage in substantial litigation, ultimately successful, over the following years to enforce the patent against infringers. By the time the patent expired there were 80 licenses. In 1843, just after it expired, 42 of the 80 furnaces in south Staffordshire were using hot blast, and uptake in south Wales was even slower.

Other advantages in using hot blast were that raw coal could be used instead of coke and in Scotland, the relatively poor "black band" ironstone could be profitably smelted. It also increased the daily output of each furnace, in the case of Calder ironworks from 5.6 tons per day in 1828 to 8.2 in 1833, which made Scotland the lowest cost steel producing region in Britain in the 1830s.

Nevertheless, early hot blast stoves were troublesome, as thermal expansion and contraction were liable to cause breakages in the pipes. This was to some extent remedied by supporting the pipes on rollers. It was also necessary to devise new methods of connecting the blast pipes to the tuyeres, as leather could not longer be used for making the connection.

Ultimately this principle was applied even more efficiently in regenerative heat exchangers, such as the Cowper stove (which preheat the incoming blast air using waste heat from the flue gas and are used in blast furnaces to this day), and in the open hearth furnace (for making steel) by the Siemens-Martin process.

Independently, George Crane and David Thomas, of the Yniscedwyn Works in Wales, conceived of the same idea, and Crane filed for a British patent in 1836. They began producing iron by the new process on February 5, 1837. Crane subsequently bought Gessenhainer's patent and patented additions to it, controlling the use of the process in both Britain and the U.S. While Crane remained in Wales, Thomas would move to the U.S. on behalf of the Lehigh Coal and Navigation Company and found the Lehigh Crane Iron Company to make use of the process.

Anthracite in ironmaking

Hot blast allowed the use of anthracite in iron smelting. It also allowed use of lower quality coal because less fuel meant proportionately less sulfur and ash.

At the time the process was invented, good coking coal was only available in sufficient quantities in Great Britain and western Germany, so iron furnaces in the U.S. were using charcoal. This meant that any given iron furnace required vast tracts of forested land for charcoal production, and generally went out of blast when the nearby woods had been felled. Attempts to use anthracite as a fuel had all ended in failure, as the coal resisted ignition under cold blast conditions. In 1831, Dr. Frederick W. Gessenhainer filed for a U.S. patent on the use of hot blast and anthracite to smelt iron. He produced a small quantity of anthracite iron by this method at Valley Furnace near Pottsville, Pennsylvania in 1836, but due to breakdowns and his illness and death in 1838, he was not able to carry the process into large-scale production.

Anthracite was displaced by coke in the U.S. after the Civil War. Coke was more porous and able to support the heavier loads in the vastly larger furnaces of the late 19th century.

Steel

For steel the hot blast temperature can be from 900 °C to 1300 °C (1600 °F to 2300 °F) depending on the stove design and condition. The temperatures they deal with may be 2000 °C to 2300 °C (3600 °F to 4200 °F). Oil, tar, natural gas, powdered coal and oxygen can also be injected into the furnace at tuyere level to combine with the coke to release additional energy which is necessary to increase productivity.