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Hot blast refers to the preheating of air blown into a
blast furnace A blast furnace is a type of metallurgical furnace used for smelting to produce industrial metals, generally pig iron, but also others such as lead or copper. ''Blast'' refers to the combustion air being "forced" or supplied above atmospheri ...
or other metallurgical process. As this considerably reduced the
fuel A fuel is any material that can be made to react with other substances so that it releases energy as thermal energy or to be used for work. The concept was originally applied solely to those materials capable of releasing chemical energy b ...
consumed, hot blast was one of the most important technologies developed during the
Industrial Revolution The Industrial Revolution was the transition to new manufacturing processes in Great Britain, continental Europe, and the United States, that occurred during the period from around 1760 to about 1820–1840. This transition included going f ...
. Hot blast also allowed higher furnace temperatures, which increased the capacity of furnaces. 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. Hot blast was invented and patented for
iron Iron () is a chemical element with symbol Fe (from la, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, right in ...
furnaces by James Beaumont Neilson in 1828 at
Wilsontown Ironworks The ruins of the Wilsontown Ironworks are located near the village of Forth in Lanarkshire in Scotland, approximately to the south east of Glasgow. The works were founded by the three Wilson brothers in 1779, and operated until 1842. The works ...
in Scotland, but was later applied in other contexts, including late
bloomeries A bloomery is a type of metallurgical furnace once used widely for smelting iron from its oxides. The bloomery was the earliest form of smelter capable of smelting iron. Bloomeries produce a porous mass of iron and slag called a ''bloom''. ...
. Later the
carbon monoxide Carbon monoxide ( chemical formula CO) is a colorless, poisonous, odorless, tasteless, flammable gas that is slightly less dense than air. Carbon monoxide consists of one carbon atom and one oxygen atom connected by a triple bond. It is the simpl ...
in the flue gas was burned to provide additional heat.


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 of coal to 5.16 tons of coal per ton of produced iron with further reductions at even higher temperatures.W.K.V. Gale, ''British iron and steel industry'' (David and Charles, Newton Abbot 1967), 55-8. He, with partners including
Charles Macintosh Charles Macintosh FRS (29 December 1766 – 25 July 1843) was a Scottish chemist and the inventor of the modern waterproof raincoat. The Mackintosh raincoat (the variant spelling is now standard) is named after him. Biography Macintosh was ...
, patented this in 1828. Initially the heating vessel was made of
wrought iron Wrought iron is an iron alloy with a very low carbon content (less than 0.08%) in contrast to that of cast iron (2.1% to 4%). It is a semi-fused mass of iron with fibrous slag inclusions (up to 2% by weight), which give it a wood-like "grain" ...
plates, but these oxidized, and he substituted a
cast iron Cast iron is a class of iron– carbon alloys with a carbon content more than 2%. Its usefulness derives from its relatively low melting temperature. The alloy constituents affect its color when fractured: white cast iron has carbide impuri ...
vessel. On the basis of a January 1828 patent, Thomas Botfield has a historical claim as the inventor of the hot blast method. Neilson is credited as inventor of hot blast, because he won patent litigation. Neilson and his partners engaged in substantial litigation to enforce the patent against infringers. The spread of this technology across Britain was relatively slow. By 1840, 58 ironmasters had taken out licenses, yielding a royalty income of £30,000 per year. 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 of hot blast were that raw
coal Coal is a combustible black or brownish-black sedimentary rock, formed as rock strata called coal seams. Coal is mostly carbon with variable amounts of other elements, chiefly hydrogen, sulfur, oxygen, and nitrogen. Coal is formed when ...
could be used instead of coke. In Scotland, the relatively poor "black band"
ironstone Ironstone is a sedimentary rock, either deposited directly as a ferruginous sediment or created by chemical replacement, that contains a substantial proportion of an iron ore compound from which iron (Fe) can be smelted commercially. Not to be con ...
could be profitably smelted. It also increased the daily output of furnaces. 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. Early hot blast stoves were troublesome, as thermal expansion and contraction could cause breakage of pipes. This was somewhat 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. Ultimately this principle was applied even more efficiently in
regenerative heat exchanger A regenerative heat exchanger, or more commonly a regenerator, is a type of heat exchanger where heat from the hot fluid is intermittently stored in a thermal storage medium before it is transferred to the cold fluid. To accomplish this the hot f ...
s, such as the Cowper stove (which preheat incoming blast air with waste heat from flue gas; these are used in modern blast furnaces), and in the
open hearth furnace An open-hearth furnace or open hearth furnace is any of several kinds of industrial Industrial furnace, furnace in which excess carbon and other impurities are burnt out of pig iron to Steelmaking, produce steel. Because steel is difficult to ma ...
(for making steel) by the Siemens-Martin process. Independently, George Crane and David Thomas, of the Yniscedwyn Works in
Wales Wales ( cy, Cymru ) is a country that is part of the United Kingdom. It is bordered by England to the east, the Irish Sea to the north and west, the Celtic Sea to the south west and the Bristol Channel to the south. It had a population in ...
, 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 US. While Crane remained in Wales, Thomas moved to the US on behalf of the
Lehigh Coal & Navigation Company The Lehigh Coal & Navigation Company was a mining and transportation company headquartered in Mauch Chunk, Pennsylvania, now known as Jim Thorpe, Pennsylvania. The company operated from 1818 until its dissolution in 1964 and played an early and i ...
and founded the
Lehigh Crane Iron Company The Lehigh Crane Iron Company (later simply the Crane Iron Company) was a major ironmaking firm in the Lehigh Valley from its founding in 1839 until its sale in 1899. It was founded under the patronage of Josiah White and Erskine Hazard, and fi ...
to utilize the process.


Anthracite in ironmaking

Hot blast allowed the use of
anthracite Anthracite, also known as hard coal, and black coal, is a hard, compact variety of coal that has a submetallic luster. It has the highest carbon content, the fewest impurities, and the highest energy density of all types of coal and is the hig ...
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 Coking is the heating of coal in the absence of oxygen to a temperature above 600 °C to drive off the volatile components of the raw coal, leaving a hard, strong, porous material of high carbon content called coke. Coke consists almost en ...
coal was only available in sufficient quantities in Great Britain and western Germany, so iron furnaces in the US were using
charcoal Charcoal is a lightweight black carbon residue produced by strongly heating wood (or other animal and plant materials) in minimal oxygen to remove all water and volatile constituents. In the traditional version of this pyrolysis process, ...
. 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 Anthracite, also known as hard coal, and black coal, is a hard, compact variety of coal that has a submetallic luster. It has the highest carbon content, the fewest impurities, and the highest energy density of all types of coal and is the hig ...
as a fuel had ended in failure, as the coal resisted ignition under cold blast conditions. In 1831, Dr. Frederick W. Gessenhainer filed for a US 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 Pottsville is the county seat of Schuylkill County, Pennsylvania, United States. The population was 13,346 at the 2020 census, and is the principal city of the Pottsville, PA Micropolitan Statistical Area. The city lies along the west bank of t ...
in 1836, but due to breakdowns and his illness and death in 1838, he was not able to develop the process into large-scale production. Anthracite was displaced by coke in the US 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.


References

{{Iron and steel production Blast furnaces Metallurgy Steelmaking Scottish inventions British inventions