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wrought iron. 719
From I860 to 1860, several improvements were made in the construction of
blast furnaces. Up to that time the building material used was slate and granite,
which, however, necessitated heavy walls, and the whole structure preferably to be
built in a parallelipipedical shape. As the height of the blast furnace was increased
a lighter construction was desirable, and for that reason only the lower part was,
as previously, built of cut stone while the upper part was all made of brick and
of a round shape.
When the blast furnaces were made larger and the hearths
wider, it became necessary to increase the number of tuyeres in
order better to distribute the blast in the furnace. The older
blast furnaces had only one tuyere, which was afterwards, the
fifties, increased to two or three; a larger number could not
suitably be applied in a furnace of a rectangular section. On
account of this, the stacks in 1870/80 were placed on beams
resting on cast iron columns or trestles, as had been customary
for some time at the foreign coke furnaces. The first blast furnace
of this construction was built in this country in 1876. The
number of furnaces of this type has since been steadily
increasing, although not very rapidly, as this design could be adopted
only when a new furnace was erected, but at present, however,
the greater number of Swedish blast furnaces are of this type. In
these blast furnaces the number of tuyeres were increased to 4,
sometimes to 6, according to the size of the furnace, and the
furnace walls were made considerably thinner than was previously
customary.
The blast furnaces erected during the last decenniums are 16
to 18 meters high, the latter being the maximum for a charcoal
furnace, as the attempts to increase the height still more have
not given satisfactory results. The furnaceshaft is now at the
bottom and top made in the shape of a truncated cone, the
intermediate part being cylindrical.
When blast furnace gas in 1830/40 began to be used in
hot-blast stoves for heating the blast and in roasting kilns, the gas was
taken out through one or more ports in the furnace walls, 4 or 5
meters below the top of the furnace. During the years 1860/70,
these gas-ports were altered in such a way that from the upper
part of the stack, i. e. the charging floor, a sheet-iron cylinder of
the depth of 3 meters was suspended, behind which the gas passed and was thence
conducted through ports and flues to the kilns where it was to be used. This
arrangement for taking out the gas from blast furnaces has proved to be very
practical and is now used in all Swedish furnaces not provided with closed charging
device. Closed charging devices have only of låte come into use. Previously
there have certainly been made several attempts to use such feed devices of
different types, but these attempts have not proved satisfactory, wherefore such devices
have been abandoned. The circumstances contributing to this are: that the
difference in the specific gravity between charcoal and the heavy and compact hard
ores makes a more accurate charging of charcoal and ore necessary than can be
accomplished by the kind of charging devices used at coke-blast furnaces abroad
and as the charcoal often contains a large amount of water, in order to get rid
of this, relatively large gas ports and gas flues are required. If the charcoal
contains much water a closed charging device is not desirable, no matter of what
type it may be. Furthermore, it is to be taken into consideration that blowing
engines and crushers at most of the Swedish works are run by waterpower,
that not all the gas produced is required for the blast furnace, and, finally, that
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