In the early days of blast-furnaces when furnaces were small and the quantity of iron made per cubic foot of furnace capacity was very small as compared with present practice, the quantity of power required to drive the furnace was relatively insignificant. In that early day the gases from the top of the furnace were allowed to burn to waste in the air, and plants were generally located with reference to a water-power site.

With increase in size and rate of driving, the amount of power required increased very rapidly. We shall see in a later chapter that this increases nearly as the square of the output of a given furnace, and that modern furnaces require from 2000 to 3000 horsepower to drive them. Consequently as furnaces were driven harder it became more and more difficult to find water powers adequate to blow them. This, with the increasing introduction of the steam engine in suitable sizes and designs, and the increasing realization of the vast fuel value of the furnace gases, led to the introduction of steam power for blowing furnaces.

This step was hastened by the advent of anthracite as a furnace fuel, the pressures required with it being so much greater than they are with charcoal fuel in a given size furnace, and under conditions similar in other respects.

The question of the quantity of furnace gas per ton of iron produced and its heating value will be treated at some length in a later chapter, but in tracing the development of the present subject we may say that in ordinary, good modern practice about one-half of the total thermal value of the fuel charged comes out of the top of the furnace in the form of its top gases. Some 6 to 10 per cent. of this heat is in the sensible condition, the rest is contained in the gas in the shape of CO, whose combustion with air gives 4370 B.t.u. per pound of CO, or 10,200 B.t.u. per pound of carbon contained.

In good modern practice there are produced per ton of iron on the average about 140,000 to 200,000 cu. ft. of gas, of which about 60 per cent. by volume is nitrogen, and the remaining 40 per cent. is made up of CO, CO2 and hydrogen in varying quantities; this gas has a thermal value ranging from 90 to 115 B.t.u. per cubic foot.

In Lake ore practice the quantity produced is from 140,000 to 160,000 cu. ft. per ton of iron; the analysis approximates N 60%, CO2 15%, CO 23%, H2 2%, and the heating value 95 B.t.u. per cubic foot.

' In districts where leaner ores are used and correspondingly more coke is required the percentage of nitrogen does not alter much, but the CO increases at the expense of the CO2 with proportional enrichment of the gas and increase in its thermal value.

In the early furnaces, this gas was allowed to escape freely from the top of the furnace, from which it burned continuously in a great flame, visible at night for miles; this is the case at a few cold-blast charcoal furnaces to this day.

The first attempts to utilize the heat of the waste gases were directed toward heating the blast rather than to generating steam. Furnaces were quite commonly built, 30 or 40 years ago, with a small pipe stove, of the general type to be described later, on top of the furnace and immediately over the throat, the gas rising from the latter mixed with the air and the resulting flame was drawn up through the stove. One or two charcoal furnaces built on this plan are still in operation now (1916). This construction, at least in some cases, necessitated the filling of the furnace entirely on one side and wonder is that furnaces could operate at all under this handicap, in fact, it is perhaps the greatest tribute to the hot blast that a furnace with hot blast produced in this way would do enough better than a cold-blast furnace symmetrically filled to make the change a profitable one.

The next development in the utilization of the furnace gas was to put both boilers and stoves on a masonry structure as high as the top of the furnace, and lead the gas off through flues opening into the stack several feet below its top. The resistance of the stock column above the flues and the draft of a powerful draft stack at the far end of the system served to draw the gas off through these flues. Air drawn in from the top of the furnace through the inversities in the stock mixed with the gas so that it was drawn through the stoves and boilers, not as gas but as flame.

The idea that the gas kept from contact with the air could be led about and burnt at practically any desired distance from the top of the furnace seems to have been rather slow in taking hold. The introduction of the bell and hopper enabled the top of the furnace to be sealed tight except when the bell was opened to charge the stock, ordinarily only about 5 per cent. of the time. Finally the gas lost during this interval was saved by the introduction of the upper bell. Under these conditions the top of the furnace is never opened to the air and a constant pressure is maintained sufficient to force the gas through the downcomer and mains to the boilers and stoves, or to the cleaning system, as the case may be.

In this system, therefore, the mains carry not flame but gas, which may even be reduced to atmospheric temperature and still remain an excellent combustible.

Under these conditions the gas can be led without difficulty to very considerable distances and consequently the stoves and boilers can be located with reference to other considerations. They have accordingly, for many years past, been placed on the ground and in positions best suiting the convenience of the plant as a whole.