The first question to be asked and answered is as to the value of power for purposes outside the blast furnace. If no market is at hand or can be created this is obviously zero. If on the other hand there is a market it is obvious that surplus power produced by the furnace must meet the competition of power produced by the cheapest means otherwise available. This will generally be determined in a broad way by the cost of coal at the given locality. Granting the existence of the market for the power at a price so determined, and knowing the quantity of gas to be produced by the given furnace plant we deduct from the former 35 per cent. to represent the consumption of the hot blast stoves, and 5 per cent. for other auxiliaries around the plant; the remaining 60 per cent. is the gross amount which must supply the power required for blowing the furnace and that which is to be sold.

Fairly reliable figures are to be had as to the number of thermal units required per horse-power, by each of the different types of prime movers. In the case of the gas engine this can be converted directly into cubic feet of gas required, based on the thermal units per cubic foot of the gas after it is cleaned and cooled and ready for the gas engine cylinders. But in the case of the steam engine and the steam turbine the efficiency of the boiler enters as a factor of much importance. It has been claimed that this may be put as high as 80 per cent., but the best and the most recent data concerning boilers and their operation with blast furnace gas, indicate that they often reach in practice something less than 60 per cent. This also, however, is capable of investigation and determination for the detailed case in contemplation, and a figure must be taken corresponding to the results which may reasonably be expected. Due allowance must be made for the steam pressure to be used, and particularly for the degree of superheat desired in the steam, and for the fact that the actual quantity of heat required in proportion to the theoretical, is greater as the degree of super-heat rises.

With this information we may figure the number of cubic feet of gas required to produce a horse-power in the case of the steam engine and the steam turbine, and in this case the thermal value of the gas per cubic foot in a partly cleaned and warm condition may be taken instead of that for its cold condition, which makes a difference of between 5 and 10 per cent. in favor of the steam apparatus. This helps to offset the initial advantage of the gas engine, that it only requires about two-thirds as many thermal units per brake-horsepower as do the steam driven motors.

We can now figure the quantity of gas needed to blow the furnace in each case and subtracting this from the total net heat available we obtain the amount available for the development of power to sell. From this we may determine the amount of such power we can produce in each case and its value, from which the operating cost of its production, exclusive of fuel, must be deducted. Against this we must set up the cost of the investment, the proper rate of amortization, and the proper rate of interest on the capital in each case. By comparing these with the net earnings for the power sold in each of the three cases, we can determine which is the best investment, always remembering that the amount of money available, or which can be raised, is more often than not strictly limited, and that the fact that a fine interest can be earned on an extra two or three millions is of little interest or value if the management cannot raise the extra two or three millions. It is also very necessary to remember that the type of motor determined upon as best to blow the furnace is not necessarily the best for developing power for sale. In fact in my judgment the probabilities are that the reverse is the case. The steam turbine, owing to rotary motion, high steam economy, and ease of supervision has great advantages in driving electric generators, whereas for driving blowers it has no better means to apply to the purpose than the turbo blower whose efficiency is only 70 per cent. as a maximum as compared with 85 per cent. or more as the over-all efficiency of the piston blower. This gives the reciprocating engine, whether steam or gas driven, a long lead over the turbine for blowing purposes, while the latter has, as outlined above, many advantages over both the former in the generation of electric power.

For all these reasons it seems well to reiterate that each case presents a problem of its own which must be handled without prejudice by someone competent to weigh not only the technical considerations involved, but also that more serious and more difficult question which has wrecked so many engineers and their brilliant schemes: How much interest will it pay on the capital invested?