From what has been said it will be seen that we have the choice of three separate and very distinct types of blowing engines. It would be desirable to make a definite pronouncement as to the superiority of one of these types, but it is not possible to do this for two reasons. First, the whole matter is in a state of active controversy at the present time, with the advantage shifting from side to side as one receives additional light upon one or another phase of the situation. Second, there is no definite solution for all cases because the conditions vary greatly.

Fig. 123. Net pressure diagram.

At a steel plant which has within itself a vast market for power, the power producing capacity of the blast-furnace gas must obviously be utilized to the fullest possible extent, so that the purchase of other fuel may be minimized or eliminated, and for this object an investment may be made of from four to eight times the annual net saving, depending upon the conditions and the judgment of the owner.

On the other hand a furnace Bet down in an isolated location with no other manufacturing or mining operation nearby to supply a market for its power is justified in putting in machinery only so good as will secure for it virtual immunity from the purchase of coal for power purposes, and will give the lowest operating coat exclusive of fuel.

Fig. 124. Combined net pressure diagram of air and gas cylinders.

Under these conditions it is obvious that no decision of this question for all cases is possible even if the actual physical data of operation available were complete, and were unanimously admitted by all parties in the controversy, which is by no means the case. The problem must be solved for each case on its own merits, and the lines of this solution may in a general way be laid down. It may be said in advance that recent investigation of this subject with details of actual capital and operating costs of gas engine plants by Messrs. H. J. Freyn, formerly of the Allis-Chalmers Company; C. J. Bacon, of the Illinois Steel Company, and Arthur West of the Bethlehem Steel Company, with data on the steam turbine and turbo blower, submitted by Mr. Richard H. Rice of the General Electric Company, have indicated that the fuel cost of power production is by no means the only cost, and in many cases is not even the largest item, but that as we pass from the plain and simple types of machinery of old, to the more refined and economical but expensive types of to-day, the fixed charges due to investment may rise faster than the value of the fuel saved. It is only by considering all these factors that we can arrive at the proper solution in any given case.

Fig. 125. Combined diagram of air, gas and inertia.