I. Pulsation Of Discharge

Under certain conditions of operation at considerably less than the rated capacity of these machines, they are liable to a very disagreeable pulsation of the discharge, the theory of which I do not completely understand, though I believe it has been worked out mathematically. It seems to be due, however, to the fact that the volume is too small to maintain the delivery velocity necessary to overcome the pressure over the whole area of discharge, continuously, and the discharge therefore breaks and a backflow from the discharge pipe into the turbine casing takes place at short intervals, producing this pulsation. It is claimed to be overcome in the later types of these engines by the introduction of a butterfly throttle valve in the inlet pipe above the diaphragm.

II. Inefficiency

The efficiency claimed for these machines by their builders under best conditions is 70 per cent., and of course this falls off on both sides of the maximum along the characteristic curve of practically all centrifugal pumping machinery.

The diagram factor of the air card from a well-designed blowing engine as a whole is about 95 per cent., and the mechanical efficiency of the engine as a whole, as I have previously stated, should be about 92 per cent. By diagram factor is meant the ratio of the actual diagram produced by a given engine under a given discharge of pressure to the theoretical indicator card with an adiabatic compression for the same pressure. The product of these two is about 87 1/2 per cent., the difference between this and 70, or 25 per cent., is the handicap which the turbo blower must overcome to equal the performance of the reciprocating blowing engine.

Turbines have been brought to a very high state of perfection, particularly in very large units, such as are used in electric generating stations. But a turbine of, say, 3000 h.p., with an efficiency of 70 per cent. in the blower end, which would deliver 2100 air-h.p., can certainly not do 25 per cent. better in steam consumption than a steam engine properly designed and running with the conditions of boiler pressure, superheat, and vacuum which are admittedly indispensable to the economy of the turbine.

If it be impossible for the turbine to do 25 per cent. better than the reciprocating engine under these conditions, then obviously the superior efficiency of the turbine will not offset the low efficiency of the blowing end, and the turbo blower as a whole will be a less economical unit than the reciprocating steam engine.

The steam expert of one large and extremely well-managed company has publicly expressed a doubt as to whether the turbo blower could get along with as little steam as the reciprocating engine, his company having both reciprocating engines and a turbo blower in the same building and operating under identically the same conditions.

III. Failures

There have been several wrecks of turbo blowers in this country, but these apparently have been due to defects in the design of new and larger units than had before been attempted, rather than to any inherent defect in the principles involved. This we have long since learned is the price which must be paid in the development of new equipment, and must therefore not militate unduly against the turbo. At the same time, it cannot be denied that turbines in other service have more than once "gone through themselves," to use the expressive phrase of the operator, with consequences so disastrous as to leave little of the structure of any value, even for repair purposes. Before the turbo blower can establish the supremacy towards which its builders are casting their eyes, it must establish a high degree of immunity from accidents, and also an economy not only eclipsing that of the best reciprocating steam engines, but rivaling that of the gas engine, which increasingly, as time goes on, must become a powerful competitor in blowing machinery.