The internal construction of the Rateau machine as built by the Southwark Company is shown in Fig. 103. The U-shaped discharge passages by which the air discharged from one fan is delivered into the central aperture of the next are clearly shown. In this turbine there are fans or impellers in two groups of four each, the inlet of the first group being next to the central bearing between the two sets. The air then progresses to the left and is discharged into a receiver pipe which carries it over into the inlet of the second group next to the inside bearing, and it then progresses outwardly through this group to the discharge at its right-hand end. In this way the end thrusts against the two sets of impellers are balanced against one another so as to throw as little work as possible on the thrust bearing.

Fig. 104. Steam end governing apparatus and thrust bearing adjustment, Rateau turbo blower.

The turbine which drives the blower is shown in half-section at the extreme left. The shafts for the turbine and for the two sections of the blower are all separate, and coupled together by flange couplings to permit an absolute alignment of each shaft in its own bearings. The shaft is located endwise by a thrust bearing at the left-hand end of the turbine shown in Fig. 104. This thrust bearing is capable of being moved and can be adjusted by a hand-wheel and screw to secure proper clearance in operation. The general arrangement of this machine is well shown by Figs. 105 and 106.

The details of the design of the impeller, casings, etc., are highly technical matters in which mathematical theory and elaborate experimentation must go hand in hand to get the best results. To the great majority of users the only question of importance is how much steam is required to deliver a given quantity of blast at a certain pressure, and the details of the methods by which the guaranteed results are obtained is a matter of indifference to them and I make no attempt to treat of these matters, because it seems to me outside the field of the present work.

Fig. 105. Southwark Rateau turbo blowers.

Fig. 106. Southwark Rateau turbo blower.

The turbo blower is of course entirely valveless, and it therefore escapes the troubles of valve design and operation which I have briefly described in connection with reciprocating engines. It is also entirely without anything comparable to the piston displacement of the reciprocating blowing engine. That is, it has no metering effect, and its speed is accordingly no gage whatever of the quantity of wind it is delivering under given conditions. It is the function of a blower or centrifugal pumping apparatus of any kind to produce pressure and the flow which results is proportional to the difference maintained between the pressure at the discharge of the apparatus and the pressure in the vessel into which it discharges. If a reciprocating blowing engine, running at a certain speed, delivered the desired amount of air to a furnace at say 15 lb. pressure, and if the resistance of the stock in the blast furnace suddenly fell so as to let the pressure drop down to 14 lb., it would make little or no difference in the quantity of wind delivered by the engine, but with the turbo blower it would make an enormous difference, because it would endeavor to maintain the same discharge pressure, and with the pressure difference between this and the resistance of the furnace increased by the drop in the latter, the flow would be greatly augmented. For this reason the turbo blower requires a governing device capable of controlling the quantity of air passing through it.

In the Rateau system the well-known principle of the Venturi meter is used. In this an inlet pipe is provided which has a relatively small throat approached by a gradual taper and followed by an expanding taper to the normal diameter of the pipe. The difference in the velocity of the air at this throat and in the large area behind it, give rise to a difference in pressure at these points which depends upon the velocity of the flow, and by measuring the pressure with a differential gauge the flow can be accurately determined and controlled. The arrangement is shown diagrammatically by Fig. 107. The partial vacuum at the throat of the Venturi meter is maintained constant by means of a relay working on the steam governor of the turbine. If the flow increases the vacuum in the throat of the Venturi tube increases, and this acting through the relay cylinder operates on the steam valve of the turbine and reduces its speed, which in turn reduces the difference in pressure maintained by the blower, until the quantity of air flowing through it is reduced to the amount desired. A centrifugal governor is interconnected with the same system so as to close the steam valve entirely in case of excessive speed.