The sectional diagrams (Fig. [2]) show the four successive stages in the movement of the rotors. In position (1), chamber D has been filled with air, while chamber E is discharging air against pressure in the delivery pipe. In position (2), chamber D is cut off from the inlet, and the air in it is being carried round. Blade C has entered pocket Z, which is filled with air under pressure; this air, however, in turn is released into pocket Y through leakage passage O. Continued rotation carries the rotors A, B and C to position (3), and the remaining pressure in Z is now being transferred to X by leakage passage N.
As the fourth position is reached, chamber F is filling and pocket Y is discharging its air. When further rotation brings impeller blade B into the discharge passage, the air in space D will be delivered from the blower. After leaving position (4), the rotors again reach a position similar to that shown at (1), and the cycle is then repeated.
Fig. 2.—Sturtevant Rotary Blower.
A study of the above will convince the reader that this blower is ingenious and very suitable for the class of work under investigation.
For certain conditions the Roots blower and the multifarious types of drum pumps or blowers can be used, seeing that it is quantity rather than pressure that is required, but it is essential that the most reliable and efficient exhauster should be installed in accordance with the conditions for each installation, bearing in mind always the questions of dust, speed, lubrication, etc.
Nash Hydro-Turbine. An entirely new type of rotary exhauster was recently illustrated in the Chemical Age, and a study of Fig. 3 will show the principle of this pump. This pump was built just before the war by Messrs. Siemens-Schuckert, and similar pumps are now being introduced by the Nash Engineering Co. (U.S.A.), and are known as the Nash Hydro-Turbine.
This exhauster has a cylindroid external casing, inside which is a shaft carried on the two end brackets and having mounted upon it a crude type of water-wheel. It should be noted here that this arrangement has the effect of bringing the edges of the wheel into a position of eccentricity in relation to the inside of the external casing.
The wheel shaft is connected directly to a high speed electric motor, and when the pump is running the water which is fed into the casing is thrown by centrifugal force to the periphery of the casing and thus forms certain air pockets into which the air of the system is drawn. The air is now locked between the hub of the wheel and the sheet of water surrounding the outside of the wheel. As the wheel revolves, the air in each pocket in turn is compressed into a smaller capacity and eventually, when it arrives opposite the outlet point, it at once escapes into the atmosphere due to its additional pressure.
