L = steam piston.
M = air piston.
Note.—If preferred, the steam cylinder may be omitted and the air pump driven by electric motor direct-coupled or geared to the main shaft. Belt drive on to the flywheels or special pulley is possible but not advisable.
Typical Reciprocating Pump. Fig. 1 shows the general construction of a vertical steam-driven air pump such as is generally used in present day plants. The pump shown has a stroke of 14 ins., with an air-cylinder diameter of 28 ins. The machine is a dry air pump, and is fitted with Corliss inlet valves and special leather exhaust valves: it is fitted with ring-oiling bearings and automatic continuous lubrication, and under test it shows a mechanical efficiency of 78 per cent. from power input to air horse-power delivered. Such a pump is suitable for a plant handling 20 to 25 tons per hour and, if preferred, the pump may be driven through gearing from an electric motor on the same bed-plate.
Rotary Blowers and Exhausters. The turbine blower and exhauster depend entirely upon centrifugal force for their power to compress or exhaust air or gas, etc. The use of centrifugal force makes this type of machine resemble the centrifugal water-pump, but radical differences in design have to be introduced, seeing that water is not compressible, whereas air is capable of compression, and alterations are also necessary, due to the great difference in specific gravity of the two substances.
Owing to the very low specific gravity of air, the machine must run at a much higher speed than would be required with water to develop a given pressure. The high speed of the steam turbine has given impetus to the design of large exhausters on the turbine principle.
It will be recognized that, when air is made to flow steadily along a conduit or pipe of gradually diminishing cross-sectional area, the velocity of the air must increase correspondingly, in order that the constant quantity may flow through the smaller section of pipe. Increasing the velocity will diminish the pressure due to the greater kinetic energy to increase the velocity. The converse is the result of passing air at constant pressure through a channel gradually increasing from a smaller to a larger area, the velocity being then reduced and the pressure increased.
Now if a number of impellers be mounted turbine fashion on a high speed shaft, and the casing be so designed that the area between stages gradually increases, the air will enter the first stage and will be caught up by the impeller and accelerated until it leaves No. 1 impeller at a higher pressure and velocity. Leaving the casing through a diffuser which gradually increases in area the velocity is transformed into pressure in the diffuser. The air therefore travels to the second stage with its initial pressure plus the pressure due to the conversion of velocity in the first diffuser. This process may be repeated stage by stage until almost any desired pressure is obtained.
As there are no rubbing surfaces in this type of machine it is particularly suitable for the work under consideration, and when developed for efficient running in small sizes it will be very effective in “booming” pneumatic conveying.
Sturtevant Blower or Exhauster. The Sturtevant Engineering Co., Ltd., has developed a special rotary blower or exhauster suitable for use with pneumatic conveying installations and, although this machine has not a water-seal for surfaces under pressure (as in the Nash Hydro-Turbo described below), it has a number of distinctive points, and the discharge of the air, or the intake of air, as the case may be, occurs at a more nearly constant pressure and with smaller pulsations than with any other rotary blower known to the writer.