Fig. 3.—Nash “Hydro-Turbo” Exhauster.
Tracing the wheel right round we then find that another pocket is formed into which the air is again sucked, and so the cycle of operations continues, two compressions and extractions occurring per revolution of the spindle. For many materials such a pump has the great advantage that the water acts as a wet filter and traps all the dust in suspension which can be washed out in the form of sludge. Where the dust is valuable this characteristic would be a disadvantage, but it is very useful in plants dealing with corrosive and abrasive materials.
Lubrication. The lubrication of cylinders in the reciprocating type of pump has been mentioned in connection with the necessity for taking special care in filtering all dust from the air pulled over from the discharger. Ordinary oil makes a sticky surface to which any dust adheres readily, and the two combined make an abrasive mixture which will quickly score and damage the walls of the cylinders. One firm, at least, has gone a long way towards removing this trouble by inserting pieces of solid graphite into the piston: this provides dry lubrication and produces a very smooth surface on which dust finds great difficulty in obtaining a “footing.”
The use of “Aquadag” is also fairly successful: this lubricant consists of deflocculated graphite in such very fine particles that they will remain in suspension in water even without mechanical agitation. “Aquadag” is fed into the cylinders in the same way as oil and deposits its graphite in the pores of the cylinder walls, whilst the water is atomized and blown out with the air.
The turbine exhauster solves the problem by eliminating the necessity for internal lubrication, and the perfect filtering of the air is then not so important.
Air Filters. It is impossible to deal with practically any granular material without carrying over more or less dust beyond the discharger, and to obviate the damage and inconvenience which would result from allowing this to enter the exhauster, an air filter is fitted between the discharger and the pump. Many types of air filters have been introduced, and representative examples are described below.
Fig. 4.—Sturtevant “Cyclone” Dust Separator.
“Cyclone” Separators. For such materials as grain, malt, etc., the ordinary type of cyclone separator is frequently mounted inside the receiver. This separator consists of an inverted cone: the dust laden air enters at the top and the heavy material circulates inside, and gradually falls to the bottom from which it is discharged into suitable containers (see Fig. [4]).
The cyclone is an excellent separator, and has the advantage that it is self-cleaning, and offers little or no resistance to the flow of air, but so large a cone would be required to separate very fine dusts that it frequently becomes impossible to use this type of plant.