A pneumatic conveying plant of practically any type comprises the following five main components: (1) A pump or exhauster to create a partial vacuum in the pipe line and so induce a high velocity air current in which the material will be conveyed. (2) An air filter in which any light dust carried over beyond the receiver is trapped, to prevent its entering the exhauster where it would quickly damage the piston rings and cylinder walls of reciprocating pumps. (3) A main receiver or discharger, and occasionally one or more subsidiary receivers, in which the conveyed material is extracted from the system and discharged into the receptacle or on to a dump as required. (4) The pipe line, junctions, etc., to connect the point of supply with the desired point of delivery. (5) The suction nozzle through which the material enters the system, together with the “free air” which is to act as the conveying medium.

Pumps and Exhausters. The type of apparatus used for creating the flow of air varies according to the ideas of the numerous makers of the plants. It must be remembered that for a pump working under the conditions required for pneumatic plants it is not a high vacuum that is required; the most important function is to handle very large quantities of air at a comparatively low vacuum.

The most efficient type of plant for dealing with large quantities of air is certainly the reciprocating pump, although several makers are now devoting a lot of attention to the multi-stage turbine type of blower, or exhauster. The probability is that this type may shortly be as efficient as the reciprocating pump, and if so it is almost sure to be used extensively, as it has distinct advantages in other directions compared with the cylinder and piston type. This remark has special reference to the high pressure system for conveying sand, coal, sugar, etc., and does not apply to the “small pipe system” detailed later.

In dealing with ashes, flue dust, crushed iron ore, and similar abrasive materials, it is essential that the air passing through the pump should be filtered thoroughly so that no dust should enter the cylinders and cut the walls and piston rings. The turbo-blower, having no rubbing surfaces, would suffer little or no damage from a small percentage of dust, hence the filtering equipment might be less elaborate and less costly. No lubrication would be necessary in the rotary pump (except, of course, at the bearings), hence no difficulties would arise owing to oil acting as a dust trap, as is sometimes the case in reciprocating pumps with defective air filters.

Key to Fig. 1.

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.

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.

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.

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.

“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.

Bag Filters. An alternative type of air filter is the ordinary bag filter, which consists of a number of closely woven fabric “stockings” in a cast iron container. The air is led into the casing so that it passes down through the inside of the tubes, through the fabric, where it deposits the greater proportion of the dust, and then out to the atmosphere.

Naturally, after working a certain period, the fabric becomes choked with deposited dust, and it is then necessary to dislodge the dust by shaking the “stocking” somewhat violently. This is usually carried out by a mechanically operated vibrating apparatus, but cleaning is done more effectively and time is saved if the air is shut off from the filter while the dust is dislodged. This is done most conveniently by having two similar filters installed, working in parallel; then, when cleaning becomes necessary, all the air is passed through each filter in turn, whilst the other is cleaned.

The bags or tubes hang vertically in the casing, and as the air is brought in at the top and discharged in a downward vertical direction, it naturally discharges the heavier material straight into the bottom of the casing, owing to the inertia of the heavy particles and the high velocity of the incoming air.

Fig. 5 illustrates a Sturtevant automatic bag filter. In this case the air is brought in at the bottom. The cleaning of the bags is effected automatically. At frequent intervals and in rotation, each section of bags is cut off automatically from the supply of dust-laden air by closing the outlet valve of that section. At the same time, that section is opened to the atmosphere at the top, causing a reverse current of air downwards. The bags are then agitated automatically and the dust adhering to the fabric drops, and is blown into the discharge hopper below. The opening and closing of the valves is accomplished by a simple mechanism driven by the pulley shown at the top of the illustration. Where the amount of dust to be handled is large and continuous it can be extracted from the hopper by a worm or screw conveyor, or, as in some other patterns, by a rotary valve placed in the bottom of the hopper.

Mollers’ Air Filter. This apparatus consists of separate fabric pockets mounted on a frame. The pockets are rectangular and taper towards the top. Referring to Fig. 6, A is the frame on which the fabric is stretched, and B is the fabric pocket. As many of these units as necessary are mounted side by side in an adjustable frame, each unit having fixing and tension bolts. This arrangement permits a filter of any desired capacity to be built up quickly, and facilitates repairs and cleaning of filter bags.

Wet Filters. The type of filtering apparatus used for dealing with poisonous material, emery dust from grinding wheels, sand from sand-blasting apparatus, etc., is usually of the wet type, of which Fig. 7 is a good example. This apparatus consists of a tank having a wire shelf on which a layer of coke is supported. The tank is partially filled with water, the level of which is regulated by an overflow.

The dust-laden air impinges on the surface of the water, and the major portion of the dust is trapped by being driven actually into the water. The dust particles too light to be brought into contact with the water are compelled to pass through the coke screen and are there arrested. If necessary, the scheme can be made to deal with finer dusts by having the coke constantly sprayed with water. The dust is reclaimed from the tank in the form of sludge or mud through the door or valve provided for that purpose.

Another type of air filter which might be developed in connection with the pneumatic handling of material consists of a very slowly revolving drum or cylinder, which is fitted with a continuous corrugated tape running spirally from the centre to the extreme edge of the casing. The space between the corrugated sheets is very small, and as a stream of water is continually running over and around the divisions, the air passing through the very tortuous path provided is bound to come into contact with these wet surfaces and give up its dust or other contamination, which is washed off when it arrives at the bottom of the cylinder. Naturally, this or any other wet filter would not be used where the recovery of dust in dry form was desired.

Another form of wet filter consists of a chamber of suitable proportions (according to the amount of air to be cleansed), fitted with racks in which are placed strips of glass at an angle of 45° to the flow of air, and at 90° to one another. The glass strips have a serrated or prismatic face, and the air carries the dust forward into the angles of the glass. A very fine water spray keeps the glass moist and eventually trickles down to the drain channels, washing the glass in its course.

The development of apparatus for air-washing has received considerable attention during recent years, owing to the necessity of having pure, dust-free air for ventilating turbo-generators, etc., and no difficulty should present itself in obtaining satisfactory results for pneumatic conveying plants, except in cases when the collection and retention of dust is required. In these cases the bag or fabric sheet filter is the only type available.