Dischargers. The advocates of “blowing” material, instead of “sucking” it through the pipe, often lay great stress on the alleged difficulties of extracting the material from the system without allowing air leakage. This, however, has been overcome successfully by several designers and is not the serious difficulty so frequently suggested, providing that the material is suitable for this means of treatment. The only exception to this is, when the high velocity at which the material enters the discharger—say, 45 to 50 ft. per sec.—causes it to pack or bind so that it will not fall by gravity into the rotary valves and then out into the storage bins.

The function of the discharger is to cause the incoming material to lose its velocity and to fall into a compartment which can eventually be discharged after it has been moved from the low pressure to which the chamber itself is subjected.

This is best accomplished by the use of a rotary valve, somewhat similar to a paddle-wheel, which is revolved slowly but continuously. This wheel can be placed either vertically or horizontally at the bottom of the receiving chamber; the material enters the large chamber above, loses its velocity, drops by gravity, is caught in the box formed by the revolving paddle-wheel, and gradually is carried forward out of the chamber, eventually passing over an aperture through which it again gravitates to the bunker, silo, or other container. Probably the horizontal type of rotary valve is preferable because, owing to the increased surface exposed to the vacuum, the “suction” effect assists in holding the valve up to its seat.

When dealing with such materials as malt and grain, it is an advantage to be able to inspect the material entering the receiver, and at least one maker secures this advantage by constructing the chamber of a glass cylinder to which are bolted cast iron top and bottom pieces carrying the necessary pipe connections and discharge valves. Fig. 8 illustrates this construction and also shows fairly clearly the method of driving or revolving the discharge valves. The top flange of the valve has a worm wheel tooth cut around its periphery and the actuating worm engages in this wheel, thereby obtaining a large reduction in speed. In other words, the worm can be driven by a light, high speed belt and pulley, and still revolve the valve at a very low speed; such gearing is smooth, silent running, and altogether admirable for such a purpose as the one under consideration.

A very common form of separator, which is used almost invariably on plants dealing with wood shavings, sawdust and similar materials, is known as the cyclone or centrifugal separator. This is usually constructed with a sheet steel body with the inlet for the dust-laden air at the top, and so arranged that the air enters tangentially. Inside there is a smaller cylinder of sheet steel forming an air outlet, and the laden air sweeps round the annular space between the body and the inner cylinder. This results in a whirling action and the material entrained in the air is projected by centrifugal force against the side of the separator body. In some instances an internal ledge, or plate of “corkscrew” form, leads the material downwards towards the outlet at the bottom of the separator.

In the case of some of the denser materials which can be conveyed by air, it is sufficient to connect the discharge pipe to an open bin or chamber, the material in such cases being heavy enough to separate itself by the action of gravity.

Mr. Gordon S. Layton, describing dischargers in his paper, before the Engineering Group of the Society of Chemical Industry (Birmingham, April, 1920), stated—

The rotary type of discharger is preferable to the tipping box type, for the following reasons: because the rotary discharger is more easily kept air-tight, works without vibration, and gives a practically continuous stream, whereas the discharge from the tipping box occurs as large isolated masses of material.

It is impossible to give specific details concerning the discharger because, in all cases, the conditions under which the plant has to work affect the whole design. For instance, where the working is only intermittent, e.g. the removal of ashes from a boiler-house, the discharger can be eliminated, provided that the ash bunker is large enough to hold the quantity of ashes to be dealt with at each operation. In such a case, the ash container would be capable of being exhausted, and the material entering as before would simply drop by gravity into the container and remain there until the pump was shut down; it would then be allowed to gravitate into the truck or other conveyance for disposing of the ashes.

Hand-holes for cleaning, and easy access to the interior are essential in the design of a discharger, especially if the material to be handled is liable to “pack” when entering at a high velocity.

Pipe Lines. One of the most important points in the designing of a pneumatic conveying system is the correct lay-out of the pipe line.

A fatal mistake often made in low pressure and exhausting systems is that the numerous branch pipes are added to or altered after the makers have left the original installation. Almost invariably, branches thus added are made to approach the main trunk at too great an angle, with the result that eddies and whirlpools are created within the pipe, seriously reducing the output of the main trunk. So essential is it that this junction should be correct and that the two streams should run as nearly parallel as possible, that the Sturtevant Engineering Co. has patented a special junction (Fig. 9) to bring together the two streams of air in the main and the branch pipe practically parallel, as shown.

Bends. In connection with high pressure systems, the following points are of great importance. All vertical and horizontal straight lengths may be of a light section constructed in steel. Bends should be avoided whenever possible, and those which are inevitable should be made in hard cast iron, with every possible provision for easy replacement of wearing parts.

The wear takes place at the point of actual contact which, in elbows, is practically confined to one place only. The material rushes to the end, strikes the bend, and—suddenly changing its direction of travel—whirls off down the next straight length. The impact and the resulting wear on the pipe, as well as the breaking of the material conveyed, are naturally much greater in elbows than in easy bends, but if the breaking of the material is not detrimental, elbows should be employed, as they are less costly and can be replaced more quickly and easily.

Certain raw materials—such as salt, soda, lime and various chemicals—which have to be ground before use, may be prepared to a considerable extent for this operation by the use of elbows. On the other hand, easy bends should be employed for material which it is desired to convey without damage, e.g. malt, coal, and granular substances, which are finally required in granular form and not as powder.

The wear in bends is only on the external radius of the bend, and then is inclined to be localized at certain points rather than distributed over the full sweep of the bend (see Fig. 10); this being so, it is often desirable so to construct the bends that the back is in segments which can be renewed easily (see Fig. 11). Alternatively, the bend may be constructed on the “lobster” principle (Fig. 12), only the worn sections being replaced when overhauling. It is not necessary always to take a bend at an angle of 90°, and if the small short angle sections are interchangeable, then almost any angle can be constructed by building up with the necessary number of sections.

Fig. 10.—Course taken by Material Round Bend.
Fig. 11.—Segment-Back Bend.
Fig. 12.—Lobster Bend.

With regard to the straight lengths of pipe, it is necessary to ensure a smooth internal bore, especially at the joints. It is therefore desirable that the joints should be self-aligning as, if this is not the case, eddies will be formed which will cause the material to deviate from the centre of the pipe, striking the side at one or more definite points where holes will eventually be worn through the pipe.

In vertical pipes the evidence of wear is negligible, in fact the conveyed material presumably does not touch the pipe at all, but travels up the centre of it as a core.

Capacity of Pipes. The velocity of the air passing up the pipe should be from 40 to 50 ft. per sec., equivalent to about 35 miles per hour.

The conveying capacity of an efficient pipe line is approximately 15 per cent. of the total capacity: in other words, if the total cross-sectional area of the pipe be taken as 100, the effective cross-sectional area as regards conveying is 15.

Flexible Connections. The flexible connections attached to the permanent pipes may be ordinary tubing, as made by the Flexible Metallic Co. Phosphor bronze and other metals have been tried, but the extra cost is not justified. A loose screw collar connection makes possible easy fixing, and permits the flexible connection to be removed easily to prevent damage when not in use. Rubber tubing, reinforced with steel wires, would be best where it is important that the material conveyed should not be damaged, but the wear on such tubing is so rapid that the extra cost is not recovered by the saving in damage to ordinary materials. The length of the flexible pipe should be such that the movement over the greatest area to be covered does not put too great a strain on the bending properties of the tubing. On the other hand, unnecessary increase in the length of the flexible pipe merely increases the cost of one of the shortest lived portions of the plant, the “scrap value” of which is almost negligible.

Valves, etc. For use where branches are inserted in the main pipe line for convenience in either lifting over, or discharging over, a large area, special appliances have been designed and these should be used, as they do not create eddies or increase the pipe friction appreciably, or reduce the carrying capacity of the pipe line. The King patent full-way junction valve is an excellent example, and is shown in Fig. 13, from which it will be seen that a full bore circuit can be completed in any of three directions. This valve has no corners where the material can collect, hence the pipes are sucked perfectly clean the moment the feed is shut off.

Another convenient fitting of this description is the Boby patent pipe switch. This device is similar to a switch as used on a railway track, and by its use three separate side positions may be connected with one part on the main transport line. When the switch is thrown over so as to connect to any one particular branch, all other branches are disconnected.

Telescopic Pipes. When the unloading of ships is carried out by “suction” it is necessary to make provision for lengthening or shortening the vertical suction pipe, or pipes (see Frontispiece), because the ship will rise as relieved of its cargo, and the suction nozzle will simultaneously move towards the bottom of the hold as the cargo is discharged.

A still greater difficulty is encountered in tidal rivers, where the rise and fall may be many feet and must be allowed for continuously. This is best done by the introduction of telescopic pipes in the vertical downright pipes. These must be so constructed that while it is fairly easy to increase or decrease their length, the pipes must remain air-tight at the joints and connections.

Where the rise and fall is small the difference in level may be compensated by a ball and socket joint, and a counter balance on the jib arm, but this method has its limitations.

Pipes for High Pressure Systems. Coming now to the “small pipe,” high pressure systems as used for vacuum cleaning plants, the pipe lines must be designed and installed carefully and on a liberal basis. It is mistaken policy to attempt to economize by using a smaller main on branch pipes. Small diameter pipes cause excessive losses by friction, and naturally are less efficient as regards power consumption.

The frictional losses in a system of this description vary directly as the length of the pipe, and inversely as the fifth power of the diameter. Large pipes are therefore very desirable, not only because of their greater carrying capacity—which is very desirable—but also because such things as matches, hairpins, etc., are picked up every day by an installation as fitted in hotels, restaurants and theatres. Such material quickly clogs small pipes and causes endless trouble and delay.

The flexible hose should be as short as is consistent with ease of working, because the frictional losses in this class of tubing are very great. It would be preferable to increase the number of wall plugs, rather than have to use very long lengths of flexible hose.

Suction Nozzles. Probably more patents have been taken out on new suction nozzles than on any other portion of a pneumatic suction plant. The chief desiderata for a nozzle on a high pressure system for wheat, coal, ashes, etc., are that it be of light construction to allow of easy manipulation by the operator, and that it have some means of allowing a “free air” inlet, making it impossible to choke the nozzle by burying the end. It is an advantage to be able to regulate the free air inlet according to the conditions existing with different materials. The same nozzle that will act best while dealing with a large bulk of material, may be quite unsuitable when it becomes necessary to “clean up” in the corners of the hold or waggon. Fig. 14 shows different types of nozzles for high pressure plants, but as the efficiency and capacity of the plant can be affected seriously by the design of this portion of the apparatus, it is highly advisable to allow the designer to have a free hand and to make use of the experience already gained.

Nozzles designed for low pressure systems, dealing with dust, shavings, etc., have to be built to suit the machine to which they are attached, and they therefore vary indefinitely in details of design and construction. The same remarks apply to the nozzles for use on suction cleaning plants. Figs. 15 and 16 show how the suction nozzles are adapted to the machines.

It must be remembered that in low pressure systems handling shavings, dust, etc., the problem is quite different from that in high pressure systems handling wheat, etc. In the case of removing dust or shavings from a machine, the material is already in motion, and only requires drawing forward and into the pipe system, but in the case of conveyors for wheat, coal, etc., and in the case of suction cleaners, the material to be moved is heavy and stationary and has to be lifted and started in motion before it can be carried away. This necessitates a much higher air velocity through the collecting nozzles.

Idle Nozzles should be Closed. It is perhaps advisable to draw attention at this point to the disadvantages of using more than one suction nozzle on one receiver at one and the same time. The reader is asked to recall the fact that the material is not lifted by vacuum, but that the production of a partial vacuum causes a stream of air to pass up the pipe at high velocity. The material to be conveyed is entrained with the air, and due to the frictional contact between the particles of air and the particles of material, the latter is lifted and carried forward.

If the conveying plant is to be efficient and of reasonable capacity, the pipes must be relatively large, and in order that the desired partial vacuum may be maintained in them (establishing a vigorous air current) without the use of an unduly large pump, it is important that air be admitted only through those nozzles which are actually in use. Also, when more than one nozzle is in use at the same time, it is necessary to keep each nozzle covered with material to such an extent that the same amount of air passes into each pipe. Unless this is done a large quantity of air will pass up one pipe, and a small quantity up the other, and the amount of material taken in at each nozzle will vary as the quantity of air varies. To consider an extreme case, suppose that the man operating at one of the pipes allows his nozzle to become exposed. Air will rush in at this nozzle to the full capacity of the pump, with the result that little or no air will pass up the second pipe, and consequently no material either. Thus, if one man is sufficiently neglectful to leave his nozzle idle and open, he renders practically useless the other nozzle or nozzles on the same main.

Even with care this is bound to occur to a certain extent, as is shown by the figures given by makers for the estimated power consumption, viz., about 1 h.p. per ton on single-nozzle plants, and 1½ h.p. on double-nozzle plants.

Under these conditions it should be considered whether it is more advisable to install one large plant with two nozzles, or two small plants, each with only one nozzle. The decision depends upon the extra cost of power for the double-nozzle plant compared with the higher capital charges on the two single-nozzle plants.