CHAPTER VI
THE INDUCTION CONVEYOR
Numerous means have been devised to cause the necessary current of air to flow along the conveyor pipe, but the ideal method is probably yet to seek. Probably the most satisfactory and economical system, until recently, was the positive pump exhausting a vacuum chamber; the latter receiving the material, and discharging it into the receptacle provided for that purpose.
The difficulties arising in practice, however, incited the inventive genius of engineers responsible for the operation of these plants, and a number of attempts were made to induce an air current by other means.
Ejector Systems. Steam ejectors were fitted to the closed tank provided for the reception of the material, thus converting the tank into a vacuum chamber, and eliminating the discharger. In other cases injectors, also operated by steam, were placed at intervals along the conveyor pipe, usually at such convenient points as 90° bends, and the slight vacuum created by the condensation of the steam and also by the velocity of the jet, induced an air current which swept the material along with it into the receiver chamber.
Although both these methods are in practical use, their applications are strictly limited to materials which do not suffer by contact with heat and moisture; the methods are therefore used principally for conveying ashes and soot from boiler furnaces and flues. Ashes formed by the combustion of coal contain large amounts of abrasive matter, and it is very important that all this matter should be extracted from the air, before entering the exhauster of the suction system. The steam jet cuts out the exhauster entirely, but absorbs an excessive amount of power in the form of steam. It has the advantage, however, of quenching the ashes on their way to the settling tank. For flue dust, however, the steam jet is unsuitable, as the condensed vapour causes the material to cake in the pipes, and the latter rapidly become choked, involving considerable delay and trouble in cleaning out. The ejector system is used for this material, the tank being of the closed type, and the necessary vacuum being created by a steam ejector fixed in a branch at the side near the top, the dust striking a baffle and falling by gravity to the bottom of the tank. A special air-tight gate or valve is opened to empty the tank.
Air Induction. The cardinal feature of the induction system is the ease with which materials may be handled which cannot be conveyed by the suction method.
Sand, sugar, salt, soda ash, and many other substances of a granular nature, which are very troublesome when conveyed by the suction method, may be dealt with economically by the induction system and, although the latter is only in its experimental stages at the moment of writing, it is possible that it may displace all other systems in the near future.
The induction system differs from the suction system in that the air flow along the pipe is induced by a jet of air, at very high velocity, fixed at any convenient distance from the intake nozzle of the conveyor pipe, and the material conveyed is discharged either from an open end into an open container, or by some form of cyclone. The closed discharger or container, with its baffles and rotary valves or air locks, is eliminated, and the substance to be handled has a free and unrestricted flow throughout the length of the pipe.
Advantages of the Induction System. The advantages of the induction conveyor may be summarized as follows: (1) Low first cost, the power unit being the only expensive item. (2) Low maintenance cost, there being no moving parts and little wear. (3) Low labour cost, practically no attendance being required. (4) Flexibility and ease of handling. (5) High efficiency of power unit and reliability of system. (6) Ability to handle materials which are easily damaged.
In the case of a suction plant handling grain or coal, the intake end of the conveyor is fairly flexible, and the nozzle may be operated over a fairly large radius, say, all over the floor of a vessel’s hold. The discharge end, however, is fixed, unless a cumbersome and expensive gantry is provided to permit of the discharge apparatus being moved about. Even should the discharger be mounted on rails, the area over which it can operate is limited by the rails on which it runs. With a large plant, this would mean that a number of dischargers would be required to lift from a ship into a warehouse or store, from the latter into bunkers or silos, or perhaps into trucks or waggons. Each discharger would require a separate exhauster and a separate intake, and valuable space would be occupied by the plant and expense incurred for machinery which would not be in use for a considerable part of the time.