The Mechanical Handling and Storing of Material

220 THE MECHANICAL HANDLING OF MATERIAL hour. At the Surrey Commercial Dock, in a similar pneumatic installation, 2-16 I.H.P. was consumed per ton per hour. The lift in this case is 12 ft. less than at Millwall, and the cylinders of the air-pumps are water-jacketed. In comparing the power consumed by pneumatic elevators, it must be borne in mind that in some cases of unloading there are 80 ft. of vertical and 100 ft. of horizontal pipes, which means conveying for some considerable distance, and entails consequent expenditure of power. Mr F. S. Tuckett states that with the last machine built on the Duckham system at the East Ferry Road Engineering Co.’s Works, working under ordinary conditions with steamer cargoes of 3,000 to 4,000 tons of bulk grain, an average quantity of 1 97 tons per hour can be removed with a crew of seventeen men and an engine-room staff of five, whose total wages amounted to 6s. 8d. per hour. The coal consumption is 475 lb. for every ton of grain lifted. Allowing for coal at per ton, and for stores at |th of a penny per ton of grain lifted, the cost of unloading these cargoes amounts to 2^d. per ton of grain.1 Mitchell’s Improved Pneumatic Elevator.—Mr A. H. Mitchell, for many years engineer to the late London Grain Elevator Co., and now on the engineering staff of their successors, the Port of London Authority, has had unique opportunities for experimenting with and investigating practically every known system of grain handling by pneumatic means. He has diagnosed the reason of the failures of some, and followed up or further developed the latent good points in others, and he is undoubtedly one of the best authorities of the day on this subject. The writer is indebted to him for a number of interesting interviews, and the following description of the latest developments introduced by him on this important method of grain handling is the outcome of these interviews. In the original elevators a tendency was discovered for the speed of the grain in the pipes to accelerate, during the whole of its journey, and thus enter the “ canister ” at an extremely high velocity. To rectify this obvious fundamental defect, the diameter of the pipes was increased as they approached the “canister” in order to reduce gradually the velocity of the air. This idea was developed on the lines that if the pipes receiving the grain from the hold were kept practically vertical, the initial velocity in the lower portion of the pipe would be sufficient to carry the grain a very considerable height without the further expenditure of energy. The existing machine was altered, therefore, in such a way that the expanded pipes were carried far down into the hold of the ship, and the upper end was joined to a more or less sharp bend of considerably enlarged cross section, and from here the pipe was conducted at a downward gradient to the “ canister.” This down pipe is of a diameter sufficiently large to reduce the speed of the air current as the grain slides down by gravity. As a further development the large vertical pipe below the bend was made telescopic, so that without the addition of more pipe the grain could be lowered down in the hold. This utilisation of the kinetic energy in the grain has led to a considerable decrease in the amount of power required. A typical example of a plant on these lines, built by' Henry Simon, Ltd., of Manchester, is shown in diagram Fig. 301, which represents a floating pneumatic elevator with a capacity of about 200 tons per hour. It will be seen that the pipes are not only telescopic but that the inclined down pipe forms a boom which can be slewed sideways, and at the same time luffed vertically. The plant is mounted on a steel pontoon. The suction pipes are of small diameter 1 For a comparison of the merits of the pneumatic elevator and the bucket elevator for unloading ships, see pages 213 and 474.