The Mechanical Handling and Storing of Material

CHAPTER XL COAL, COKE, AND ASH HANDLING PLANTS FOR BOILER- HOUSES, GASWORKS, ETC. The Handling of Coal.—-The coal-handling equipment of a boiler-house usually consists of an elevator in conjunction with one of several types of conveyors, or of a gravity bucket installation combining the work of an elevator and conveyor. In general, slow speed machinery is recommended for such work, as it is more immune from breakdowns, has a. longer life, and smaller maintenance expenses ; on the other hand, the initial cost of such machinery is enhanced. The driving mechanism should be substantial and yet as simple as possible. It should be borne in mind that the detail of construction of any one type of conveyor, important as it may be, is not the all-important factor, but rather the general scheme of the equipment and the design in its broader features should be adapted to the work required, so that the upkeep and working cost will be a minimum, and that the system as a whole may do the work as efficiently as possible. The mechanical appliances for handling coal and ashes have been fully described in the previous chapters, but the bunkers holding the coal for immediate use above the boilers justify more than a passing notice.1 Ordinary rectangular hopper-bottomed bunkers are well known and frequently used, but the type of self-cleaning suspension bunker, either of boiler plates or reinforced concrete, require a few general remarks. Such bunkers have come to the fore of late years, and are likely to be used universally in installations in the future. The construction is very simple. From two main supporting girders steel straps are suspended at close intervals, and they are covered on the inside by steel plates, the steel straps taking the full weight of the bunker and contents and transmitting it to the longitudinal gilders on each side. The maintenance of a steel plate bunker is comparatively high on account of corrosion, and hence the reinforced type has been introduced. With this latter system the steel straps forming the support are laid with ferroinclave (a special crimped steel sheet) or other reinforcements, with an inside lining of cement and an outside cement sheathing, the ferroinclave sheets forming the reinforcement for the concrete linings. Such bunkers are practically indestructible. Fig. 882 gives an illustration of such a bunker in the general power plant of the United States Navy Yard at Norfolk, in conjunction with a travelling weighing hopper for the coal distribution from the bunkers. The required capacity of bunkers depends upon the interval during which the supply may possibly be interrupted, but generally provision is made for no longer a period than one week, and longer interruptions may be provided for by stock heaps in close proximity to the boiler-house, from which the bunkers may be replenished. Coal bunkers of large capacity were formerly often placed in a separate building along- side the boiler-house, or the retort-house of gasworks, but the great advantage and saving in labour in having the bunkers suspended above the boilers or retorts—to feed them by 1 See also the introductory remarks in Chapter XLI., “ Storing of Coal and other Minerals in Stock Heaps and Silos.” 630