Modern Gasworks Practice

308 MODERN GASWORKS PRACTICE is that the capacity of a grab to fill is in no way increased by dropping it down heavily on to the material. It is a fallacy to suppose that the jaws dig more effectively into the material if this is done; for, with. properly designed grabs, the action of the jaws is a gathering one, and they have a natural tendency to dig themselves in. ELEV ATING AND CONVEYING MACHINERY Only in exceptional cases is the gasworks engineer in a position to design in detail mechanical plant for the handling of his Materials, and as there are to-day many contractors wlio are specialists in this type of work there should be no hesita-tion in placing the matter in their hånds. If this is done, standardized details will not be departed from, and the system will gain in efficiency and economy. In obtaining tenders or settling upon new plant there are several points whicli the engineer with the erection of machinery in view should bear in mind. He should supply to the contractor figures showing the capacity of plant in tons per hour, stating the conditions of working. In return, the contractor should be asked for the following particulars :— (o. ) Breadth. of elevator buckets or conveyor band. (b) Pitch of buckets, and speed of chain. or band. The engineer should pro-visionally fix the speed, laying down a certain limit. (c) Detailed specification of buckets and chain; or, (d) Detailed specification of belt. (e) Particulars of arrangement for taking up elongation of the chain or belt, and the facilities afEorded for the renewal of working parts. (f) Details of framing and supporting girders, with the thickness of all sheets and plates in hoppers, etc. 1 (g) Power required for driving; also types and sizes of motors or engines supplied, and surplus power provided. (m) Arrangements for lubrication ; also what provision is made for the lesseiiing of friction. (n) If electrically driven, details of cabling, starters, and accessories (if any) supplied. No general ride can be laid down as to the limiting size of a works on which coal or coke-handling plant can be profitably employed. It may be taken, however, that a minimum size plant (including buildings) would cost from £4 500 to £5,500 ; and 17| per cent, on this outlay represents about £875. Accordingly a 60 million per annum works would have about 3s. per ton of coal used before the machinery could be shown to be profitable. There is a certain similarity between elevators and conveyors in that, so far as types employed on gasworks are concerned, they consist almost solely of endless belts or chains passing around two terminal pulleys. To such chains are attached suitably shaped buckets or draw-bars. The length of the conveyor or elevator is 1 The engineer will generally prefer to purchase his own motors, arranging for the surplus of power he thinks advisable. (See Chapter X.)