298 MODERN GASWORKS PRACTICE
The machine was originally introduced at a time when many engineers still favoured the light charge of coal, and one of the few disadvantages it embraced was its inability to completely fill the ordinary retort with coal. This was due to the fact that a certain amount of room must be left in the crown of the retort for the return of the chain plates. In the latest type of machine (Fig. 215), however, this drawback has been overcome, and retorts may now be completely filled if so desired. The main polygonal driving-wheel is still retained, and the modification has chiefly occurred in the construction of the chain. All push-plates are dispensed with, and instead there is a chain carrier inside which runs a flexible conveyor. The chain carrier and conveyor are set in motion and enter the retort together, the conveyor depositing
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Fig. 215a.—The New Type of Fiddes-Aldridge Machine, for delivering Full Charges. SeCTION OF THE CHAIN.
coal along the bottom of the retort until the far end is reached. The speed of the flexible conveyor is then increased as both are withdrawn, and coal is thrown over the end of the conveyor at such a rate as to fill the retort completely. By varying the relative speeds of the chain carrier and the running chain or conveyor any sized charge can be placed in the retort. The flexible conveyor runs the full length of the carrier chain, so that t'h.ere is in reality one conveyor inside another. As ex-plained above, a thin layer of coal is laid on the bottom of the retort during the outward discharging stroke, but this is not essential, and, if so desired, the whole of the charge may be delivered over the end of the conveyor during the return stroke. The construction of the machine will be followed by reference to Fig. 215a, in which. the flexible and carrier conveyors are clearly seen.