454 MODERN GASWORKS PRACTICE
of the bell is counterbalanced by loading the wheels on the shaft, as shown. This
floating bell is composed of a cast-iron polygonal top, having cast-steel posts depending from each corner. The spaces between the posts are filled in with
Fig. 281. —Dempstek’s Primary Extractor.
bundles of perforated and slotted plates spaced. a short distance apart, the perforations being so arranged as thoroughly to baffle and break up the gas on its outward passage. Safety valves are fitted on the drum top so that in the event of the suspension chain breaking a gas outlet will be pro-vided. The action of the machine is automatic, in that the extractor surface exposed to the gas varies in accordance with the quantity of gas passing through the mains.
Several forms of apparatus for tar removal, many of which are employed as primary extrac-tors, are now to be found. Tliere is no question, of course, that it is preferable to remove tlie sus-pended tarry vesicles at as early a stage as possible, and while the gas is still warm ; although this coadition does not apply in the case of the light oil fog which should be permitted. to remain in the gas up to the condenser outlet. A description has already been given of a favourite form of primary extractor, namely, the “ Cyclone.” Another type, operating on a different principle, is shown in Fig. 281. This extractor consists of a central chamber around which is a sub-divided annular space. The central chamber is of large area, and is
provided with special screens through which. the gas must pass as it ascends. The suspended vesicles in the gas are deposited on the screens, the liquid thrown down
dripping from one screen, to another and assisting in. the process of washing the gas. In addition., a central spray is fitted at the top of the chamber. Thin tar is passed