706
MODERN GASWORKS PRACTICE
merely to stand at his station, and by the turn of a lever can change from cycle to cycle. The working floor of the Beckton plant is seen in Fig. 433.
Single Superheater Plants
A. good deal of attention has been given to the single superheater type of plant, some examples of which now exist in this country. In such plants the carburettor and superheater are merged into a single vessel of rather larger capacity than the superheater of the more common plant. There is some conflict of opinion as to which method is to be preferred from the point of view of getting the utmost efficiency from the oil, but the single superheater plant is certainly less costly in the first place.
In this type of plant, it has been said that by introducing the oil at an intermediate point in the superheater the more volatile constituents separate out im-mediately, ascend, and make their escape before undergoing decomposition. Meanwhile, the lieavier constituents of the oil fall to the base of the vessel and are then swept forward again. by the current of water gas enter-ing from the generator, traversing the entire height of the chamber.
The most important plant operating on this principle in this country is that known as the Merrifield-Westcott-Pearson. As before explained, it <liffers from the double superheater plant in that the carburettor and super heater consist of one vessel, and the oil is injected at three distinet points round the periphery of the vessel instead of fiom one spray at
the top. The oil, moreover, is sprayed in a direction contrary to that of the gas, whereas in the Humphreys plant it travels with the gas. An additional feature is that the hot gas passes through the condenser before being serubbed, while in the original installations the main valves were cooled by a system of water circulation. The advocates of the double superheater system point out that the primary con-sideration affeeting the economical gasification of oil is that the oil shall be exposed,at a high rate of travel, to a proper fixing surface at a moderate and carefully adjusted temperature. Accordingly, this surface must consist of length in the direction in which the gas travels, rather than of cross-section : for the vapours must never approach a