Engineering Wonders of the World
Volume III

378 ENGINEERING WONDERS OF THE WORLD. generated to produce the forced draught for the furnace. Continuous improvements in the furnaces have entirely reversed the proportions of the furnace and the boiler, and whereas in the early days a boiler of 25 or 50 horse-power was considered sufficient for a row of eight or ten large furnaces burning at a comparatively slow rate, we now find boilers of 200 or 300 horse-power attached to a battery of two or three furnaces, the boiler taking up almost as much room, and costing almost as much money, as the destructor itself. So far from merely providing the steam for their own forced draught, modern destructors produce a vast surplus which is used for many purposes, the production of electric light and power being one of the most important. Striking examples of such destructors on modern lines may be found in Liverpool, Not- tingham, Glasgow, Greenock, London, and many Continental towns and cities. Some of these plants are provided with a complete electrically-driven equipment for handling the refuse, so that there is neither raking, shovel- ling, nor handling of the material by the work- men until after it has passed through the puri- fying process of fire. We describe as an example a plant recently- erected at Greenock, and may mention that plants on precisely similar principles have been erected in the borough of Poplar, London, and the cities of Melbourne (Australia), St. Peters- burg and Warsaw (Russia), and Zürich (Switz- erland). The plant at Greenock will serve as a type of the rest. This consists of six cells or fur- naces, divided into three batteries, each bat- tery consisting of two cells, and Greenock havjng attached to it a water- Plant. , , , -i p k tube boiler of 2o0 horse-power. Forced draught is produced by means of elec- trically-driven high-pressure fans, which draw the air from various parts of the building where ventilation is required, and, after pre- liminary heating, blow it into the ash-pits of the cells. An air pressure equal to about five inches water column of water is maintained under the grate. The rate of combustion is about 100 lbs. per square foot of grate per hour, which is about double the rate usually obtained in the boilers of battleships under forced draught, this with a fuel consisting en- tirely of rubbish, and popularly supposed to contain nothing of value whatever. The steam produced is sufficient, when used in engines of a modern type, to produce about 100 electrical units (kilowatt hours) for every ton of refuse burnt. In other words, from six to seven tons of refuse produce an amount of steam equivalent to that obtained by burning a ton of good coal. The stoking of these furnaces is done by means of an overhead electric crane. The carts, on arriving at the destructor, tip their con- tents into a series of boxes, , , . r i i J- £ Automatic each capable of holding from stokjng one to two cart loads. As the carts come in at irregular times, and the refuse has to be burned with strict regularity, these boxes are kept ready filled until needed, and are then lifted by the crane, and placed in a cradle on the top of the furnace, so arranged that the weight of the box opens the door of the furnace, thereby permitting the contents to be dropped bodily into the destructor, the door being automatically closed by the lifting of the box. When closed, the furnace door is sealed by dipping into a water trough on the same principle as the ordinary gasholder. The labour of the furnacemen is thus con- fined to the removal of incombustible residue from the destructor. This residue, known as clinker, consists chiefly of silica, and is broken up for making concrete, ground up with lime to make an excellent mortar, or used after fine grinding and mixing with a small proportion of lime in the manufacture of artificial bricks, or (using cement instead of lime) for the manu- facture of paving flags.