The Locomotive Of Today

The Boiler : Variable Biast Pipes. 31 Variable biast pipes are also largely used, and the advan- tages of being able to vary the opening of the exhaust orifi.ee are obvious, as, when an engine is provided with but one outlet, this has to be designed of a size small enough for the hårdest forcing required of the engine, and irrespective of the faet that a large proportion of its work will be performed under mueh more favourable conditions. When a driver has facilities for using a larger nozzle he can considerably reduce the back pressure when opportunities occur and lessen the draught through the fire. One style of pipe which has been largely adopted is of cast iron, and has a cap hinged at the front, that is nearest the smokebox door, controlled by levers, and workeel by a rod passing to the cab, where a handle and clamp enable either large or small outlet to be used at will. The diameter of the cap is that of the ordinary pipe for the same class of engine, and that of the pipe is about 40 per cent, larger. D, Fig. 6, shows a variable nozzle of this description in section, known as Macallan’s pipe, with the cap moved off its seat, allowing the steam to escape from the larger opening. E shows a plan of the pipe with cap on, so that the exhaust steam has to pass the smaller opening. It will be seen from these views that the pipe has two lugs cast on it, and the cap has one, which is made to fit between them, and a pin passes through, made square in the hole through the cap lug. We describe this form somewhat fully owing to its larjje application, but we can only find room to mention one or two others in actual use, as the designs for variable biast pipes are simply legion, and descriptions would fill pages of letterpress. A simple device has the nozzle made up of strips of elastic Steel arranged conically, and encircled by a cast iron band; the size of the outlet can be altered at will by raising or lowering this band. Providing a solid taper plug in the interiör of the nozzle, the raising or lowering of which will allow of more or less opening, is a third arrangement; whilst a fourth has double concentric openings at the nozzle, the inner being shut off from the outer when desired by a ring working inside the pipe ; for a large outlet and soft exhaust both nozzles are open, whereas for a sharp draught the con- nection may be closed, and steam only permitted to escape from the inner pipe. In the majority of variable biast pipes sliding surfaces are relied on, which must be more or less exact. These are not to be recommended for use in such a position as a biast pipe occupies, as they so easily become choked with soot, etc. An arrangement mueh used on the Continent is to provide the outlet with two hinged flaps, one on each side of the pipe,