. A text book of physics, for the use of students of science and engineering . made of hollow rim section, and cooling water is run intothe rim and scooped out again by means of a sharp-edged pipe. Mechanical efficiency of an engine.—The difference between theindicated and brake horse-powers represents the horse-powerrequired to overcome frictional resistances of the mechanism ofthe engine. Writing and respectively for thesepowers, we have: Power wasted in the engine mechanism = given to the piston per minute=\ x given out by fche engine per
. A text book of physics, for the use of students of science and engineering . made of hollow rim section, and cooling water is run intothe rim and scooped out again by means of a sharp-edged pipe. Mechanical efficiency of an engine.—The difference between theindicated and brake horse-powers represents the horse-powerrequired to overcome frictional resistances of the mechanism ofthe engine. Writing and respectively for thesepowers, we have: Power wasted in the engine mechanism = given to the piston per minute=\ x given out by fche engine per minute= x 33000. XXXIX STEAM TURBINES 515 The ratio of these quantities of energy gives the efficiency of theengine mechanism, and is called the mechanical efficiency of the engine. ,T , . , „, . x 33000 Mechanical efficiency = nnnnn J x 33000 Steam turbines.—It is only during the last twenty-five years thatthe steam turbine has been developed. The action consists inblowing steam against blades fixed to a wheel, and thus causing the. Fig. 472.—Action of the de Laval steam turbine. wheel to revolve. Part of a de Laval steam turbine is shown inFig. 472. In this type of turbine, the steam is brought to a nozzleand allowed to expand in passing through it. The expansion isapproximately adiabatic, and the work which would have been doneby the steam had it been expanded behind a piston is convertedinto kinetic energy in the steam. The result is that the steam leavesthe nozzle with a very high speed. In Fig. 472 there are fournozzles, each of which blows a jet of steam against the blades whichare fixed round the circumference of the wheel. If steam is expanded in a properly designed nozzle from a pressureof 150 to 1 lb. wt. per square inch absolute, it will leave the nozzlewith a velocity of nearly 1000 feet per second (assuming no losses).Theoretically the speed of the rim of the wheel should be one half of 516 HEAT CHAP. »**- Fig this, viz.
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Keywords: ., bookcentury1900, bookdecade1910, bookpublishe, booksubjectphysics