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third stage, partial vacuum, and fourth stage, condenser vacuum.
Ques. 731.–What are the diameters of the wheels?
Ans. The diameters of the wheels vary according to the size of the machine, that of a 5,000 kilowatt unit being 13 feet.
Ques. 732.—What amount of clearance is there be, tween the revolving and stationary blades?
Ans. The clearance between the revolving and stationary blades is from 32 to 1 inch, thus reducing the wastage of steam to a very low percentage.
Ques. 733.—Describe the action of the steam in a two-stage Curtis turbine.
Ins.--The steam enters the nozzle openings at the top through valves that are controlled by the governor. After passing successively through the different sets of moving blades and stationary blades in the first stage, the steam passes into the second steam-chest. The flow of steam from this chamber to the second stage of buckets is also controlled by valves, but the function of these valves is not in the line of speed-regulation, but for the purpose of limiting the pressure in the stage-chambers, in a manner somewhat similar to the control of the receiver pressure in a two-cylinder or three-cylinder compound reciprocating engine. The valves controlling the admission of steam to the second and later stages differ from those in the first group in that they partake more of the nature of slide-valves and may be operated either by hand or automatically; in fact, they require but very little regulation, as the governing is always done by
the live-steam adınission-valves. As previously stated, the steam first strikes the moving blades in the first stage of a two-stage machine at a pressure of about 15 pounds
above atmospheric pressure, lit with great velocity. From this wheel it passes to the set of stationary blades between it and the next lower wheel. These stationary hlades change the direction of flow of the steam and cause
it to impinge against the buckets of the second wheel at the proper angle.
Ques. 734.-How is speed-regulation accomplished in the Curtis steam turbine?
Ans.-The governing of speed is accomplished in the first set of nozzles, and the control of the admission-valves here is effected by means of a centrifugal governor attached to the top end of the shaft. This governor, by
a very slight movement, imparts motion to levers, which in turn work the valve mechanism. The admission of steam to the nozzles is controlled by piston-valves which are actuated by steam from small pilot-valves which are in turn under the control of the governor. Speed-regulation is effected by varying the number of nozzles in flow, that is, for light loads fewer nozzles are open and a smaller volume of steam is admitted to the turbine wheel, but the steam that is admitted impinges against the move
ing blades with the same velocity always, no matter whether the volume be large or small. With a full load and all the nozzle sections in flow, the steam passes to the wheel in a broad belt and steady flow.
Fig. 198 5,000 KILOWATT GENERATING UNITS Comparison of space occupied and size of foundations. Modern Engine Type Unit and a Westinghouse-Parsons Turbine Type Unit of similar rating and overload capacity.
Ques. 735.-What great advantage does the steamturbine as a prime mover for an electric generator Dossess over the reciprocating engine?
Ans. -The advantage of a high speed of revolution,
whereby there can be a great reduction in the size. weight, and cost of the direct-driven generator.
Ques. 736.–Give approximately the over-all dimensions of a Westinghouse-Parsons turbo-generator unit of 5,500-kilowatt, 11,000 volt capacity, of the revolving field type, speed 750 revolutions per minute, vacuum to be 277/2 inches.
Ans.-Length 47 feet, width 13 feet, and height 14 feet to top of gallery-ring.
Ques. 737.—What amount of floor-space would a reciprocating engine ar.d direct-connected generator of equal capacity with the above occupy?
Ans.—The generator would be 42 feet in extreme diameter, its weight would be 445 tons (speed to be 75 revolutions per minute) and it, together with the fourcylinder piston engine, would fill a space 40 feet wide by 60 feet long, and tower 45 feet in height.
Ques. 738.-Describe in general terms the construc