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stage or section of buckets in the De Laval turbine, why such good economy in the use of steam?
Ans.—The static energy in the steam as it enters the nozzles is converted into kinetic energy by its passage through the divergent nozzles, and the result is a greatly increased volume of steam leaving the nozzles at a tremendous velocity, but at a greatly reduced pressurepractically exhaust pressure-impinging against the buckets of the turbine wheel and thus causing it to revolve.
TABLE No. 11
Ques. 728.—Describe in general terms the Curtis steam-turbine.
Ans.—The Curtis turbine is built by the General Electric Company at their works in Schenectady, N. Y., and Lynn, Mass. The larger sizes are of the vertical type, and those of small capacity are horizontal. In the vertical type the revolving parts are set upon a vertical chaft, the diameter of the shaft corresponding to the size of the machine. The shaft is supported by and runs upon a step-bearing at the bottom. This step-bearing
consists of two cylindrical cast-iron plates bearing upon each other and having a central recess between them into which lubricating oil is forced under pressure by a steam or electrically driven pump, the oil passing up from
beneath. A weighted accumulator is sometimes installed in connection with the oil pipe as a convenient device for governing the step-bearing pumps, and also as a safety device in case the pumps should fail, but it is seldom required fro the Intter purpose, as the step-bearing pumps
have proven, after a long service in a number of cases, to be reliable. The vertical shaft is also held in place and kept steady by three sleeve bearings, one just above the step, one between the turbine and generator, and the other near the top. These guide bearings are lubricated by a standard gravity feed system. It is apparent that the amount of friction in the machine is very small, and as there is no end-thrust caused by the action of the steam, the relation between the revolving and stationary blades may be maintained accurately. As a consequence, therefore, the clearances are reduced to the minimum. The Curtis turbine is divided into two or more stages, and each stage has one, two or more sets of revolving blades bolted upon the peripheries of wheels keyed to the shaft. There are also the corresponding sets of stationary blades, bolted to the inner walls of the cylinder or casing.
Ques. 729.-What is the diameter of the vertical shaft for a 5,000 kilowatt turbine and dynamo?
Ques. 730.—How is the heat energy in the steam imparted to the wheel of the Curtis turbine?
Ans. Both by impulse and reaction. The steam is admitted through expanding nozzles in which nearly all of the expansive force of the steam is transformed into the force of velocity. The steam is caused to pass through one, two, or more stages of moving elements, each stage having its own set of expanding nozzles, each succeeding set of nozzles being greater in number and of larger area than the preceding set. The ratio of expansion within
these nozzles depends upon the number of stages, as, for instance, in a two-stage machine the steam enters the initial set of nozzles at boiler-pressure, say 180 pounds. It leaves these nozzles and enters the first set of moving blades at a pressure of about 15 pounds, from which it further expands to atmospheric pressure in passing
thru.gh the wheels and intermediates. From the pressure in the first stage the steam again expands through the larger area of the second stage nozzles to a pressure slightly greater than the condenser vacuum at the entrance to the second set of moving blades, against which it now impinges and passes through, still doing work, due to velocity and mass. From this stage the
steam passes to the condenser. If the turbine is a fourstage machine and the initial pressure is 180 pounds, the pressure at the different stages would be distributed in
Fig. 195. Diagram of the nozzles, moving blades and stationary blades of a two-stag Curtis steam turbine. The steam enters the nozzle
openings at the top. controlled by the valves shown, two of the valves are open, and the course of the steam through the first stage is indicated by the arrows.
about the following manner: Initial pressure, 180 pounds; first stage, 50 pounds: second stage, 5 pounds;