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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 four-stage machine and the initial pressure is 180 lbs., the pressure at the different stages would be distributed in about the following manner: Initial pressure, 180 lbs.; first stage, 50 lbs.; second stage, 5 lbs.; third stage, partial vacuum, and fourth stage, condenser vacuum.
The Curtis turbine is built by the General Electric Co. at their works in Schenectady, New York, and Lynn, Mass. The larger sizes are of the vertical type, and those of small capacity are horizontal.
Fig. 134 gives a general view of a 5,000 K. W. turbine and generator. The generator is shown at the top, while the turbine occupies the middle and lower section. A portion of the inlet steam pipe is shown, ending in one nozzle group at the side.
There are three groups of initial nozzles, two of which are not shown. The revolving parts of this unit are set upon a vertical shaft, the diameter of the shaft corresponding to the size of the unit. For a machine having the capacity of the one illustrated by Fig. 134 the diameter of the shaft is 14 in.
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 for the latter 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 con
sequence, 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. As in the Westinghouse-Parsons
type, the function of the stationary blades is to give direction to the flow of steam.
Fig. 135 illustrates one stage of a 500 K. W. turbine in course of construction. It will be observed that there are three wheels, and that in the spaces between these wheels the stationary buckets or vanes are
placed, being firmly bolted to the casing. Fig. 136 shows sections of both revolving and stationary buckets ready to be placed in position. The illustration in Fig. 135 shows the lower or last stage. The clearance between the revolving and stationary blades is from 3 to it in., thus reducing the wastage of steam to a very low percentage. The diameters of the
wheels vary according to the size of the turbine, that of a 5,000 K. W. machine being 13 ft.
Fig. 137 shows a nozzle diaphragm with its various openings, and it will be noted that the nozzles are set at an angle to the plane of revolution of the wheel.
Fig. 138 is a diagram of the nozzles, moving blades and stationary blades of a two-stage Curtis steam turbine. The steam enters the nozzle openings at the top, controlled by the valves shown, the regulation of which will be explained later on. In the cut Fig. 138 two of the valves are open, and the course of the steam through the first stage is indicated by the arrows.
After passing successively through the different sets of moving blades and stationary blades in the first stage, the steani passes into the second steam chest. The Aow 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 threecylinder 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