Studying the Blueprints
Fig. [8] shows an elevation and part-sectional view of a 1500-kilowatt Curtis steam turbine. If one should go into the exhaust base of one of these turbines, all that could be seen would be the under side of the lower or fourth-stage wheel, with a few threaded holes for the balancing plugs which are sometimes used. The internal arrangement is clearly indicated by the illustration, Fig. [8]. It will be noticed that each of the four wheels has an upper and a lower row of buckets and that there is a set of stationary buckets for each wheel between the two rows of moving buckets. These stationary buckets are called intermediates, and are counterparts of the moving buckets. Their sole office is to redirect the steam which has passed through the upper buckets into the lower ones at the proper angle.
The wheels are kept the proper distance apart by the length of hub, and all are held together by the large nut on the shaft above the upper wheel. Each wheel is in a separate chamber formed by the diaphragms which rest on ledges on the inside of the wheel-case, their weight and steam pressure on the upper side holding them firmly in place and making a steam-tight joint where they rest. At the center, where the hubs pass through them, there is provided a self-centering packing ring (Fig. [9]), which is free to move sidewise, but is prevented from turning, by suitable lugs. This packing is a close running fit on the hubs of the wheel and is provided with grooves (plainly shown in Fig. [9]) which break up and diminish the leakage of steam around each hub from one stage to the next lower. Each diaphragm, with the exception of the top one, carries the expanding nozzles for the wheel immediately below.
FIG. 9
The expanding nozzles and moving buckets constantly increase in size and number from the top toward the bottom. This is because the steam volume increases progressively from the admission to the exhaust and the entire expansion is carried out in the separate sets of nozzles, very much as if it were one continuous nozzle; but with this difference, not all of the energy is taken out of the steam in any one set of nozzles. The idea is to keep the velocity of the steam in each stage as nearly constant as possible. The nozzles in the diaphragms and the intermediates do not, except in the lowest stage, take up the entire circumference, but are proportioned to the progressive expansion of steam as it descends toward the condenser.