. Applied thermodynamics for engineers. es FC, CD, DB, BO, are then laid off, equal respectively to eo, and Wo of FO. At any point like G, then, the steam velocity is ZG and the blade velocity is that for the drumin question: for G, for example, it is 181 ft. per sec Knowing the steam velocity and peripheral velocity for any state like G, weconstruct a velocity diagram as in Fig. 249, choosing appropriate angles of entranceand exit. In ordinary practice, the expansion in the buckets is sufficient, not- PRESSURE TURBINE 383 withstanding friction, to make the relative exit and absolute entrance
. Applied thermodynamics for engineers. es FC, CD, DB, BO, are then laid off, equal respectively to eo, and Wo of FO. At any point like G, then, the steam velocity is ZG and the blade velocity is that for the drumin question: for G, for example, it is 181 ft. per sec Knowing the steam velocity and peripheral velocity for any state like G, weconstruct a velocity diagram as in Fig. 249, choosing appropriate angles of entranceand exit. In ordinary practice, the expansion in the buckets is sufficient, not- PRESSURE TURBINE 383 withstanding friction, to make the relative exit and absolute entrance angles andvelocities about equal. (This equalizes the amounts of work done by impactand by reaction.) In such case, we have the simple graphical construction ofFig. 260. Since ah = he, dh = he, and ad = ec, we ob-tain u(ah-\-he) adQic -\-hd) work=- Drop the perpendicular hh, and with h asa center describe the arc aj. Draw dg per-pendicular to ac. Then dg^ = = ad{dh+hc), and ■1-2 work = ^^ foot-pounds, or Fia. 260. Art. 534, Prob. 18. —VelocityDiagram, Pressure Turbine. heat be laid off to some convenientother states give the heat drop curve This result represents the heat convertedinto work at a stage located vertically inline with the point G, Fig. 259. Let thisscale, as GH. Similar determinations forIJKHLMNOP. The average ordinate of this curve is the average heat drop orwork done per stage. If we divide the total heat drop obtained by the averagedrop per stage, we have the number of stages, the nearest whole number being taken.* Suppose the machine to he required to drive a 2000 kw. generator (2400 kiv. overloadcapacity) at 175 lb. initial absolute pressure and 50° of superheat, the condenser pressurebeing 1 lb. absolute, the r. p. m. 3600, the generator efficiency and the losses as follows:steam friction, ; leakage, ; windage and bearings, ; residual velocity inexhaust, The theoretical heat drop is 1227-890 = 337 B.
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