Steam turbines; a practical and theoretical treatise for engineers and students, including a discussion of the gas turbine . ed till the best efficiency is obtained. This will be whenthe steam leaves the last blades nearly at right angles to theplane of the wheel; that is, when the absolute velocity of the steamleaving the blades is, in the diagram, nearly perpendicular to theline showing the blade velocity (see pages 76 and 78). Design of Blades for Impulse Turbines. We shall continue withthe discussion of the design of blades for an impulse turbine withnozzles and with a single rowof blades,
Steam turbines; a practical and theoretical treatise for engineers and students, including a discussion of the gas turbine . ed till the best efficiency is obtained. This will be whenthe steam leaves the last blades nearly at right angles to theplane of the wheel; that is, when the absolute velocity of the steamleaving the blades is, in the diagram, nearly perpendicular to theline showing the blade velocity (see pages 76 and 78). Design of Blades for Impulse Turbines. We shall continue withthe discussion of the design of blades for an impulse turbine withnozzles and with a single rowof blades, assuming now thatthe entrance and exit angles(/? and y) have been deter-mined. We shall assume alsothat the total area of the noz-zles at their largest section hasbeen calculated as it has beenexplained on pages 44 to 49. To avoid losses by eddies,nozzles are often arranged ingroups placed symmetricallywith respect to the periphery fig. the blade wheel. Usuallythe nozzles would be arranged in two groups diametrically opposite in a circular plate, called adiaphragm, as in Fig. 48. We shall assume that each nozzle. Diagram Showing Location ofNozzles in a Diaphragm. 98 THE STEAM TURBINE group covers one-fourth of the circumference of the blade if the blades in the wheel were removed so that theycould not obstruct the flow of steam, the area through whichthe steam can pass is approximately J 7rDh for each nozzlegroup, where D is the mean diameter of the blade wheel and h isthe height of the opening from which the blades have been re-moved. When, however, there are blades on the wheel theheight h must be increased, because the effective area for thepassage of steam is reduced. Fig. 49 shows two views of a small segment of a blade pitch of the blades is marked p and the blade angle is 0. Ifthere are no blades, the area for the passage of steam in a lengthp is approximately p X h. With the blades in the wheel thearea is only h X p sin 0.* It follows then, w
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