Steam turbines; a practical and theoretical treatise for engineers and students, including a discussion of the gas turbine . nsion.* Practical conditions, however, as stated * A nozzle with a circular section (perpendicular to the axis) has hss surfaceexposed to the flow of steam than a nozzle of any other form of the same lengthand expansion. For this reason this form should give minimum friction practice, however, this type is not often used when the section at the mouth ofthe nozzle is made rectangular, at least when the nozzles are arranged in groupswith the mouths of the several
Steam turbines; a practical and theoretical treatise for engineers and students, including a discussion of the gas turbine . nsion.* Practical conditions, however, as stated * A nozzle with a circular section (perpendicular to the axis) has hss surfaceexposed to the flow of steam than a nozzle of any other form of the same lengthand expansion. For this reason this form should give minimum friction practice, however, this type is not often used when the section at the mouth ofthe nozzle is made rectangular, at least when the nozzles are arranged in groupswith the mouths of the several nozzles close together. There are obvious advan-tages from this last construction, as first pointed out by Professor Riedler, becauseif the nozzle mouths are made rectangular and close together a long continuousband of steam is secured which is approximately homogeneous and of constantvelocity. The flow from the end nozzles is, of course, affected by excessive eddyingand other irregularities just as single nozzles. Efficiency of the end nozzles istherefore considerably less than that of any of the others in the Fig. Distance along Axis of Nozzle 23. Expansion Curve of a Nozzlewith an Elliptical Axial Section. 52 THE STEAM TURBINE before, make the nozzle shown in Fig. 24 with expanding straight-line walls preferable if the throat and mouth areas are properlydesigned. Fig. 24, however, is intended to show primarily theeffect of using a nozzle for non-condensing service, which wasdesigned to be used condensing. For this reason the expansion in the nozzle is greaterthan it should be for thepressures with which itis operating; and for thisreason th^ pressure insidethe nozzle, as illustrated bythe curve, falls below theexhaust pressure. Thisis called over-expansionor over-compounding and is always accompa-nied by a loss in fact, as will be shownagain later, the effect ofover-expansion, or makinga nozzle too large at themouth, reduces nozzle effi-ciency much mor
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