Minutes of Proceedings of the Institution of Civil Engineers Volume 140 (Paperback)

This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1900 Excerpt: ..."orifice in a thin plate" and that above 100 lbs. per square inch Iib, having a sharp inlet, gives a greater velocity than II, which has a rounded inlet and the same outlet. Apparently a rounded inlet admits a greater weight of steam to the narrowest section than the orifice or nozzle can deal with efficiently. The advantage of I over Iia thus arises from its smaller discharge, the smaller quantity of steam being able to expand with greater freedom and consequently to develop a greater velocity than the denser steam issuing from Iia. From the point of view of the kinetic energy developed per pound of steam, the velocity curves may be taken to represent the "efficiency" of the various nozzles. The effect of a sharp inlet is then to reduce the density of the steam at the narrowest section, and hence less steam is passed, but the steam that does pass is fully or almost fully expanded, and hence, though the discharge is reduced, the efficiency is increased. In consequence of this conclusion all the later nozzles were designed with an inner edge only slightly rounded off. The curves in Fig. 11, though they do not actually become horizontal within the range of the experiments, appear to be asymptotio to horizontal lines, but they do not entirely exclude an inclined asymptote. On comparing the nozzle No. Iib with those in actual use on Laval turbines, it was found to lie midway between extremes. The Author therefore designed nozzles Nos. Ill and IV, which represent more extreme cases. Then, in order to separate the effects of taper and length of the expanding cone, No. Ill was cut down until its expansion was equal to that of No. IV; this nozzle is called Iiia. Fig. 1'2 shows the velocity curves of nozzles Nos. Ill, Iiia, and IV. Figs. 6, 9, an...