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It is usual to design the high-pressure end so as to do one-quarter of the total work. Hence the number of rows of 160 blades on this end will be ---- = 40. The 4 intermediate blade speed being V 2 times that of the high-pressure, the value of for this portion will be 80; whilst for the low-pressure end we have n = 40. The intermediate portion is also pro- portioned to do one-quarter of the total work, hence the number of rows of 80 blades on it will be -- = 20. The low- 4 presstire end, on the other hand, accounts for one-half the total work, whence the 40 number of rows on it will be -- = 20. Z Some makers subdivide the low pres- sure end of the turbine into two parts, increasing the mean diameter once again in the ratio V2 to 1. In this case each section of the turbine is rated to do one- quarter of the whole work. It is more common, however, to have three diam- eters only to the rotor, in which case the low-pressure end is, as stated, usually de- signed to do half the total amount of work, Practice varies somewhat as to the speed with which the steam is allowed to flow through the first stage of an elec- tric-light turbine. Some makers He at much higher than others, with the result that the work at full output is more evenly divided up between the high, low, and intermediate sections of the turbine. With 64 as the assumed number of ex- pansions, it may, as in in the case of 2700 marine turbines, be made equal to ---- Vn ft. per second; whilst if 81 is the assum- ed expansion ratio, it may be made equal 3000 to ----. n is, of course, obtained from Van the relation already given -- viz. °n= constant. Some designers work out the speeds through the whole turbine in great detail, apportioning to each group of Stages, or "expansion," as it is commonly termed in the shops, its due fraction of the total heat theoretically available for conversion into work. The areas avail- able for flow are then adjusted, not by the empirical rule given, but by direct calculation from the actual volume of the steam at each successive point of the tur- bine. Some very excellent results have been obtained in this fashion, but we are concerned here only with certain empiri- cal tules which yield fair results in prac- tice, and are very easy to apply. We shall, therefore, take the nominal "'TAE MarRINE REVIEW 3000 ratio of expansion as 81, and v = ---- as Vn the speed of the steam through the first row of blades. Assuming a consumption of 17 1b. of steam is required per kilowatt hour, the total passed will be 40,800 Ib. per hour, or 11.3 lb. per second. If the steam pres- sure above the governor-valve is 175 lb. gage, it may be taken at maximum out- put as 175 lb. per sq. in. absolute below this valve. Steam at this pressure and 150 deg. superheat will have a volume of about 3.25 cu. ft. per lb.; the volume passed per second will thus be some 37 cu. ft. Taking the steam velocity as 3000 ---- = 237 ft. per second, the net area V 160 37 required will be ---- = 0.1560 sq. ft. = < 231) 22.47 sq. in. If the area of the annulus is three times the net area, or 67.41 sq. in., the 67.41 blade height required will be ----- = 1.1; 19.5 " say, 11% in. In the above the fact that some 7 per cent. of the total steam sup- plied to the turbine commonly flows through the high-pressure dummy has been neglected. At the entrance to the intermediate expansion the steam will occupy (according to the empirical rule already given) three times its original volume. The mean diameter here being V 2 times that for the high-pressure end, and the steam speed also V2 times as great, the height of blade for the first stage of the intermediate section will be 3 so ee 1S times that..of tie tae V2Z2K V2 row of the high-pressure section. Simi- larly, the blade height for the first row of the low-pressure end will be 1.5 times that of the first row of the intermediate section. Each of these sections is com- monly sub-divided into from two to four "expansions," or groups. The practice of makers here varies considerably. Some have only two "expansions" per diameter, while others have four, and the number 3 is also frequently used. Adopting the number 4, the blade height of the high- pressure and intermediate groups may be 1.31 times the previous blade height. We then get the following table of blading, the heights having been adjusted to even sixteenths,, though the more common practice is to work to eighths :-- Group No. t. 0 iV ight--high res- eee ce ee ee ee Blade height, intermediate 11% 2% 2% 3% To correspond with the high-pressure 49 and intermediate sections of the turbine, the low-pressure end should have eight groups, with the following blade heights: Grotip No. 1.2, 3: 4 5. 6 Te 8, Bila dé height (inches) '2.53 3.31 4.38 5.73 7.60 9.95 13.25 17.25 Here, however, two difficulties make their appearance. The most serious of these is the excessive length of the blades at the condenser end of the tur- bine. In practice, the blade height is not made more than one-sixth of the mean diameter, corresponding in this case to 6% in. Hence the area required at the low-pressure end of the turbine must be obtained by the use of "wing" blades. These are blades of small curvature, the tangent to the curve of the back of the blade at entrance being at an angle of about 90 deg., whilst at discharge it is 45 deg. This gives a passageway to the steam equal to 0.71 of the annular area in place of the standard 1/3. Hence, using these blades, the height of the final rows may be 6% in., corresponding to a blade height of 13.8 in., for the standard _ blades, which, it will be seen, is less than it should be; but this constriction of the steamway cannot well be avoided here without making the rotor in four diam- eters. On the Mauretania, however, wing blades giving a passageway of 0.86 per cent of the annulus have been used, but so large a degree of "winging" as this is not adopted in turbo-generator practice. The other difficulty referred to lies in the fact that the total number of rows in the low-pressure rotor is only 20, which, if divided into eight groups, could give only 2%4 rows in each, which is, of course, impossible. Hence the Groups I. and II. may be combined into one of the same height--say, 134 in.--consisting of five rows of blades on both rotor and casing. The increase of area necessary to give the expansion due to Group II. 's -- obtained by "gaging" the last two rows of blades in both the rotor and on the casing. This is done by forcing a piece of metal between the blades so as to twist them more parallel to the axis of the turbines. By thus changing the angle of discharge from about 20 deg. to aboiti 2514 deg., the desired increase in the pas- sage for the steam is provided. Groups III. and IV. may be similarly combined, the height adopted being, say, 434 in., the final rows being "gaged" in precisely the same way. The four groups remaining may then be made into one of ten blades 614 in. long, the increase of area requir- ed from row to row being obtained part- ly by "gaging" and partly by the use of wing blades. The steam consumption and the pres- sure below the stop-valve at smaller our- puts are readily obtained by another em- pirical formula, or, rather, diagram. At