Canadian engineer . ow heads and attendant low May i6, igi2. THE CANADIAN ENGINEER r/,1 Velocities permit of a design of bucket to handle largequantities of water. In such a runner, thin warped bucketsof ample size and large openings can be used. Thesebuckets are, relatively speaking, structurally weak. Highheads and consequent high velocities make necessary asimple design of bucket, thicker material, and, therefore,relatively small quantities of water. Strength, therefore,must be carefully considered in determining the diameterand speed of a runner. In general it may be said thatfor proper st
Canadian engineer . ow heads and attendant low May i6, igi2. THE CANADIAN ENGINEER r/,1 Velocities permit of a design of bucket to handle largequantities of water. In such a runner, thin warped bucketsof ample size and large openings can be used. Thesebuckets are, relatively speaking, structurally weak. Highheads and consequent high velocities make necessary asimple design of bucket, thicker material, and, therefore,relatively small quantities of water. Strength, therefore,must be carefully considered in determining the diameterand speed of a runner. In general it may be said thatfor proper strength and design, the following specific speedsmay be used for any head below the maximum head Speed. Maximum Head. K rev. per min. feet. meters. 68 tc/ 75 50 15 50 to 68 100 30 35 to so 200 61 25 to 35 400 122 10 to 25 ..600 183 This tabulation is not by any means fixed and is merelygiven as a guide. If the specific speed figures out for anygiven head as greater than the values given above, two run-. 30jr25?20jl5jn^?53r0 5^ I0)t 15:< XH 23^ 30) EEO Fig. 9.—Effect on Power ofChange of Speed, Basedon Constant Head. sor£ EQUIV. SPEED VARIATION Fig. 10. (See Fig. g.) ners should be used. If two runners are used, the specificspeed must again be figured out with a horse power one-half of that used before. If head were the only determin-ing factor, the specific speed could be chosen from the abovetable and the rev. per min. calculated from the formula. (b) Characteristic Efficiency.—It has already beenshown that different values of K result in a different effic-iency characteristic. From our formula it is noted thatspecific speed varies with rev. per min., hence for anygiven horse power and head the rev. iper min. fixes thespecific speed, and tJie efficiency which is obtained from arunner with this specific speed both for full load and forpartial loads. The particular speed characteristic desired,therefore, has much to do with selection of
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