Page 206 - Centrifugal Pumps Design and Application

180 Centrifugal Pumps: Design and Application

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paper, question arose as to whether inclusion of the Ui /2g term in the
static head expression recognizing the blade inlet diameter is appropriate.
Low through flow and a strong forced vortex might well combine to ex-
tend rotation to the impeller center!ine, i.e., might introduce prerotation
of inlet flow. Measurements indicating that static pressure at the tip is
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close to u 2 /2g reinforce this view. Also, inlet prerotation is indicated by
quite respectable suction performance despite radial blade inlet geome-
try. Thus, the theoretical head normally used in practice simplifies to

When the subscript is dropped, u is taken to indicate the impeller tip
speed. Actual head for P.E. pumps is then stated as:

To further understand the workings of the RE. pump, a somewhat sim-
plistic exercise involving a mixture of theory and experience is put forth
to establish how actual head is generated. As has been indicated, tall
blade geometry produces a strong forced vortex resulting in a static head
coefficient near unity, so the actual static head produced by the impeller
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is simply u /2g. Tests have shown that diffusion efficiency is nearly flat
through much of the flow range, so we state that *? d = .8. Diffusion re-
covery potential is in accordance with the diffuser area ratio

And finally, the P.E. flow coefficient is in the vicinity of 0 = .8. So ac-
tual head generation may be written

Say, then, that the diffuser terminates in two throat diameters, i.e., has an
area ratio of 4, so the actual head should be