Two kinds of dissimilarities are discussed: (a) Planned dissimilarities between the different hydraulic types as expressed by the performance specific speed; (b) deviation from geometrical similarity which is “forced” to accommodate some special condition at the same performance specific speed. Experience has shown that irrespective of a continuous variation of the design element of a consistent row of hydraulic types of impellers, there are certain factors or criteria which remain constant for different specific speeds. The best known of these is the suction specific speed. From the second group of dissimilarities the most common examples are mismatched impellers and cases. In solving such problems the designer’s judgment is based on a knowledge of causes and effects in selecting certain design elements to meet the desired performance, because these follow certain rules which are “laws” of Nature. The object of this paper is to review the most common of these laws as applied to dissimilar pumps and blowers.

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