by Joe Evans, Ph.D.
January 20, 2012

The affinity laws tell us quite a bit about the inner workings of an impeller. We know that, for any given impeller, the head it produces varies as the square of a change in speed. Double the speed and the head increases by a factor of four. If you keep speed constant, the same rule holds true for small changes in its diameter. 

The flow through an impeller follows a similar rule, but in this case, its change is directly proportional to the speed or diameter change—double the speed or diameter, and the flow doubles. Actually, a change in rotational speed or impeller diameter refers to its peripheral speed or the speed, in feet per second, of a point at its periphery. It is this speed that determines the maximum head and flow attainable by any impeller.

The head produced by an impeller is almost entirely dependent upon its peripheral velocity, but flow is influenced by several other factors. Obviously, the width and depth (cross sectional area) of the flow passages (vanes) and the diameter of the impeller eye are important considerations as they determine the ease with which some volume of water can pass through the impeller. 

Other factors such as vane shape also influence an impeller’s performance. But where do you start if you want to design an impeller from scratch? Do you take a wild guess about dimensions and shapes, make some samples and then test them? 

In the early days, that is exactly what pump designers did. Today, however, they can draw on years of experience and, at a minimum, find a suitable starting point for the design. That starting point is specific speed. Next month, we will investigate specific speed and how it can predict the performance of a particular impeller.

Where Can You Learn More?

There are many great pump books available today, but one of the classics is now available as a free download at Google Books. Pumping Machinery was authored by Arthur M. Green, a professor of mechanical engineering at Rensselaer Polytechnic Institute and was published by John Wiley & Sons over 100 years ago. It begins with a comprehensive history of pumps and ends with a detailed review of centrifugal pumps and their advances over the previous twenty years. You will be impressed at the level of knowledge possessed by the author. The number of illustrations is amazing and accounts for a significant portion of its 725 pages. This one gets an A+ rating from me.

Pumps & Systems, February 2012

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