Page 205 - Centrifugal Pumps Design and Application

High Speed Pumps 179

some portion of the exit flow path; for example, plugging some of the
diffiiser passages in a vaned diffuser. This, of course, results in impeller
passages that are oversized for the lower flow rates according to conven-
tional design practice, but in fact can produce efficiencies superior to
those attainable with the very narrow passages that would result in EE,
design procedures. The term partial emission (P.E.) arose to describe
such pump geometry, apparently coined by Balje.
The Barske pump is correctly classified as a partial emission type,
since the emission throat area is much smaller than the impeller emission
area. More to the point, net through-flow in the Barske pump can occur
only in a path extending generally from the inlet eye to the vicinity of the
emission throat. This is true for the simple reason that the remainder of
the case cavity is concentric with the impeller and is filled with incom-
pressible fluid, precluding any possibility of a radial flow component.
High circumferential fluid velocities exist in the forced vortex created by
the impeller, which are superimposed on the through-flow stream ex-
tending from eye to throat. Through-flow is then in essence a fluid mi-
gration, where a given element of fluid makes a number of circuits within
the forced vortex and moves to successively higher orbits in the eye-to-
throat flow region.
Alternatively, the Barske pump can be referred to generically and
geometrically as a concentric bowl P.E. pump or simply a concentric
bowl pump. This is convenient for easier differentiation of the original
pump type from its evolutionary offshoots to be described later.

Partial Emission Formulae

Use of tall, radial-bladed impellers in P.E. pumps results in flow con-
ditions that must be described as disorderly. No attempt is made to match
inlet geometry to the flow streamlines. Very low mean radial flow veloc-
ities combined with high tip speeds reduce the discharge vector diagram
to essentially the tangential tip speed vector, U2. Calculation procedures
for P.E. pumps then are based on simple algebraic expressions involving
impeller tip speed rather than on the vector diagrams used in EE. design.
Barske starts with the assumption that the fluid within the case rotates
as a solid body or forced vortex, and neglects the negligibly low radial
component, resulting in a theoretical head of:

The first term represents the vortex or static head and the second term
represents the velocity head or dynamic head. Even within the Barske