Handbook of Turbomachinery Second Edition Revised - Ventech!

alternative frequently employed. If the relative angle of the flow leaving an
impeller were parallel to the impeller blades at the trailing edge, the absolute
value of the tangential component of the exit velocity would be Vy;i. The
ratio of Vy;2, the tangential velocity actually realized, to Vy;i is called the slip
factor. Various formulas for estimating slip factor have been published, but
most are unreliable. Most of the values were deduced by measuring the
temperature rise in the impeller and calculating the value of DH and the
value of Vy;2 by using Eq. (7). In nearly all impellers, however, some flow is
recirculated between stations somewhere between the inlet and discharge.
The actual value of Vy;2 is lower than that calculated. In fact, slip factors
greater than unity have been reported, and this is absurd. A practical
solution is to estimate the slip factor by Eq. (22), and then increase the
impeller diameter by about 5%. The impeller tip is gradually cut back to the
proper value during development testing. Chapter 4 of Balje [19] has a good
discussion about slip factor. One of the equations noted there is
Slip factor ¼ 1 ð3sinb b;2Þ=Z ð22Þ
STATORS OR DIFFUSERS
Four types of stators or diffusers have been used. They have been designated
as vaneless, vane island, cascade, and pipe diffusers. Most often, nearly all
the diffusion takes place while the gas velocity has a significant radial
component. The flow then is usually either turned to the axial direction, as
shown in Figs. 1–4, or collected by ascroll, which changes the flow direction
to a tangential one. Chapter 6 of Balje [19] reviews the design of scrolls.
Vaneless Diffusers
These diffusers are rarely used in industry except when some efficiency can
be sacrificed to reduce manufacturing costs, when size is not a criterion, or if
a wide range of flow rates between the maximum flow and surge is
mandatory. Scrolls are generally used with these diffusers. Chapter 6 of
Balje [19] reviews the design of these diffusers. As previously noted,
deceleration of the tangential component of velocity leaving the impeller is
automatically accomplished as the radius of the flow increases. Deceleration
of the radial and axial components, which is controlled by the space between
the two walls, must be kept small.
Short, vaneless diffusers also offer a cost-effective way to execute part
of the development testing of centrifugal and mixed-flow compressors. The
Copyright © 2003 Marcel Dekker, Inc.