3.9.2Centrifugal pumps
The most common type of pump used in the chemical industry is the
centrifugal pump, though its performance deteriorates rapidly with increasing
viscosity of fluids even with Newtonian fluids. The underlying principle is the
conversion of kinetic energy into a static pressure head. For a pump of this
type, the distribution of shear within the pump will vary with throughput.
Considering Figure 3.34 where the discharge is completely closed off, the
highest degree of shearing is in the gap between the rotor and shell, i.e. at
point B. Within the vanes of the rotor (region A) there will be some circulation
as sketched in Figure 3.35, but in the discharge line C, the fluid will be
essentially static. If the fluid is moving through the pump, there will still
be differences between these shear rates but they will be less extreme. For
a pseudoplastic material, the effective viscosity will vary in these different
regions, being less at B than at A and C, while a shear-thickening material
will exhibit the opposite behaviour. Under steady conditions, the pressure
developed in the rotor produces a uniform flow through the pump. However,
there may be problems on starting, when the very high effective viscosities of
the fluid as the system starts from rest might result in the overloading of the
motor. At this time too, the apparent viscosity of the liquid in the delivery line
is at its maximum value, and the pump may take an inordinately long time
to establish the required flow. Many pseudoplastic materials (such as food
stuffs, pharmaceutical formulations) are damaged and degraded by prolonged
shearing, and such a pump would be unsuitable.
As mentioned previously, it is generally accepted that the performance of
a centrifugal pump deteriorates increasingly as the extent of non-Newtonian
characteristics increases. Both head and particullarly efficiency, are adversely
affected, and the performance of small pumps is impaired to the greatest extent.
Severe erosion of the impeller and pump casing is also encountered especially
with particulate suspensions. Some guidelines for using charts based on water
for non-Newtonian materials by selecting a suitable value of the effective
viscosity are, however, available [Duckham, 1971; Walker and Goulas, 1984].
Carter and Lambert [1972], on the other hand, have found helical gear pumps
more suitable for viscous Newtonian and non-Newtonian fluids.
المادة المعروضة اعلاه هي مدخل الى المحاضرة المرفوعة بواسطة استاذ(ة) المادة . وقد تبدو لك غير متكاملة . حيث يضع استاذ المادة في بعض الاحيان فقط الجزء الاول من المحاضرة من اجل الاطلاع على ما ستقوم بتحميله لاحقا . في نظام التعليم الالكتروني نوفر هذه الخدمة لكي نبقيك على اطلاع حول محتوى الملف الذي ستقوم بتحميله .
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