Flat Blade Disk Turbine / RUSHTON TURBINE

Flat Bladed Disc Turbine or Rushton Turbine

 

Introduction & History

  • In 1950, J. H. Rushton invented an impeller called a flat bladed turbine. It is
    commonly known s the Rushton turbine or a flat bladed disc turbine (or disc
    turbine). The Rushton Turbine is considered a generic impeller today.
  • There are probably more mixing studies done and articles written using the
    Rushton Turbine (we name it RT) than any other impeller. The casual mixing
    investigator will think that mixing is only about Rushton Turbine. Well, with
    the huge amount of work done on the RT, there is also a wide disparity of data
    on the Rushton Turbine.

Construction & Working

The figure shows a six bladed Rushton turbine.

It resembles multi-bladed paddle agitators, turning at high speeds on a shaft
mounted centrally in the vessel. However, with the Rushton turbine, the blades
are attached to a disc, which is again mounted on a shaft.

The blades are vertical and flat. The impeller may be open, semi enclosed or
shrouded. The diameter of the impeller is smaller than with paddles, ranging
from 30 to 50 percent of the diameter of the vessel.

It is a radial flow impeller i.e. it is one of those, which generate current sin
tangential or radial directions.

Number of impeller blades ranges from 4 to 16, but is generally 6 or 8. Some
data is presented here which is quoted from J. H. Rushton’s very famous work way
back in 1950. Mixing theory was in its infancy back then. The 5th exponent on
impeller diameter was still being questioned. Rushton studied in his report many
different Rushton turbines and other impeller configurations, many that aren’t
used any more today.

Power number (Np) for wB/D=0.1, is 6.0 and that for wB/D=1/12, is also 6.0. Here
wB is the baffle width and D is the diameter of the impeller. For this
calculation, the impeller off bottom distance, OB, is equal to the impeller
diameter, OB/D=1.0. The liquid level is equal to the tank diameter, Z/T=1.0. The
impeller diameter, D, is about 1/3 of the tank diameter, D/T=1/3. The fully
baffled condition is assumed. Rushton studied the impact of the different baffle
standard widths, 1/10th for the metric system and 1/12th for the
American/British method.

Factors affecting the performance of a Rushton turbine

The power of a six bladed Rushton turbine is a function of viscosity, Reynolds’
number, geometry and baffles. With increase in viscosity, the power requirement
of the turbine increases. In other words, more power is required in order to
agitate a fluid having more viscosity.

The size and rotation of the trailing vortices were found to be influenced by
both blade number and blade curvature. Therefore, the blade shape may affect the
ease of formation of the ventilated cavities and reduce their size in such a way
that, even under gassing, power can remain high. Since the early papers on
mixing research, different designs other than the traditional flat-blade Rushton
turbine have been considered, like the arrowhead disperser, the curved-blade
turbine, or, more recently, the divided, inclined blades turbine.

In one of the earliest studies on a modified Rushton impeller, van’t Riet et al.
presented results on the performance of convex and concave designs. Flattened
power curves and improved gas handling capacity were obtained with a
concave-blade turbine, so it is proposed as an attractive alternative.

With decrease in viscosity, baffling becomes important and the baffle width gets
larger. Because the Rushton turbine is mainly used in low viscosity
applications, it is important to consider the effect of baffling o the
performance of the impeller. These turbines generate strong currents, which
persist throughout the vessel, seeking out and destroying stagnant pockets. Near
the impeller is a zone of rapid currents, high turbulence, and intense shear.

The principal currents are radial and tangential. The tangential components
induce vortexing and swirling, which must be stopped by baffles or by a diffuser
ring if the impeller is to be most effective. In an un-baffled tank, there are
strong tangential flows and vortex formations at moderate speeds. With baffles
present, the vertical flows are increased and there is more rapid mixing of the
liquid.

Most vessels have at least 3 baffles. 4 is the most common and is often referred
to as the “full baffled condition”. This means that any more baffles doesn’t
significantly add to the power consumption of the turbin

Applications

In general, turbines are effective over a wide range of viscosities. But the
Rushton turbine is normally employed on gas dispersion applications. Agitation
is one of the important factors in the chemical and biochemical reaction
processes.

Gas-liquid processes in particular, like fermentation and a variety of
oxygenation and hydrogenation processes, need a large gas handling capacity and
an effective gas dispersion for generating as large an interfacial area as
possible.

Disk turbines are radial-flow impellers that are particularly suitable for gas-
liquid dispersion through mechanical agitation. This is so not only because the
disk collects the gas underneath and forces it into the high shear zone near the
blades where formation bubbles occurs, but also because it eliminates the flow
instabilities shown by open-blade turbines.

The Rushton turbine has proved to be a good cost effective impeller for low
concentrations of immiscible liquid or gas. Two very strong trailing vortices
are shed from each blade. These areas of high shear are responsible for breaking
the larger droplets to smaller droplets.

They are primarily used for very high intensity mixing applications where low
power number impellers would tend to be very large.

In synthesis of Glycidyl methacrylate (GMA) (Sartomer, USA) and divinylbenzene
(DVB) (Merck), the discontinuous organic phase is introduced into the aqueous
phase, constantly stirring with a 6 bladed Rushton turbine.

Limitations of the Rushton turbine

The standard Rushton turbine is one of the most usual impellers found in the
industry. Some weaknesses of the Rushton turbine as an ideal gas disperser have,
however, been identified.

First, there is an important fall in power demand after gas is introduced,
usually more than 50% of the ungassed value. This represents an inconvenience
for power prediction, but above all it means a loss of potential for heat and
mass transfer.

Second, flooding occurring at relatively low gas flow numbers handicaps the gas
handling capacity of the Rushton turbine. Both disadvantages are due to the
formation of high-speed, low-pressure trailing vortices at the rear of the
blades, which are associated with the phenomenon of boundary layer separation.

Under gas dispersion, the low-pressure core of the trailing vortices attracts
gas bubbles, which coalesce to form ventilated cavities behind the blades.

These cavities not only lead to an increase in pressure and thus to an important
fall in power relative to the un gassed situation, but also control turbine
hydrodynamics and dispersion characteristics.

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