Smith Turbine

Smith Turbine

Description

-This impeller was found in 1980’s by smith and coworkers. It is most commonly
referred to as the Smith Turbine, after Prof. Smith. The RS was never patented
and therefore is considered generic. In different company, It’s name is
different.

RS6

Chemineer CD-6

Lightnin R130

Philadelphia Mixers Smith Turbine

Hayward Gordon RDC

Np=3.2, Nq=0.61*

-This impeller is similar to the Rushton turbine except curved blades are
substituted for the vertical flat blades resulting in a lower power number and
some other advantages.

Construction and working

-This impeller has 6 blades which are fixed on a disk. The diameter of the
Rushton turbine should be 1/3 of the tank diameter. The RS is a radial turbine
with 1/2 pipe blades. It is most commonly referred to as the Smith Turbine,
after Prof. Smith. The original Smith Turbine did not use a full 1/2 pipe, but
the commercialization of this impeller dictated this. The impeller was designed
to disperse gases better than a Rushton Turbine (RT) and without the huge power
drop.

Turbulent Power Number Np=3.2, Flow Number Nq=0.61

-The flow number is estimated by Post Mixing using the 1/3 power law and
comparing it to the 6 bladed Rushton Turbine, RT6.

Why curved blades?

-Researchers recognized that the impeller blade itself plays the major role in
gas dispersion and postulated that large blade can performs better in terms of
gas dispersion.

-Van’t Riet and Smith pointed out that the vortex pair behind the impeller blade
plays the lead role in gas dispersion in a stirred vessel. Thus, 6 blade disk
turbine impeller has become the most important because the impeller blade
affects not only the pumping capacity of the impeller but also the strength of
the vortex. It has also impact on mass transfer rate of the aerated agitated
vessel.

Advantages

-This impeller was designed to disperse gases better than a Ruston turbine.

-It is able to handle more gas before flooding and does not experience as great
a power drop-off due to gas loading than the Rushton turbine. However, at
elevated power levels it produces similar mass transfer as the Rushton design.

-At each blade two strong starling vortices are formed. These areas of high
shear are responsible for breaking the larger droplets into smaller droplets.

Applications

-It is good effective impeller for low concentration of immiscible liquid or
gas.
-This is most commonly used in microbial fermentation to provide high shear
condition requirement for breaking bubbles and thus increasing the oxygen rate.

Limitations

-power demand in the flooded regime may be significantly higher than in the
absence of gas. Furthermore, once flooded, there is a considerable hysterics
that delays the return to loaded operation when the gas rate is reduced.

-The work has also shown that for a given off-gas rate, the relative power
demand of an aerated impeller is almost always greater in a hot system than at
ambient temperature, though it is not yet possible to quantify the increase.

-Bioprocess research and development , Merk research laboratories, USa examined
that CD 6 suggested minimal power drop with aeration, improved mass transfer
rates with airflow increases.

-These arrangements may warrant further study under a wider range of production
consideration.

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