Fluid Division Mixing
(FDM)
TECHNOLOGY - Low
Shear |
Mixing Principle: Inter-Cavity
Transfer 
Fig A. FDM
rotor
Fig. B FDM mixer in Low Shear
Mode
and
stator
shown with dosing injectors
FDM is a concentric
rotor-stator system with indentations or cavities machines
into the rotor and stator surfaces (Fig. A). The rotor
and stator are based on a stepped conical form so that the
diameter of the cone increases from inlet to outlet. The
cavities in the rotor and stator are typically spherical
segments which are open on both vertical and horizontal
faces.
Medium to high viscosity fluid
is pumped into the inlet of the mixing head by an external
pump such as a gear pump or extruder. As fluid moves
through the mixer, it is repeatedly cut and folded by the
action of material transfer between the rotor and stator
cavities. As there is no closure to the flowpath through
the mixer and as the rotor-stator gap is normally large, the
pressure drop through the head is very low. This means
that highly effective blending is possible without applying
shear and extensional stressing to the material. The
low-shear FDM is therefore ideal for blending and conditioning
shear-sensitive materials such as rubber, plastic, certain
edible fats, adhesives, etc. Very good temperature
control of both rotor and stator surfaces is possible using a
combination of stator jacketing and internal channels within
the solid rotor. This increases the range of
applications still further.
Benefits
The
particular geometry of the FDM mixer has some particular
benefits when compared to older cylindrical-type inter-cavity
transfer mixers:
-
the stepped cavity rows allow
double the number of cavity interactions for a given mixing
length, making the machines more compact and less
expensive
-
The rotor and stator can both
be made as single, externally machined or cast components
rather that the "clam-shell" type of stator arrangement
normally used. This again reduces cost and makes
cleaning the machines easier.
-
The use of quarter-spheres
rather than hemispheres as cavities minimises the potential
for caking and bake-on of materials within cavities.
Cleanability is therefore much improved.
-
The stepped cavity rows ensure
that the shortest flowpath through the mixer is through the
cavities rather than through the rotor-stator gap.
This remedies a problem seen with cylindrical mixers where
low viscosity additives are often not properly incorporated
in the mixture.
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