Facts and Fiction About
Swirl Separators and Hydroclones
Aquacultural literature will sometimes use the
terms swirl separator and hydroclone
interchangeably.
However, although swirl separators and
hydroclones function to remove solids from a
fluid, the process in which this is done is
distinctly different between the two.
HYDROCLONES
Rely primarily on centrifugal force
and unique hydrodynamics to
separate and remove solids.
Inlet water enters the
hydroclone tangentially at the
top of the unit.
At high water velocity,
considerable g forces of up to
7,500 are created.
This centrifugal action
separates particles by pushing
them to the boundary layer
along the outside wall of the
hydroclone.
Picture taken from “Aquacultural
Engineering”, Wheaton, 1985
As water spirals downward into the
conical area, it becomes restricted.
At the apex of the hydroclone some
water leaves via the undeflow while
the remaining water is forced back
upward.
This upward moving water forms a
“free vortex” that spirals upward
through the center of the hydroclone.
Formation of the free vortex is critical
to proper solids removal.
Picture taken from
“Hydroclones”, L. Svarovsky,
1984.
Centrifugal force separates the
solids, while the hydrodynamics
created by the free vortex removes
the separated solids.
The main features that affect hydroclone
performance are:
Diameter- Larger the diameter, the larger the cut size.
Inlet Diameter-Larger the diameter, larger the cut size.
Conical Angle- Greater the angle, larger the cut size, especially
beyond 25o
Vortex Finder Diameter- Larger the diameter, larger the cut size.
Note: Cut size refers to the particle size where 50% of
particles at that size will be removed by the hydroclone.
Clearly, it can be seen that hydroclone
efficiency to remove small particles is
reduced as hydroclones increase in size.
Small hydroclones with high velocity rates
are capable of removing particles <10 um in
diameter.
However, in commercial aquaculture
operations, a properly designed hydroclone
should remove approximately 80% of
particles above 77 um.
SWIRL SEPARATORS
Unlike hydroclones, swirl separators
remove particles through sedimentation
NOT centrifugation.
Therefore, a well designed swirl separator
should consider particle settling velocity,
volumetric flow rates and design features
that enhance sedimentation.
Unlike hydroclones, the higher the
volumetric flow rate, the less efficient
swirl separators become.
Out Let Flow
Settling Particle
Velocity
Inlet Flow
Upward Water
Velocity
Important design features:
Inlet design- Recent studies have shown that tangential
inlets (as are commonly used) result in less efficient solids
removal versus non-tangential inlets.
Tangential inlets increase water velocity and create
turbulence, both of which are counterproductive features in a
swirl separator.
Inlet designs that show the best removal efficiency consist of
a downward spout situated in the centre with a deflection
plate.
Another efficient inlet design incorporates a rectangular
shape.
Central inlet with deflection plate
Picture taken from “Solids Removal in
Freshwater Recirculating Aquaculture
Systems”
Rectangular Inlet
Picture taken from “Solids Removal in
Freshwater Recirculating Aquaculture
Systems”
Regardless of what inlet structure is chosen, the key
elements that should be considered for inlet design are:
1. Reduce turbulence.
2. Slow and disperse in-flow.
3. Minimize short circuiting.
Outlet Design- Outlets that offer the best solids
removal are weir type designs that encompass the entire
periphery of the swirl separator.
Whatever, outlet design is chosen, the main concern is
to choose a design that slows the water velocity (by
having a large volume capacity), creates quiescent
conditions and promotes water flow over the entire
volume (inhibits short circuiting).
Modeling shows that a well designed swirl separator
should be approximately 88% efficient at removing
aquacultural solids and display a cut size of
approximately 150 um.
In field studies, however, have found swirl separators to
be only 44-48% efficient. This poor performance may,
however, be due to the fact that swirl separators have
been designed with centrifugal separation in mind, when
in fact the primary mode of removal is via
sedimentation.