Page 10 - GlobalEyes Edition 6 Dec 2011

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Bradken is a recognised
authority in casting design,
specification and manufacture
of high chromium iron ball
and ring consumables.
Typically used in vertical
spindle mills for coal fired
power applications, the
vertical spindle ball and ring
mill consists of two rings
separated by a series of
large balls. The lower ring
rotates, while the upper ring
presses down on the balls via
a set of pressurised rams.
Coal is introduced into the
centre as the lower ring
rotates which forces the balls
to roll and pulverise the coal.
The pulverised material is
carried out of the mill by the
flow of air moving through
it. The size of the pulverized
particles released from the
grinding section of the mill
is determined by the air flow
feeding into the main chamber
through the throat and the
efficiency of the classifier
separator. If the coal is fine
enough to be picked up by
the air, it is carried through the
classifier. Coarser particles
return to be further pulverized.
Feedback from Bradken’s
customers identified that
rotating air throats were
becoming increasingly popular
as retro-fitted modifications
to coal mills as they improved
burning efficiency and reduced
emissions by improving
the size distribution of coal
particles fed into the classifier
and subsequently the burners.
Many of these new rotating
throats fit directly to the
lower grinding ring using
either mechanical fasteners
or welding, neither of which
would be possible with the
high chromium iron products
Bradken specialises in. The
business case for developing
an in house design to
help to secure the future
supply of the Bradken rings
became a high priority.
It was clear that any new
design would require
objective assessment and
benchmarking before being
put into use- this was a
perceived weakness of the
alternative product available
in the market. Consequently
Computational Fluid Dynamics
(CFD) modelling was
chosen as a proving tool.
globaleyes / edition 6
TECHNOLOGY
CFD SIMULATION
RESULTS IN
IMPROVED DESIGN
FOR GRINDING
MILL THROATS
10
Air and Coal Out
Coal In
Air In
Throat
Sweeper Region
Cross Section of the CAD model used in the CFD study, showing coal
input (black arrow) direction of airflow (white arrows), direction of
coal/air mix (red arrows)
Classifier