Chemical Engineering November 2010 - 66

Solids Processing
Controlling particle size
The most common methods of controlling
particle size in mechanical
centrifugal air classifiers are rejector
speed, cage aperature size of rejector
elements, airflow velocity and the
ratio of feedrate to air.
Rejector speed controls the impact
or collision force on the air-entrained
particles as they attempt to exit
the air classifier. Higher speed allows
only the finest particles to pass
through the rejector for collection.
This increases the rejection of larger
particles (Figure 4).
Airflow velocity generated by a fan
controls the drag force on particles
as they enter the classification zone.
Higher airflow allows larger particles
to be removed from the feed, while
lower airflow allows only the finest
particles to pass through the rejector
cage for collection.
Rejector elements / cage-aperture
controls impact the collision force on
FIGURE 4. The
most common
ways to control
particle size are
rejector speed,
rejector elements
and aperature
size, airflow
velocity and feedrate-to-air
ratio
the air-entrained particles as they
attempt to exit the air classifier. A
greater quantity of rejector elements
(blades or rods) makes the cage aperture
smaller and allows only the finest
particles to pass through the rejector
for collection. This increases the rejection
of larger particles.
Feedrate-to-air ratio controls the entrainment
of particles in the airflow. A
higher feedrate allows only the finest
particles to pass through the rejector
cage for collection. This increases the
rejection of larger particles.
Measuring performance
The performance of mechanical centrifugal
air classifiers can be evaluated
by analyzing the cutpoint, tolerance,
yield and efficiency.
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ticle size that is intended to be classified.
This value can be measured in
millimeter mesh or micron size. Tolerance
is the percentage of oversized
or undersized particles allowed in
the finished product. Yield is the percentage
of production rate per unit of
feedrate. Efficiency is the percentage
of the desired particle-size fraction
recovered as product from the total
amount available in the feed.
■
Edited by Scott Jenkins
Author
Joseph Muscolino is product
manager for air classifiers
at Sturtevant Inc. (348 Circuit
St., Hanover, MA 02339;
Phone: 800-992-0209; Email:
jmuscolino@sturtevantinc.
com; Web: www.sturtevantinc.
com). Muscolino has 26 years
of industrial experience with
air classifiers and mills. He is
a member of various professional
societies, including the
   
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50 ChemiCal engineering www.Che.Com noVemBer 2010
National Stone, Sand and Gravel Assn., and is
the author of several technical articles and case
histories. He received a B.S. in mechanical engineering
from Northeastern University in 1981.
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Chemical Engineering November 2010

Table of Contents for the Digital Edition of Chemical Engineering November 2010
