World Grain - February 2014 - 84

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Milling oPeratIonS

absorb moisture and can be stretched to
a high tension while maintaining a uniform thread opening. In a purifier, these
characteristics allow for less sagging
and better product flow, and distribution
for better stratification and improved
separation of stock. Polyester also tends
to have a rougher surface as compared
to nylon, which also improves the efficiency of the purifier.
In a centrifugal sifter, the stock is
typically fed into the sieving space by a
screw feeder. The stock is then thrown
about the cylindrical sieving space by
rotating beaters or paddles. The beaters accelerate the stock to aid the separation and movement of fine particles
through the sieving material. The accelerated stock comes into contact with
the sieving surface several times. This
action requires high energy. Enclosing
this force causes the sieve surface to
flex repeatedly.
While this action helps prevent the
sieve from blinding over without additional sieve cleaners, the continuous flexing increases thread fatigue and reduces
the life of sieving material more than
other sieving motions. For this reason,
synthetic sieving material is preferred to
wire clothing in a centrifugal sifter.
Maintenance
From sieve tension to sieve cleaners,
maintenance also plays a role in selecting the best sieving material. Maintaining proper tension on the sieve keeps
the stock properly stratified. The right
method of attaching the sieving material
to the sieve frame is a vital part of maintaining the right sieve tension. The attaching method affects the performance,
sanitation and ease of maintenance for
the sifter. There are two primary attachment methods for sieving material: adhesives or stapling.
Both methods require stretching the
sieving material over the frame, manually or with mechanical or pneumatic
equipment. The use of staples is quickly
being eliminated in the flour milling industry. The use of pneumatic stretching
equipment and adhesives allows for the

best tension. Better tension improves
sifting performance and increases the
life of the sieving material, reducing
downtime and maintenance cost. The
elimination of staples in the sifter also
improves food safety.
No discussion of sieving material
is complete without mentioning sieve
cleaners. Sieve cleaners come in several
shapes, sizes and materials and include
the traditional rubber balls and plastic
cubes along with a multitude of molded
shapes with metal balls or brushes.
A combination of ball cleaners and
cubes are effective for preventing the
blinding over or buildup in the sieving
material, while at the same time scrape
the corners and bottom of the sieve. In
newer-style sieves, the combination
sieve cleaner and pan cleaner, made of
molded plastic with rubber or brushes
to gently clean the sieving material and
assist with the movement of the fines
through the sieve, improve sifter efficiency. Whichever method is preferred,
it is important to pair the appropriate
sieve-cleaning mechanism with the
sieving material to maximize clothing
life and sieving efficiency.
SuMMary
When selecting sieving material, never forget the primary objective, which
is to maximize milling performance.
Selecting the best sieving material will
maximize milling performance by optimizing sifter capacity and efficiency and
properly distributing product throughout the mill. The right sieving material
improves extraction of the mill as well
as flour quality. Material type, micron
size, and percent open area are important considerations, but maintaining the
balance of sieving efficiency with material durability is critical to maximize
mill performance.
Mark Fowler is associate director of the
International Grains Program at Kansas State
University. He may be reached at
mfowler@k-state.edu.
We want to hear from you - Send comments and
inquiries to worldgrain@sosland.com. For reprints of
WG articles, e-mail reprints@sosland.com.

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World Grain - February 2014

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