World Grain - December 2007 - 36

FEATURE: INCREASE AQUAFEED PRODUCTION THROUGH PLANT OPTIMIZATION

ditioning, one or more of the following
steps can be employed:
* increasing preconditioner size;
* increasing existing preconditioner
fill by one-time adjustment of beater
configuration;
* adding the automatic Retention
Time Control (RTC ) system;
* increasing energy inputs in the
extruder.

Figure 2: Screw speed versus output

Extrduer feed rate (kg/hr)

6,000
5,000
4,000
3,000
2,000
1,000
0
200

400

600
800
1,000
Extruder screw speed (rpm)

come the extruder's limitations in volumetric conveying capacity, it can significantly contribute to energy input
and retention time. Retention time in
the extruder barrel can vary from as
little as five seconds to as much as two
minutes, depending on the extruder
configuration. Average retention time

1,200

in the preconditioner can be as long as
five minutes. For some high moisture
processes, the energy added by steam
in the preconditioner can be as much
as 60% of the total energy required by
the process.
To increase preconditioner capacity
or to compensate for inadequate precon-

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2.1 to- 3m length,
r $ spear:
f 14lSample
T c ` 41
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AVAILABLE POWER TO THE EXTRUDER
When power is the limitation to more
throughput, the options to remove this
bottleneck are more obvious. Factors to
consider include the following:
* install larger extruder drive motor
(more available kilowatts);
* check with extruder manufacturer
to determine maximum allowable installed power based on system design
limitations;
* factor in the effects of removing
other bottlenecks (improved preconditioning, etc.).
Extrusion systems in the industry
are available with power trains of more
than 2,000 kilowatts. Lack of power is
the most common bottleneck to higher
production rates for existing process
lines. An extrusion system operating at
or above full load for most products is
an indication that power is the limitation
to higher throughputs.
VOLUMETRIC CAPACITY
OF THE EXTRUDER
Volumetric capacity is based on the
free volume geometry of the extruder
screw and the screw speed. Plotting the
Table 2: Buoyancy properties of
feed for common aquatic species
Floating
Alligator
Carp
Catfish
Eel
Frog
Koi
Milkfish
Tilapia

Slow-sinking
Bluefin Tuna
Flatfish
Mai mai
Salmon
Sea bream/
bass
Artic Char
Tilapia
Trout

Sinking
Cod
Flounder
Halibut
River crab
Sea bream/
bass
Abalone
Sea urchin
Shrimp

Trout

Yellowtail

Turbot

For more information, see Page 54.

December 2007 / World Grain / www.World-Grain.com

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World Grain - December 2007

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