World Grain - December 2007 - 37

FEATURE: INCREASE AQUAFEED PRODUCTION
THROUGH PLANT OPTIMIZATION

Table 3: Final product bulk density
correlation with buoyancy properties
Pellet buoyancy

Sea water
@ 20ºC
(3% salinity)

Fresh water
@ 20ºC

Fast sinking
Slow sinking
Neutral buoyancy
Floating

> 640 g/l
580-600 g/l
520-540 g/l
<480 g/l

> 600 g/l
540-560 g/l
480-520 g/l
<440 g/l

screw speed (revolutions per minute) versus potential output
(kilograms per hour) indicates screw performance or efficiency (see Figure 2, page 38). In most cases, actual output is
lower than the potential volumetric capacity due to backwards
pressure or leakage flow. However, when the extruder is designed with a cooled, grooved inlet feed throat and barrel sections, the output can be higher than the expected, calculated
volumetric capacity of the screw.
A bottleneck due to volumetric capacity is usually manifested by the extruder operating in a "choked" or full condition.
Barrel fill will be great enough to plug or partially plug the
barrel steam and water injection ports. In extreme cases, infeed material will visibly fill the extruder inlet and overflow
the throat.
Force-feeding devices are sometimes disguised as tools to
increase volumetric capacity, but their main function is to
eliminate product bridging in the extruder inlet due to poor
mixing in the preconditioning stage.
The volumetric capacity for an extrusion line can be increased in one or more of the following ways:
* installing a larger extruder screw diameter;
* increasing extruder screw speed;
* configuring the extruder with screw geometries designed
for maximum conveying efficiency;
* utilizing grooved barrel liners;
* controlling extruder barrel temperatures with heating/
cooling systems.
OPEN AREA OF DIE ASSEMBLY
A specific die open area is required to develop the proper
back pressure and barrel in the extruder during processing.
This open area requirement remains rather constant for a
product having a distinct buoyancy. If the die area is insufficient, products may expand too much and extruder loads are
excessive as a result of increased barrel fill.
Increasing the die open area to increase throughput potential
is a straightforward relationship. Many die design techniques
are employed to increase the number of die openings and the
total die open area.
The most common arrangement of die orifices is on the die
face, which is axially positioned with the extruder center line.
A substantial increase in the number of die orifices can be
realized when they are arranged on the periphery of the die

Manufacture high quality

AQUATIC
FEED
L"'t1/0

s
with Clextral

twin screw extrusion

Clextral extruders make high quality aquatic feeds that
provide complete nutrition for healthy growth:
- Optimized protein, carbohydrate & fat content
- Precise control of gelatinization
- Flexibility for sinking and floating characteristics
- Large range of productsizes and densities
Schedule a test in our pilot plant and benefit from
our 30 years of experience in feed extrusion.

Complete processing lines and services

C

LEXTR AL

NORTH AMERICA: + 1813 854 4434 SOUTH AMERICA: + 56 2 335 59 76
EUROPE: + 33 4 77 40 31 31 ASIA PAOFIC: + 86 21 64 74 78 06

www.clextral.com/contact
For more information, see Page 54.

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World Grain - December 2007

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