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Loss-in-Weight,
Gain-in-Weight Feeders
by Joseph Marinelli
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It seems appropriate at this
time of year to discuss loss-in-weight and gain-in-weight approaches to
feeding bulk solids. In previous articles, we have discussed volumetric
feeding vs. gravimetric feeding. If you require feed accuracy in the ¼%
to ½% range, a gravimetric system will provide it (depending on the
material being handled).
A gravimetric feeder relies on
weighing the material to achieve a required discharge rate or batch. This
approach should be used when the above stated accuracy is required, when
the material's bulk density varies, or when you simply need to record the
weight of material used for a particular process. A disadvantage of a
feeder that weighs material is that it is usually more expensive than a
volumetric device.
Loss-in-weight (LIW) systems
are the state of the art when feed accuracy is essential. In a
loss-in-weight system, everything is weighed. Load cells are attached to
the bin or hopper that are capable of weighing the entire bin, feeder, and
contents. The load cells sense a change in weight that occurs while your
product is discharging and sends a signal to a controller that speeds the
feeder up or slows it down.
| Figure 1. Loss-in-Weight,
Continuous |
Figure 2. Loss-in-Weight, Batch |
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A LIW system does exactly what
it states, measuring the loss in weight of material discharged from the
system. As such, it can be used both in continuous and batching systems.
They can even be used for liquids simply by replacing the feeder with a
pump. A continuous LIW system is shown in Fig. 1. Fig. 2 shows a batch LIW
system that incorporates a bulk mode (fast feed) and a trim mode (slow
feed to top off the batch).
A disadvantage of LIW systems
is that obviously they cannot weigh while they are being filled. Fill time
is critical in that a typical LIW system will switch to volumetric feeding
while the filling process continues. When the bin is filled, it switches
back to gravimetric. Screws, belts, rotary valves and louvered or
vibratory pan feeders can be used to control discharge.
Gain-in-weight (GIW) systems
are used only for batching applications. The receiving container rests on
a scale or on load cells and the system controls the discharge from the
filling bin, which can use a volumetric feeder to control rate. In a
gain-in-weight application, batch accuracy of 1/4% at two standard
deviations is not unusual.
| Figure 3. Gain-in-Weight
Feeder |
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The disadvantages of this type
of system are: headroom requirements, the fact that only one material can
fill the GIW hopper at a time and the GIW hopper must empty completely
with little or no buildup. A GIW system is shown in Fig.3. As with the LIW
batch application, the GIW system incorporates a bulk mode (fast feed) and
a trim mode (slow feed to top off the batch).
Help others by posting your comments, suggestions and
experiences with bulk solids feeding or any other materials handling concerns you may have
on our On-Line Help Forum. For
past Ask Joe ! Articles, visit the Ask Joe! Archived Articles.
Guest articles for the Ask Joe! Column are always welcome,
for more information please contact Joe Marinelli directly at his email address:
joe@solidshandlingtech.com.
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