How to Design a Volumetric Belt
Feeder
by Joseph Marinelli
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When using a hopper with a slotted
outlet configuration, you are pretty much limited to using a belt
feeder or screw feeder to withdraw your product. Belts and screws
can be designed as volumetric devices, since they discharge a
particular volume of material per unit time. This is done by control
of belt speed or screw rpm. We discussed screw feeder design in
previous articles, recognizing that a screw is a good device when
you require it to be enclosed or you have a headroom constraint. A
belt feeder is preferable to a screw when handling certain bulk
solids, and you would typically use a belt when:
- You require a large slotted
opening to prevent arching
- You are handling large particles,
sticky or fibrous materials
- Dust is not a problem
Assuming that your hopper is designed
to give you reliable mass flow, the belt feeder must be capable of
maintaining the mass flow pattern and withdraw material uniformly
over the entire outlet cross-sectional area. The key to this
approach is to ensure that the feeder increases in capacity in the
discharge direction. This is an extremely important statement that
will be made several times during our discussion.
Figure 1: Adjustable Gate
Hopper on Belt Conveyor
In this article we will suggest ways
to design belt feeders. Take, for example, the approach shown in
Fig. 1. This design shows the material being discharged from the bin
or hopper, directly onto the belt below. Notice that a gate is used
to regulate the amount of product that is allowed onto the belt.
Also notice that the material is being withdrawn preferentially from
the front of the hopper. Obviously, if mass flow is required because
of the flow properties of the material, this approach serves to
create a funnel flow pattern (some material moving, the rest
remaining stagnant). A stable rathole can form, causing a flow
stoppage, erratic feed, flushing and or segregation. As well, this
preferential flow channel can cause solids compaction, wear on
belts, and excessive power required to move the belt.
Figure 2: Optimum Design
Does this mean we cannot use a belt
under a mass flow bin? Absolutely not! As with screws, the capacity
of the feeder must increase in the discharge direction in order for
the material to be withdrawn uniformly over the entire outlet. In
order to reliably deposit solids onto a belt, a properly designed
interface between the bin and belt is recommended. The optimum
design of an interface is shown in Fig. 2. Notice that the bin or
hopper is not modified but a properly designed interface is used to
achieve the increased capacity in the discharge direction. Also
notice that the interface increases in capacity in elevation view as
well as in plan view.
The slant nose provides stress relief
as material is transferred to the discharge end. Flat or troughed
idlers can be used to support and train the belt. Troughed idlers
will provide a more rigid belt. If flat belts are used, the idlers
should be closely spaced so as to prevent belt sag. Skirts can also
be used to prevent spillage. Be sure that the skirts expand slightly
in the direction of belt travel so as not to interfere with material
flow.
The rule of thumb to obtain the
increased capacity is to increase in plan view and elevation view,
approximately ¼" per foot of outlet length. Others have used
about a 3° to 5° increase to obtain the proper increase in
capacity. Also remember that width of the interface slot outlet at
the back is set by the required slotted opening to prevent arching.
Additionally, the sidewall angle of the interface should be at least
the required angle for mass flow for a wedge shaped configuration
(it can be steeper).
The above approach will allow you to
reliably discharge product onto a belt feeder or conveyor. Keep in
mind, that a belt can be converted from volumetric feeder to a
gravimetric feeder, simply by adding weigh idlers upstream of the
bin outlet. The signal from the weigh idlers will cause the belt to
speed up or slow down according to the set point.
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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