Chemical Engineering October 2017 - 48

Cover Story
Mass flow screw
Capacity over length
Capacity
(ft3 per
revolution)
Length along screw
Uniform flow across bin outlets entire area
Capacity
(ft3 per
revolution)
Length along screw
Preferential flow channel
Stagnant material
Constant
screw
pitches
Constant-pitch screw
Capacity over length
between the vanes and the housing. This
seal prevents countercurrent, high-pressure
air from permeating up into the bin and interrupting
material flow.
Conical
shaft with
constant
pitches
Increasing
screw
pitch
FIGURE 6. Varying the flight
pitches and altering the screw
shaft in a screw feeder can
maintain uniform withdrawal
of material from a container
Conveying
section with
constant
pitches
Constant
diameter
shaft
allowing the material to stream onto the belt
uniformly and maintaining a fully live bin outlet
(Figure 7).
FIGURE 7. Belf feeders, like
the one shown here, can handle
a wide range of materials
The belt feeder can handle a wide range
of materials, including friable materials, very
cohesive materials and sticky or fibrous materials.
A belt feeder is not well suited for fine
powders or other materials that are prone to
flooding or those that are extremely dusty.
Some materials may require a belt scraper or
some other cleaning device to deal with material
buildup on the belt. Belt feeders also
demand regular maintenance and cleaning.
Rotary valve feeders. A rotary valve feeder
(also known as a rotary airlock) consists of a
series of pockets attached to a rotating shaft.
The feeder has a driven rotor with vanes attached
to it that form the pockets, and is
enclosed in a cylindrical fixed housing. As
the rotor spins, material from the bin fills the
pocket by gravity. Once the pocket rotates
180 deg to the bottom of the valve, gravity
causes material to drop out of the valve.
Typically, rotary valves are used as airlocks
to feed material from a bin into a pneumatic
conveying system. They are capable of providing
an air seal due to the tight clearance
As the pockets begin filling with material,
air displaced from formerly empty pockets
may cause some erratic material flow
from the bin. Typically, this is observed as
a reduction in material flow or as material
flooding. To alleviate this problem, newer
rotary valves include a vent line designed
to send the displaced air to either the top
of the bin or to a dust collector. A vent line
may need to be added if the rotary valve
does not have one. Rotary valves typically
feed preferentially from one side of a bin,
as the pocket fills with material and cannot
take any material from the other side of the
outlet. To restore mass flow, the bin outlet
must be made fully live, where the material
is uniformly withdrawn from the bin outlet.
This is typically accomplished by adding a
vertical section between the bin outlet and
the rotary valve. The preferential flow channel
will expand upward through the vertical
section, and uniform material flow will be
established (Figure 8).
A rotary valve will be sized based on the
outlet and required feedrate. Feedrate can
be adjusted by changing the pocket size
and the rotary valve rotations per minute.
Rotary valves are suited for bins with square
or circular outlets. They are typically not applicable
for very cohesive materials, friable
materials, fibrous materials, or materials
with large chunks. Any material that can
jam or stick inside of the pockets may prove
problematic. Another concern is very abrasive
materials that can wear away at the
clearance between the vanes and housing.
Wear will diminish the air seal and change
the accuracy of the feeder over time, because
material could leak and bypass the
rotating pockets.
Pan feeders. Pan feeders (also called vibratory
pan feeders), use a pan with a vibratory
drive attached to vibrate and feed the material.
As the pan feeder vibrates, material is
thrown very slightly up and forward. Material
feeds from the outlet into the tray and can
be conveyed a short distance to some discharge
point (Figure 9a).
Vibratory feeders can handle a range of
materials and achieve a wide range of flowrates.
Increasing or decreasing the vibration
is typically used to control the feedrate. Very
fine materials may not be suited to vibratory
feeders since they are prone to flooding.
Dusty materials can be accommodated by
48
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Chemical Engineering October 2017

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