Chemical Engineering April 2023 - 44

Feature Report
Part 2
Pneumatic
Conveying Basics
Pneumatic conveying is a versatile method for transporting powders and other solid materials
within chemical process plants. Success depends on understanding science and applying
specific experience
Doan
Pendleton
Vac-U-Max
IN BRIEF
NEGATIVE VERSUS
POSITIVE PRESSURE
SYSTEM SELECTION AND
DESIGN
PRODUCT CHANGES
U
ndergraduate engineering programs
include little instruction about
powder handling. While there may
be some cursory education about
the mechanics of pneumatic technologies
and the chemistry of powders, engineers entering
industrial processing and manufacturing
industries rarely have a knowledge base
sufficient to tackle a powder-handling project
without the guidance of an experienced engineer
or the expertise of a trusted supplier (see
part 1, pp. 40-43).
The science of pneumatic conveying involves
formulas to determine parameters
like airflow and vacuum pressure. However,
after all the science and spreadsheets, the
success in designing a system that delivers
peak performance while preserving product
quality lies in the ability to understand powder
characteristics and how those characteristics
interact with equipment design.
This type of " tribal knowledge " is not formulaic
and draws upon decades of experience
with tens of thousands of different materials
and their behaviors to complement scientific
principles.
Negative versus positive pressure
Pneumatic conveying uses air to
either push (positive pressure) or
pull (negative pressure) solid material
through a conveying line.
Negative-pressure systems suck
the material through the lines,
while positive-pressure systems
blow, or push, material down the
line. Factors other than material
characteristics, such as conveying
distance and the need for
higher conveying rates, dictate
the choice between negativeand
positive-pressure systems.
Negative-pressure pneumaticconveying
systems (which constitute
the majority of pneumatic
44
conveying applications) include a pickup
point where material enters into the conveying
system, conveying tubing where material
is transferred between equipment; a vacuum
receiver (filter receiver), which serves as an
intermediate holding vessel for materials; a
vacuum source that powers the system; and
a control panel.
Pneumatic conveying systems are also
categorized by dilute-phase conveying,
where the air-to-product ratio is higher,
and dense-phase conveying, which injects
less air and more product into the conveying
line (Figure 1). Most systems use dilutephase
conveying, but dense-phase systems
are used as applications dictate. There are
some instances where semi-dense or semidilute
systems are appropriate. Dilute-phase
systems employ higher velocity rates than
dense phase systems.
System selection and design
Pneumatic-conveyor manufacturers do not
expect users to determine themselves which
type of system is needed. Expert conveyor
manufacturers determine, by compiling a
great deal of information about the application,
which type of system will best meet all
FIGURE 1. Pneumatic-conveying systems have varying air-to-product ratios
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