Chemical Engineering March 2017 - 69

Solids Processing
Particle Impact Problems in
Pneumatic Conveying
Pneumatically conveyed solid materials inevitably impact pipe walls and other
particles, which can create processing and equipment problems. Provided here is an
outline of potential problems related to particle impacts and the physics behind them
David Mills
Consultant
A
mong the defining features
of pneumatic conveying
is the flexibility that can
be achieved with pipeline
routing for the transport of bulk
particulate materials in a totally enclosed
environment. Powdered and
granular materials can be conveyed
vertically up and vertically down, as
well as horizontally, in a single continuous
routing, just by using bends
in the pipeline. By this means, any
obstructions in the routing can be
avoided simply by routing the pipeline
around them.
In traversing a pipeline, however,
there will inevitably be impacts between
the particles being conveyed
and the retaining pipeline. This will
be particularly so at each bend in
the pipeline, where the particles will
be subject to a change in direction,
regardless of pipeline orientation.
The impacts also lead to reductions
in particle velocity. In horizontal sections
of pipeline, there will be a natural
tendency for the particles to fall
to the bottom of the pipeline due to
gravity. In this case, the particle impact
is likely to be a glancing blow
and hence at a low angle of impact.
The situation with regard to bends,
however, is very different because all
the material will have to take the turn,
and, as a consequence, there will be
a considerable number of particles
impacting with the bend wall, as well
as particle-to-particle interactions in
the turbulence generated. The geometry
of the bend is an additional
variable here because the radius of
the bend will have a major influence
on the impact angle. This article provides
information about the behavior
of pneumatically driven solid materi20
Material
feed point
Air
16
12
8
4
Conveying distance, m
FIGURE 1. Pneumatically conveyed particles accelerate from the feeding point in the pipeline
als and the problems that can arise
due to impact of the solids on pipe
walls and other particles. These include
pipe wear, particle degradation,
dust formation and others.
Conveying air velocity
Conveying air velocity is a critical
parameter in pneumatic conveying.
From basic fluid mechanics, pressure
drop is directly proportional to
the square of the velocity. Therefore,
the air velocity must be kept to as
low a value as possible to avoid excessive
pressure drop. In pneumatic
conveying, however, the objective
is to have the conveying air velocity
maintained at a minimum value that
is still sufficient to reliably convey the
material. For materials conveyed in
dilute phase (suspension flow) this
velocity is likely to be in the range of
15 m/s (~3,000 ft/min), and for many
materials, dilute-phase conveying is
the only option with a conventional
pneumatic conveying system. For
materials with good air-retention
characteristics or good permeability,
however, dense-phase conveying
CHEMICAL ENGINEERING WWW.CHEMENGONLINE.COM
MARCH 2017
may be possible. In this case, the
minimum conveying air velocity may
be as low as 5 m/s (~1,000 ft/min). A
safety margin of about 20% on conveying
air velocity is generally recommended.
The margin, however, must
not be too great because pressure
drop, and hence energy required,
varies approximately with the square
of the velocity.
In pneumatic conveying, engineers
generally refer only the velocity
of the air (rather than the velocity
of particles being conveyed), since
this quantity can readily be calculated
or measured. The particles in
the air stream will be conveyed at a
slightly lower velocity. The difference
in velocity between the air and the
particles in it is generally referred to
as the " slip velocity. " The value of
the slip velocity will increase with
larger particle sizes, higher particle
densities and more complex particle
shapes (higher surface area).
The routing of the pipeline may
include sections where particles are
moved vertically up and vertically
down in order to cross roads or rail69
Acceleration
length
Slip velocity
Velocity, m/s
Particles

http://WWW.CHEMENGONLINE.COM

Chemical Engineering March 2017

Table of Contents for the Digital Edition of Chemical Engineering March 2017