Chemical Engineering March 2017 - 71

velocity to their final terminal velocity
at the discharge point from the end
of the pipeline. The acceleration pressure
drop, ∆pacc, is based on exit values
for both air density and conveying
air velocity, and can be evaluated
from Equation (1); (see box above).
This equation will take into account
the acceleration for both the conveying
air and the conveyed material.
For greater accuracy, should it be
needed, account can be taken of the
fact that the air will have a significant
value of velocity at the pipeline inlet,
since it must be high enough to convey
the material, and the particles will
be at a slightly lower velocity than
that of the air at the pipeline outlet by
virtue of the necessary slip velocity.
Because air is compressible, the
velocity of the conveying air will gradually
increase along the length of a
single-bore pipeline as the material
is conveyed to its destination point.
Problems of pipeline wear and particle
degradation, as well as power
requirements for system operation,
will all increase with an increase
in velocity. This will be the case for
both positive-pressure and vacuumconveying
systems. If high pressure
or vacuum is used for conveying, it
is essential that the bore of the pipeline
be stepped up part of the way
along its length to minimize these
problems. The location of the step
in the pipeline, however, is critical.
If it is positioned too early, the conveying
air velocity may be below the
minimum value for the material being
conveyed, and the pipeline will block.
IMPACT PROBLEMS
If the material to be conveyed is potentially
abrasive, significant wear
of the pipeline, and particularly the
bends in the pipeline, is likely to
occur. If the material being conveyed
is potentially friable, significant damage
to the material being conveyed
may occur, and it is possible that
these changes to the material could
affect the conveying performance of
the material itself. If dust generated
from the material is potentially explosive,
there is the possibility that such
degradation could result in an explosion.
As a result of particle impact,
particularly against bends, there is
potential for heat generation, which
can result in particle melting and in
the formation of streamers, often
called " angel hairs. "
As a consequence of particle impact,
particularly against bends,
there will be a significant reduction
in particle velocity. These particles
will then have to be re-accelerated
back to their terminal velocity, which
will add significantly to the pressure
drop - and hence, energy loss -
for the conveying system, as seen
by Equation (1). It is not only after the
feed point into the pipeline that an
acceleration length needs to be established.
There will be a significant
reduction in particle velocity for particles
after exiting a bend, and particularly
so after short-radius bends. This
situation is illustrated in Figure 2 [1].
Considering the conveying air
pressure, there will be a gradual fall
in pressure along the entire length of
the pipeline. Within the bend itself,
THINI
LONG TERM
Prepare for Tomorrow
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CHEMICAL ENGINEERING WWW.CHEMENGONLINE.COM
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MARCH 2017
71
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Chemical Engineering March 2017

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