Chemical Engineering March 2014 - 57

TABLE 1. COMMONLY USED CONVEYING VELOCITIES
Solids Processing
noted that these published values
are applicable to only those pneumatic
conveying systems from which
they were derived, but may or may
not be applicable for new conveying
systems. This is because the conveying
velocity for a particular conveying
system depends on the values of
various factors and variables such
as solids particle size, particle size
distribution, particle density, air
density, solids conveying rate, pipeline
diameter and more. As shown
in Table 1, the published values
may not be applicable because they
do not give any information on the
values of the variables on which
they are based.
A proposed method
This third method is based on running
a test on the as-designed and
built conveying system to determine
the true value of the solids saltation
velocity. The value of the saltation
velocity obtained by the test will be
accurate because it is based on the
properties of the solids being conveyed
and on the as-designed and
built conveying system. This value
is then used to determine the value
of the conveying velocity.
This test requires gradually reducing
the airflow that goes into the
conveying line so that the conveying
velocity continues to decrease until
it reaches saltation conditions. The
Zenz diagram (Figure 2) shows both
the dilute- and dense-phase conveying
regimes, and the saltation
velocity interface between them. As
shown, the conveying pressure is at
a minimum at the saltation velocity.
In the test, the airflow and hence
the conveying velocity is reduced
until this minimum pressure point
is reached, after which the pressure
starts to increase.
The equipment required for this
test is shown in Figure 3. A vent
line is installed in the air-supply
line at the discharge of the blower.
Its purpose is to vent off to the atmosphere
some of the conveying air
that is being supplied by the blower.
In this vent line, a flow-control valve
with a flow indicator is used to control
the airflow that is to be vented
out. The airflow that is vented out
Material
Alum
Alumina
Bentonite
Bran
Calcium carbonate
Clay
Coffee beans
Coke, petroleum
Corn grits
Corn, shelled
Diatomaceous earth
Dolomite
Feldspar
Fluor (wheat)
Flourspar
Lime,hydrate
Lime, pebble
Conveying
velocity, ft/
min
5,100
3,600
3,600
4,200
3,900
3,600
3,000
4,500
4,200
3,300
3,600
5,100
5,100
3,600
5,100
Material
Malt, barley
Oats, whole
Nylon, flake
Paper, chopped
Polyethylene pellets
Polyvinylchloride, powder
Rice
Rubber pellets
Salt cake
Salt, table
Sand
Soda ash, light
Starch
Sugar, granulated
Trisodium phosphate
2,400 Wheat
4,200 Wood flour
is then subtracted from the air supplied
by the blower to determine
the airflow going to the conveying
line. The conveying velocity is then
calculated based on this airflow and
pipeline diameter.
To run this test, the conveying
system is started and run at full
capacity for a few minutes to bring
it to steady-state conditions. Keeping
the solids flowrate constant, the
vent valve is manually and gradually
opened to start ventinga few
cubic feet per minute of the conveying
air, reducing the conveying airflow
and the conveying velocity.
A close watch is kept on the discharge-pressure
indicator installed
at the blower outlet. This pressure
will keep falling with the decrease
in airflow, but as shown in Figure
2, its value will eventually reach a
point after which it will start to increase.
The objective of the test is to
find the airflow at that point. The
vent airflow is gradually increased
until this point is reached and the
pressure, instead of falling, starts to
increase. This is the minimum pressure
point beyond which the conveying
system migrates to dense-phase
conveying. At this point, the solids
reach their saltation velocity.
The saltation velocity value obtained
by the test is increased by a
safety factor of about 30% to select
an appropriate value for the conveying
velocity. Solids velocity always
58 ChemiCal engineering www.Che.Com marCh 2014
Conveying
velocity,
ft/min
3,300
4,200
4,200
4,500
4,200
3,600
4,800
5,900
5,000
5,400
6,000
3,900
3,300
3,600
4,500
3,300
4,000
decreases when solids flow through
a bend. This decrease can be 5 to
20% depending on the properties of
the solid being conveyed. Unless the
conveying velocity is high enough,
such a decrease can result in saltation
of the solids and plugging of
the bend or its downstream conveying
line.
This test-derived optimum conveying
velocity is compared with the
velocity that is actually being used.
If the actual velocity currently in
use is lower, then the blower speed
is increased to match the optimum
conveying velocity; if it is higher,
then the blower speed is decreased.
The change in speed is determined
from the blower performance curve.
The speed change is implemented
by changing the belts and sheaves
of the blower.
■
Edited by Suzanne Shelley
Author
Amrit Agarwal is a consulting
engineer with Pneumatic
Conveying Consulting
(7 Carriage Rd., Charleston,
WV 25314; Email: polypcc@
aol.com). He retired from The
Dow Chemical Co. in 2002,
where he worked as a resident
pneumatic-conveying and solids-handling
specialist. Agarwal
has more than 40 years of
design, construction, operating
and troubleshooting experience in pneumatic
conveying and bulk-solids-handling processes. He
holds an M.S. in mechanical engineering from the
University of Wisconsin, Madison, and an MBA
from Marshall University (Huntington, W. Va.).
He has written a large number of articles and
given classes on pneumatic conveying and bulk
solids handling.
http://www.Che.Com
Chemical Engineering March 2014

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