Chemical Engineering March 2014 - 56

M
Unstable
flow
L
K
G
G
Dense
phase
regime
F
E
Dilute
phase:
strand
flow
A
Saltation velocity/minimum pressure
Log (gas velocity), V
Packed bed
Dense phase
Dilute phase:
strand flow
Dilute phase:
suspension flow
FIGURE 2. This figure shows the relationship of conveying velocity with conveying
pressure at different solids-loading rates W1 and W2. The solids-loading rate is the
solids-conveying rate divided by the internaal cross-sectional area of the conveying
pipeline. For these two loading rates, the figure also shows the transition points
(Points D and G) at which the conveying system migrates from dilute to dense phase.
For solids-loading rate W1, as the conveying velocity is reduced, the conveying system's
operating point moves from Point C to Point D in dilute phase; and then in the
dense phase, from Point D to Points E, F and G. Similarly, for the solids-loading rate
W2, the operating point moves from Point H to Point G in the dilute phase; and then
in the dense phase from Point G to Points K, L and M
Feed
bin
Vent
Pressure
indicator
Air
inlet
Blower
FIGURE 3. This figure shows the design of the vent air system for venting out a
portion of the blower airflow to determine saltation velocity
* Pressure transmitter at the
blower discharge with a pressure
indicator in the control-room control
panel. Computer storage of
pressure data
* Control room alarm for high
blower discharge pressure
* Blower motor interlocks with the
rotary-valve motor
For the rotary valve:
* Local and control-room-mounted
running lights for rotary valve
motor
* Control-room-located, manually
adjustable timer for starting and
stopping the rotary valve motor
* Interlocks with the blower motor
For the diverter valves:
* Position
lights
to
" through " and " divert " positions
* Hand switches for control room
operation of valve positions
Receiving bin:
* Low-level and high-level switches
with indicating lights for the receiving
bins
* Control room alarm to indicate
high level in the bin
Bin vent filters/dust collectors:
* Running lights for the bin vent filters
or dust collectors
* Local pressure-drop indicator
PI
Flow control
valve and
flow indicator
FLC
Rotary
valve
Finding the conveying velocity
Along with conveying pressure, conveying
velocity is perhaps the most
important variable in pneumatic
conveying. After a conveying system
has been installed and is going
through startup, its conveying velocity
should be checked to make sure
it is not too low or too high, and is
about equal to the conveying velocity
that is required. If the conveying
velocity is too low, it may cause
line plugging problems; if it is too
high, it will result in higher particle
attrition, pipeline wear, and higher
energy usage.
The conveying velocity used in
indicate
the conveying system's design calculations
may be too low or too high
because it is difficult to find a reliable
method to determine its correct
value. This value depends upon
many variables, such as solids particle
size, bulk-solids density, solidsto-air
ratio, air density, pipeline diameter
and others. Presently, there
are two methods to find the conveying
velocity. The first method is to
use equations to calculate saltation
velocity (the gas velocity at which
particles will fall out of the gas
stream). These equations have been
developed by researchers to find
the impact of the above-mentioned
variables on saltation velocity. As
they are based on research work
that is carried out in small-scale
test equipment in a laboratory, they
do not cover the entire range of solids
and all of their properties. These
equations can be found in published
books and literature.
Th second method is to use conveying
velocity values that are available
in published literature such as
those given in Table 1. It should be
ChemiCal engineering www.Che.Com marCh 2014 57
D
Saltation line,
joining pressure
minima and
saltation velocity
Dilute
phase:
suspension
flow
H
C
B
* Alarms for low- and high-pressure
drop across filter elements
(optional)
Graphic control panel:
* Graphic panel showing the conveying
system route with run
lights for the blower motor and
rotary valve motor, position lights
for the diverter valves, low- and
high-level lights for the receiving
bins, and run lights for the binvent
filters
Log (pressure drop per unit length),
∆P
L
Packed bed
Unstable regime
Solids loading = W1
Empty pipe
Solids loading = W2
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Chemical Engineering March 2014

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