Chemical Engineering January 2023 - 24

Facts At Your Fingertips
C%Air-Moving Equipment for Pneumatic Conveying
Department Editor: Scott Jenkins
n a typical bulk-solids pneumaticconveying
system, an air mover
is used to generate the necessary
air flowrate and pressure required
to transport a solid material through
a pipeline at a given rate. This onepage
reference discusses the operating
mechanisms and characteristics
of three different types of air movers
for dilute-phase pneumatic conveying:
centrifugal
fans and blowers;
regenerative blowers; and positivedisplacement
(PD) blowers (Table 1).
Centrifugal blowers and fans
Centrifugal blowers and fans are
aerodynamic-type air movers, widely
used on short-distance, very dilutephase
systems. The basic function of
a centrifugal fan/blower is to produce
kinetic energy by the action of centrifugal
force. Its impeller and channels
convert this kinetic energy of the
air into potential energy (in the form
of pressure) by efficiently reducing
the velocity of the flowing air. The air
flowrate is highly dependent upon the
conveying-line pressure drop.
The solids-to-air ratio for a fan conveying
system is typically less than
1.2 to 1. If its feedrate is steady and
smooth, the solids-to-air ratio can be
up to 2 to 1. Fans may be used for
both positive-pressure and negativepressure
conveying systems. The
centrifugal fan/blower used in pneumatic
conveying applications are normally
of the radial, flat-bladed type. At
a given speed, these centrifugal fans
or blowers will draw the same volume
of air at a given pressure.
Regenerative blowers
Regenerative blowers are also aerodynamic-type
air movers, and are
mainly used on short- to medium-distance,
dilute-phase systems, such as
drum/bag dump stations and solids
conveying eductors. A regenerative
blower impeller consists of numerous
radial blades along the circumference
of the impeller. The impeller
spins within a housing that consists of
inboard and outboard " channels. " As
the impeller passes the inlet port, air is
drawn in. As the impeller rotates, air is
24
I
Rotary lobe
blower
Rotary screw
compressor
TABLE 1. COMPARISON OF AIR MOVERS FOR PNEUMATIC CONVEYING
Mechanism Inlet capacity,
actual ft3/min
Centrifugal fan/
blower
Regenerative
blower
Dynamic
Dynamic
Positive displacement
Positive
displacement
60
- 250,000
40 - 900
15 - 30,000
120 - 58,000
Maximum discharge
pressure / vacuum
14 - 128 in. H2O
column
13 psig
14 in. Hg
5 - 25 psig
18 in. Hg
15 - 700 psig
captured between each blade on the
impeller and is pushed both outward
and forward into the channels. The air
then returns to the base of the blade.
During this process, a portion of the
kinetic energy (air velocity) is converted
to potential energy (air pressure).
This process is repeated over and
over as the impeller spins. While each
blade-to-blade regeneration " stage "
results in only slight pressure increases,
the sum total, from air entry to
outlet can yield continuous operating
pressures of up to 13 psig or vacuum
to 14 in. Hg. Regenerative blowers
have parallel inlets and outlets that
are positioned perpendicularly to the
rotation of the impeller.
The solids-to-air ratio for a regenerative
blower conveying system
is typically less than 4 to 1, and its
maximum air flowrate is about 900
ft3/min. Regenerative blower installations
typically have a direct-drive
design, but can also be belt-driven.
Among the benefits of a regenerative
blower is that it is virtually maintenance-free.
There is only one moving
part (the impeller), which does
not come in contact with the housing
channel and is essentially wear-free.
Regenerative blowers generate little
vibration and noise and can operate
for up to 40,000 hours without
requiring service.
Positive-displacement blower
Positive-displacement (PD) blowers
are probably the most commonly
used type of compressor for dilutephase
pneumatic conveying systems,
where the operating pressure typically
does not exceed about 18 psig. That
pressure is an ideal match with the
pressure capability of conventional
rotary valves. The PD blower is esOperating
speed,
rpm
800 - 3,800
300 - 4,000
1,000 -
20,000
Maximum
power, hp
1,600
2,000 - 4,500 120
500
8,000
sentially a constant-speed machine
that delivers a relatively constant volume
of air over a range of discharge
pressures. The solids-to-air ratio for
a PD blower dilute-phase conveying
system is typically from 2 to 1 to
6 to 1, up to 10 to 1 in some cases.
PD blowers have also been used for
short-distance, dense-phase pneumatic-conveying
applications,
such
as hopper truck unloading. The resistance
of the system to which the
blower is applied determines the operating
pressure.
The twin-lobe Roots-type blower
is the oldest and most widely used
PD blower. It consists of a pair of
involute profiled (shape of 8) lobes/
rotors that rotate inside an ovalshaped
casing. One lobe is the driving
lobe, which is powered by the external
power source, while the driven
lobe is driven by a pair of equal-ratio
gears. Both lobes rotate at the same
speed, but in opposite directions. As
the rotors rotate, air is drawn into the
inlet side of the cylinder and forced
out the outlet side against the system
pressure. With each revolution,
four such volumes are displaced.
The displaced air is not allowed to
return due to the small internal clearance
within the internals of the machine
(except for a very small amount
called " slip " ). No compression takes
place in the machine. However, as
the discharge end is reached, compression
occurs when the high-pressure
air in the outlet pipe flows back
and meets the trapped air. Due to
this shock compression, the thermodynamic
efficiency of the machine is
relatively low, and it generates relatively
high pulsation and high noise.n
Editor's note: The text shown here is adapted from the following
article: Liu, G., Air Movers for Dilute-Phase Pneumatic Conveying,
Chem. Eng., October 2019, pp. 56-62.
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