Chemical Engineering October 2019 - 61

In order to reduce the pulsation level
and noise, three-lobe and four-lobe
rotors have been introduced. This allows
the lobes to have a slight twist
along the rotor axes, which reduces
pulsing in the input and output. Since
the lobes run within the casing with
finite clearances, which are controlled
by the gears, no internal lubrication is
required. The air is thus delivered free
of oil.
Unlike centrifugal fans/blowers, PD
blowers deliver, practically, a constant
flowrate independent of the
discharge pressure conditions. There
is a slight flowrate reduction with the
increase of system pressure. This is
because a very small amount of air
" slip " back through the clearance at
high discharge pressure. Therefore,
in PD blowers, the flowrate is dependent
mainly on its operating speed.
The relationship between PD
blower power consumption and system
pressure drop is shown in Figure
6. Power consumption in PD blowers
is proportional to system pressure
drop. The lowest power drawn
happens when the system pressure
is the lowest (in other words, when
there is no solids loading in a pneumatic
conveying system). Therefore,
PD blowers are generally selected
for the maximum system pressure,
which they may encounter during
operation. When in operation, the
blower offers considerable power
savings, since the power consumed
by it depends upon the actual working
pressure under which it operates,
and not the rated pressure.
This is totally different compared to
other air movers, such as reciprocating
compressors, sliding-vane and
screw-type compressors, in which
the compression ratio is fixed and
the suction air is compressed, according
to the compression ratio,
regardless of the load conditions.
A fixed power is thus consumed by
these types of compressors.
PD blower performance curve
The PD blower curve supplied by
blower manufacturers typically contains
all the information you need
to choose the right blower, motor
and cooler, if necessary. In a dilutephase
conveying application, after
the desired air flowrate and system
pressure are determined, it is time to
select the PD blower to move bulk
solids to the destination reliably. First,
find a PD blower that has the right
range of air flowrate and pressure
from manufacturer catalogs. Then
use its blower performance curve
to select the right blower speed and
associated motor size.
Figure 7 shows the pressure performance
curve for the Roots Model
409 RAM blower. For instance, if the
required air flow is 600 inlet ft3/min
and system pressure is 8 psi, what
are the blower speed and motor
size? Start on the upper chart in
Figure 7 from the left vertical coordinate
and find 600 inlet ft3/min, move
horizontally to the right and find the
intersection with the 8-psi pressure
curve. Then move straight down to
find the right blower speed (rpm)
at the horizontal coordinate, 3,000
rpm. This is the speed the blower
has to run in order to deliver 600 inlet
ft3/min air flow at 8-psi system pressure
resistance.
The input power for a PD blower is
largely dependent on the total pressure
across the machine. To find the
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Chemical Engineering October 2019

Table of Contents for the Digital Edition of Chemical Engineering October 2019
