White Paper
Power Considerations of Pneumatic Convey Systems
By Jonathan Thorn, M.S.
Director of Technology
Introduction
Significant research has been conducted in regards to
understanding the gas flows and pressures required to
pneumatically convey materials, however the true power
requirements to operate a pneumatic conveying system
are less well defined. The work performed by a pneumatic
conveying system (actual work) comes from the compression of the conveying gas (volume and pressure) and
originates at the compression device. The operation of the
compression device encompasses several inefficiencies
which serve to define the needed energy input (apparent
work) to operate a pneumatic conveying system.
Rotary airlock leakage is a common source of lost supply
gas that is generated by the compression device but is
not used in the convey line to transport material. Delivery
of the convey gas from the compression device to the
feedpoint will create pressure drop (especially when gas
flow control devices are employed) and in turn elevates
the supply pressure required.
Finally, the compression device itself will carry an
absolute compression efficiency. These factors combine
to generate an apparent work that is drastically different
than the actual work required to convey the material.
Therefore, it is the application of the apparent work
analysis that will determine if a process if being operated
efficiently. Observing the apparent work required to
operate a pneumatic conveying system, whether it
be dilute phase or dense phase in nature, exposes a
power minimization curve. By gaining control over key
operation parameters such as the input gas volume (and
consequently the convey velocity), it becomes clear that
reducing the convey velocity to a point will improve the
operating efficiency of the system (for a fixed rate and
distance). However, the minimization curve shows that
eventually conveying will not be supported (plugging) or
the conveying will actually become less efficient.
The minimization curve differs significantly depending
on the type of convey system, the type of air controls
employed and the compression device. Using these
concepts to understand the operational factors that
directly impact the apparent work and power minimization
curve will allow the peak efficiency of the system to be
realized.
Apparent Work Concept
The apparent work to pneumatically convey materials
accounts for gas volume and pressure generated but not
used in directly in the conveying of materials through the
pipeline (actual work). The identification and quantification
of excess flow, pressure and subsequently power will vary
by system type and equipment used as described in later
sections. However, the basic concepts as described here
are universal.
The actual work to operate a pneumatic convey system
stems directly from the basic design parameters of the
system. Material characteristics (entrainment velocity,
friction factor) and system characteristics (rate, distance,
line size, airflow) will define the pressure required
to overcome the resistance in the pipe and transport
material. The rate of actual work (power) for compression
can then be identified by where k=1.4 for air in an adiabatic process. (1)
Assuming ideal gas laws, and substituting k=Cp/Cv and
R=Cp-Cv the expression can be rewritten as
(2)
constituting the rate of work (power) required to compress
the convey volume up to the convey pressure.
Apparent work is then the actual work influenced by any
factor involved in generating or delivering the required
airflow and pressure that does not directly effect the
movement of material. Rotary airlock leakage is a
common source of excess volume generated that allow
feeding of material into the convey line but not the actual
movement in the pipe. Rotary airlock valves will vary in
size and construction but once manufactured will have a
fixed pressure versus leakage relationship similar to an
orifice. Therefore, the convey pressure of the pneumatic
convey system will directly impact the leakage volume
and the total supply volume required.
(3)
Delivery of the air volume from the compression device
to the feedpoint is a common source of pressure drop
the compression device must overcome although
no material is moved during the pressure reduction.
Air alone pressure loss can occur simply from the
movement of air through supply lines or because the air
volume must be tightly controlled and therefore metered
through a control valve. In the case of a communal
compressed air supply, the convey air is normally
generated at pressures from 6-7 bar and then regulated
to a pressure significantly less for use.
Therefore, we must differentiate between communal air
supplies (compressed air) and dedicated air supplies
(PD blower, compressor) that are generating only the
volume and pressure needed for conveying. Clearly the
apparent work will be minimized when the air alone loss
is minimized and a dedicated compression device is
used to only generate the pressure needed to drive the
system.
(4)
The selection of the compression device and its inherent
compression efficiency ( η ) will strongly influence the
apparent power. Typically the type of compression device
will have the largest impact but the model selection
within a type and even the performance curve of a
particular model can contribute.
Taking a dilute phase example, Positive Displacement
(PD) blowers are common dilute phase compression
devices due to their low cost and simple construction.
The compression efficiency of a PD blower will vary
from approximately 50-70% depending on the discharge
pressure up to 1 bar and will be approximately 55% at
0.8 bar. A dynamic centrifugal compressor (turbo blower)
is approximately 70% efficient at 0.8 bar. Through
selection of the type of compression device a reduction
of >20% apparent work can be realized.
Within a PD blower range, to perform a certain duty, one
model may perform at an elevated efficiency compared
to another. In the example above, a range of models
may produce efficiencies from 52-58% resulting in a
10% variance in of apparent work depending on model
selection. And in the case of the turbo blower, it may
generate the same flow (constant pressure) at different
speeds using its flow modification features because of
the operating location on the performance curve.
The resulting efficiency at different speeds may vary from
60-70% efficient resulting in a 15% variance simply by
tuning. The efficiency of a compression device at a particular
operating point may not be easily identified but this value
has a large impact on the apparent work and the resulting
efficiency of the system.
Combining the influences of leakage, additional pressure
inputs and compressor efficiency the actual power can be
modified to represent the apparent power required for the
system.
(5)
Experiment
Experiments were conducted on a testing apparatus that
allows several types of conveying through a common
pipe. The piping has lengths available from 75 to 300 m in
horizontal length, 7 m vertical length and quantity (7-13) 90
deg turns depending on the route chosen. Metalicine based
polyethylene pellets weighing 550 kg/m3 and approximately
4500 micron were conveyed through a 4” Sch10 stainless
steel pipe. A high pressure rotary airlock capable of pressures
up to 4 bar was used to feed the material into the convey
line for both the dilute phase and dense phase experiments.
Pressure measurements were recorded by transducer
at the discharge of the compression device and at the
feedpoint. Total airflow values were measured by a hot wire
anemometer style mass flow meter while leakage flows (and
subsequently convey airflows and velocities) were calculated
based on the convey pressure. Power values were collected
from the digital output of a variable frequency drive.
Equilibrium was to found to be achieved when conveying
conditions were maintained for greater than 15 minutes and
followed by 15 additional minutes used to average the data.
System Configurations
Several pneumatic convey system configurations were
studied to deduce what factors most strongly influence the
apparent power to operate the system.
■■ Pressure dilute phase fed by rotary airlock and powered
by a PD blower
■■ Pressure dilute phase fed by rotary airlock and powered
by a Turbo blower
■■ Pressure dense phase fed by rotary airlock and powered
by communal compressed air
■■ Pressure dense phase fed by rotary airlock and powered
by a dedicated compressor.
Pressure Dilute Phase
Pressure dilute phase uses gas flow (QS) created by the
compression device and supplies it to the rotary airlock
where material is dropped into the stream (see Figure 1). A
portion of the gas exits the rotary airlock as leakage (QL) and
the remainder flows through the line as convey gas (QC). As
long the resulting superficial gas velocities are greater than
the minimum entrainment velocity for that material then
material will be transported. The compression device then
operates at a pressure sufficient to overcome the resulting
resistances associated with the gas and material flow.
Figure 1. Pressure dilute phase
Figure 2. Pressure continuous dense phase
The apparent power in this arrangement is influenced
by: 1) rotary airlock leakage, 2) pressure drop in gas
supply line, 3) efficiency of the compression device and
4) excess velocity above the minimum.
Rotary airlock leakage is considered a necessary
sacrifice to allow the introduction of the material into
the system. For a specific duty the leakage rate can be
considered constant (at a specific pressure) when using
equipment in good repair. The clean air line pressure
drop will vary depending on the line size but is constant
for a fixed pipe size and generally constitutes a small
percentage of the required pressure to operate the
system. Efficiency of the compression device plays
a large role in the apparent power formula. In the
efficiency comparison given above for a PD blower and
turbo blower, the required power to operate a system
can vary greatly. In Figure 3 we see the measured power
input for the two compression devices performing
under the same duties. The Turbo blower demonstrates
significantly reduced power input as compared to the
PD blower. Excess velocity in the convey line is also a
significant contributor to apparent power. In addition
to the compression of the excess gas flow, higher
transport velocities result in elevated pressure drops for
both gas and material flows.
Rate
mTPH
Total
Airflow
PD
Blower
Power
Turbo
Blower
Power
bar
m^3/hr
KW
KW
Convey Convey
m
4.2
200
0.38
332
20.8
16.3
6.1
200
0.52
358
29.4
22.4
7.7
200
0.66
391
43.1
29.0
Figure 3. Chart power requirements for PD and Turbo blowers
Figure 4 shows the measured and calculated apparent
power for a range of pick-up velocities in relation to the
actual power. These results will be further discussed in
relation to a power minimization curve.
Figure 4. Actual and apparent power curves for pressure dilute phase
Pressure Dense Phase
Pressure dense phase flow can be created using feed from a
rotary airlock if the gas flow amount is tightly controlled (often
called continuous dense phase – see Figure 2). Dense phase flow
generates significantly higher pressures than dilute phase and
requires a compression device to suit. Communal compressed
air (6-7 bar) from a compressed air header is a common to
way to drive this type of a system but a dedicated compressor
can also be used with significant differences in the resulting
apparent power. The desired convey gas volume is combined
with the expected leakage volume and delivered through a gas
metering valve. By passing the gas through a metering valve, a
resulting pressure drop occurs of which the compression device
must overcome. Losses in the clean air line are present but are
generally negligible. The apparent power in this arrangement is
influenced by: 1) supply pressure (communal or dedicated), 2)
metering valve loss (dedicated), 3) rotary airlock leakage and 4)
convey velocity.
The supply pressure of the compressed gas has a strong
influence on the apparent power. In the case of communal gas
provided at an elevated pressure, loss through the metering
valve is not relevant. The compressor has already provided
an excessively high gas pressure which is regulated down at
the use point. With a dedicated compressor the loss through
the metering valve goes directly to apparent power yet
this methodology is significantly more efficient having not
compressed the gas to an unnecessarily high header pressure.
The supply pressure for a dedicated compressor is
simply the convey losses (PC) and metering valve losses
(dPcontrol) combined. Similar to dilute phase the rotary
valve in dense phase is generally fixed for a certain duty
however the leakage volume becomes a much larger
portion of the total air (due to lower convey velocities
and higher pressures). The conveying velocity will have
a direct influence the convey pressure and thus the total
supply gas volume (QS) is reduced by the convey volume
(Qc) and increased via the leakage (QL). Figure 5 shows
the relationship between power and velocity as the
velocity is reduced.
Power Minimization Curve
Power for a specific conveying duty (constant rate,
distance & line size) can be compared to the velocity of
the conveying and will ideally generate a curve with an
inflection point yielding a minimum. The premise is that
a velocity reduction will inherently reduce the convey
gas volume and, until the convey pressure increases
drastically, there will be a net reduction in actual power.
However, the inflection point for apparent power is more
visible due to the mitigating factors of airlock leakage,
compression efficiency and the like.
around 6 m/s terminal velocity. This suggests that the
work required to convey material in dense phase flow is
approximately constant although further work would be
required to make this claim. The apparent power of the
communal gas arrangement shows a minimum around 4.5
m/s. Since supply pressure is considered constant in this
case, the total supply gas (Qs) is the controlling variable
and the inflection point occurs only when the leakage
rate increases faster than the convey volume decreases.
The apparent power for a dedicated compressor reveals
a minimum around 6 m/s; similar to the actual power.
Although rotary valve leakage significantly increases the
apparent power in the dedicated compressor scenario, the
supply pressure appears to be a controlling factor in the
minimization. In these experiments the apparent power was
2-3x higher when a dedicated compressor scenario was
employed and 7-8x higher with a communal compressor.
Conclusions
The experiments performed establish that the apparent
power to operate a pneumatic conveying system is
significantly greater than the actual work being performed;
i.e. the transport of materials through pipelines. Dilute
phase systems appear to suffer most from inefficient modes
of compression and from convey velocities in excess of
the minimum required. Dense phase systems appear to be
subject to the tendency to use communal air supplies and
the high rates of rotary airlock leakage (continuous dense
phase). In this case, using a dedicated compressor in place
of a communal supply reduces the apparent power.
For a specific conveying arrangement, it is possible to
establish a convey velocity that minimizes the apparent
power required to operate the system. For pressure dilute
phase systems the minimization occurs at or around the
minimum convey velocity. For pressure dense phase using a
rotary airlock, minimization occurs at a higher velocity when
using a dedicated compressor as compared to drawing from
a communal gas supply.
Figure 5. Actual and apparent power curves for pressure
continuous dense phase
The apparent power curve will be distinctly different for
the dense phase and dilute phase modes of conveying.
Velocity reduction in dilute phase will result in a pressure
reduction and thus a combined power reduction (until
the minimum entrainment velocity is reached). A slight
pressure increase can be observed around the velocity
minimum (see Figure 4), offsetting the gas flow reduction
in the apparent power. The actual power does not reach
a minimum before because further reductions in velocity
will fail to sustain conveying and thus actual power
minimization occurs when the velocity is minimized.
However, we see a minimization occur in the apparent
power around 18 m/s average velocity when leakage
and compression efficiency become a controlling factor.
We also see the apparent power is approximately 2x the
actual power for conveying under these conditions.
Velocity reduction in dense phase causes material to
accumulate in the convey line and results in a convey
pressure increase. In Figure 5 we see the actual power
stays approximately constant with a slight inflection
Nomenclature
Cp – heat capacity at constant pressure
Cv – heat capacity at constant volume
k – adiabatic expansion coefficient
Q – gas flow
QC – conveying gas flow
QL – leakage gas flow
PC – convey pressure
QS – total supply gas flow
PS – supply Pressure
T - temperature
P1 – compressor inlet pressure
P2 – compressor discharge pressure
dPair – pressure drop air alone
dPcontrol – pressure drop of air controls
W
– work
•
W – rate of work
– overall compression efficiency
η
References
“Compressor Efficiency Definitions”, K. Ueno, PhD, and R. E. Bye, VAIREX
Corporation, K. S. Hunter, PhD, University of Colorado, May 12th, 2003.
www.engineeringtoolbox.com, “Horsepower required to Compress Air”.
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