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Power Considerations of Pneumatic Convey Systems

February 5, 2013
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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. In this article, you will discover the results of a large-scale experiment into apparent work versus actual work required to operate a pneumatic conveying system.

Apparent Work Concept
Apparent work analysis studies the true power required to operate a pneumatic conveying system to determine if a process is operating efficiently. Actual work stems directly from basic system design parameters. Material characteristics (entrainment velocity, friction factor) and system characteristics (rate, distance, line size) define the pressure and airflow required to transport material.
    Apparent work includes gas volume and pressure that is generated — but not used — in the actual work of conveying of materials through a pipeline. Combining the influences of leakage, additional pressure inputs and compressor efficiency, actual power can be modified to represent the apparent power required for the system.
    Observing the apparent work required to operate a pneumatic conveying system, whether it be dilute phase or dense phase, exposes a power minimization curve. The minimization curve differs significantly depending on the type of convey system, the type of air controls employed and the compression device. Understanding operational factors that directly impact apparent work and the power minimization curve will support peak efficiency of the system.

Causes of System Inefficiencies
Rotary airlock leakage is a common source of excess volume that feeds material into the convey line, but does not actually move material in the pipe. Rotary airlock valves vary in size and construction. Once manufactured, they have a fixed pressure versus leakage relationship similar to an orifice. The convey pressure directly impacts leakage volume and total supply volume required.
    Delivery of air volume from the compression device to the feedpoint is a common source of pressure drop that 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 metered through a control valve.
        The selection of compression device and its inherent compression efficiency (?) will strongly influence apparent power. The type of compression device has the largest impact, but performance curves of each particular model also contribute. The efficiency of a compression device at a particular operating point has a large impact on the apparent work and the resulting efficiency of the system.
    
Types of Pneumatic Convey Technologies

Pressure Dilute Phase
Pressure dilute phase uses gas flow created by the compression device and supplies it to a rotary airlock where material is dropped into the stream. A portion of the gas exits the rotary airlock as leakage and the remainder flows through the line as convey gas. As long as resulting superficial gas velocities are greater than minimum required entrainment velocity, material will be transported.

Pressure Dense Phase
Pressure dense phase flow — or continuous dense phase — can be created using feed from a rotary airlock if the gas flow amount is tightly controlled. Dense phase flow generates significantly higher pressures than dilute phase and requires a robust compression device.

Because the supply pressure of the compressed gas has a strong influence on the apparent power, we must differentiate between communal air supplies (plant wide compressed air) and dedicated air supplies that are generating only the volume and pressure needed for conveying.
      Communal compressed air from a compressed air header is a common to way to drive this type of a system. With 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. The desired convey gas volume is combined with expected leakage volume and delivered through a gas metering valve. By passing the gas through a metering valve, a resulting pressure drop occurs which the compression device must overcome.
        A dedicated compressor realizes significant differences in resulting apparent power. With a dedicated compressor, loss through the metering valve goes directly to apparent power — yet this methodology is significantly more efficient because the gas isn’t compressed to an unnecessarily high header pressure.
    
Power Minimization Curve
The airflow value used for conveying can be adjusted to vary the velocity in a system and see how the power for a specific conveying duty (constant rate, distance, and line size) is affected. This will ideally generate a curve with a minimum. This curve can be produced for both dilute-phase and dense-phase conveying mediums.
        In dilute phase, reducing the velocity results in a pressure reduction. Therefore, we see a drop in convey air, pressure, and leakage, which combines for an overall power reduction (see Figure 1). Eventually the minimum convey velocity for the material is reached and the pressure starts to increase (~18 m/s in the example). At this point, the actual power is still dropping but the apparent power is starting to increase. The power minimization for this system has then been identified. Note that the apparent power is approximately 2x the actual power to convey the material.
    
Velocity reduction in dense phase causes material to accumulate in the convey line and results in a convey pressure increase. Therefore convey air is decreasing but the leakage air and supply pressure are increasing. In Figure 2, the actual power stays approximately constant when the velocity changed suggesting that the actual power to convey material in dense phase is independent of velocity. 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. This indicates that there is way to operate this system efficiently when the velocity is not at its minimum.

    Although rotary valve leakage significantly increases apparent power for dedicated compressors, supply pressure appears to be a controlling factor in the minimization. In these experiments, 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 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 suffer most from inefficient modes of compression and from convey velocities in excess of minimum required. Dense-phase systems have a tendency to use communal air supplies and experience high rates of rotary airlock leakage. 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 apparent power required to operate the system. For pressure dilute-phase systems, minimization occurs at or around minimum convey velocity. For pressure dense-phase using a rotary airlock, minimization occurs at a higher velocity when using a dedicated compressor versus drawing from a communal gas supply.
    For complete details of system configurations and setup used to generate the charts in this article, along with complete references and proof formulas, visit www.macprocessinc.com/PowerConsiderations.

    Jonathan Thorn is the director of technology for Mac Process in Kansas City, MO. Thorn holds a Masters degree in Chemical Engineering from the University of Pittsburgh, PA, where he specialized in bulk materials handling. He currently focuses on system design and application and has been published in various journals/conferences related to material handling over the past 14 years he has been with Mac Process.
 

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