(639c) Power Considerations of Pneumatic Convey Systems

Significant work has been performed in regards to understanding the gas flows and pressures required to drive pneumatic conveying systems, however the true power requirements to operate a system are less widely understood.  An acute study of the pressure and flow relationship exposes a power minimization curve to generate flow with a compression device.  Even a firm knowledge of the energy required to move the material does not properly cover the subject.  Efficiencies of the compression device itself, rotary airlock leakage and pressure losses through gas controls can significantly impact the power needed to operate the system.  Therefore, the Actual Work to move the material through the convey line may be significantly less than the Apparent Work to compress the air volume and subsequently the Power required to drive the system.  Once this relationship is understood, the designs of various pneumatic convey systems and the equipment selected to drive them can be analyzed for the true cost of energy usage.

Both dilute (lean) phase conveying and dense phase conveying have been shown to produce a Power minimization curve in respect to the compression device.  The dilute phase relationship is well known from a state diagram where decreasing velocities produce decreasing pressures as long as the material remains entrained in the airstream.  At the point of minimum velocity (and pressure) the power requirements have been minimized.  The dense phase relationship, also shown in the state diagram, is that decreasing velocities produces higher pressures.  Initially the velocity reduction may at first cause a reduction in power.  However, power minimization occurs just before the rate of velocity reduction causes a pressure increase sufficient to increase the power of the compression device.  This effect can be shown in experiments but may be specific to the compression device.  The desired result is a predictable ideal operating point for dilute phase or dense phase systems based on the power consumption of the compression device.

Variables extraneous to the actual conveying of material have a significant impact on the power required to operate a pneumatic convey system.  The type of compression device can have a large impact in terms of compression efficiency.  For instance, a dynamic centrifugal compressor (turbo) may use up to 30% less energy than a positive displacement pump (blower) to generate the same gas flow volume at a specific pressure.  The effect is inherent in the compression device and will vary according to device and operating point.  Leakage gas (gas lost through a rotary valve feed device) is a common phenomena in pneumatic conveying.  Leakage gas is compressed the same as the convey gas yet it does nothing to promote conveying directly thus adding to inefficiency of the overall system.  Finally, the delivery of the gas volume from the compression device to the feedpoint creates pressure loss.  This pressure loss is likewise not use in the act of conveying but is ultimately energy consumed in acts that do not promote conveying directly.  Therefore, the total energy required to drive the system may include significant inefficiencies beyond the convey mode in the pipeline.