Effect of a Localized Velocity Increase on Overall Power Consumption and Flow Characteristics in Pneumatic Conveying Systems

Abstract

Pneumatic conveying is widely used in the agricultural industry for seed and fertilizer conveying during seeding operations. Dilute phase conveying is used as it inherently has a disperse particle flow that is leveraged to promote uniform division of the flow. Due to the higher power required per unit mass for dilute phase conveying, research was undertaken to better understand the role of entrainment and conveying velocity on the particles’ behavior, entrainment level, and the power required to convey them. The two hypotheses that guided this thesis were that an increased velocity in the entrainment zone, relative to the downstream conveying zone would (1) increase the entrainment level of the product and (2) decrease overall power required for conveying. To test these hypotheses, the following were completed: • A non-invasive method to quantify the entrainment level of the particles was developed; • A lab-scale pneumatic conveying system was designed and built that allowed for testing the effect of independently varying the entrainment and conveying velocities; • These systems were used to explore product entrainment levels in the form of a probability distribution map of the products location in the conveying pipe; and • The relationship of entrainment velocity and conveying velocity on the pressure drop and energy required to convey was explored. An optical flow profiling imaging apparatus was designed and built that allowed for non-invasive imaging of the product flow behavior. A laser was used to illuminate a cross-section of the conveying line. Successive images of this illuminated section were acquired and then processed to give a probability distribution map of the particles’ location within the pipe. The centroid of this distribution was used as a proxy for the entrainment level of the product within the pipeline. To enable conveying at two different velocities in a single system, a gas extraction system was designed and constructed. This system allowed for a higher velocity during product entrainment and after a certain distance downstream air volume is bled off which in turn lowers the conveying velocity. This system was used in conjunction with the flow profiler to give a quantitative measure of the effect of gas extraction on the entrainment level of the product as defined by the centroid of the probability distribution map. The profiles taken indicated that the entrainment velocity had a significant effect on the entrainment level of the product in the downstream section. In addition to the entrainment level of the product, velocity and pressure data were acquired to explore the energy needed to convey varying mass flow rates. Specific pressure drop (ratio of air and product pressure drop to the air only pressure drop) was plotted against the mass loading ratio (ratio of the mass flow rate of the solid to the mass flow rate of the fluid) and grouped by the velocity ratio (conveying velocity over the entrainment velocity). When the velocity ratio was one the slope was positive with an intercept of approximately one. The data agreed with previously published results. As the velocity ratio was lowered the slope was reduced as well. At velocity ratios of 0.6-0.7 the slope of the relationship was approximately zero. This indicated that entraining at the higher velocity and then conveying at a lower velocity at this ratio required no additional pressure drop for conveying compared to the air only pressure drop. In addition, the specific energy required to convey the product was calculated. At the aforementioned velocity ratios there was an 8-16% energy savings compared to conveying at the same entrainment and downstream velocity. There will be an efficiency cost to accelerating the product in the entrainment zone, however if a system could be designed such that the efficiency gains of low-speed conveying are greater than the extra energy required in the entrainment region, a net energy savings can result