An Investigation on Pressure Drop for Gas-Solids Flow through Bends

Abstract

This thesis presents the results of an ongoing investigation into the modelling of pressure drop in bends due to the pneumatic conveying of fine powders. Two grades of cement and fly ash have been pneumatically conveyed from fluidised dense- to dilute-phase through three different radii of curvature (1000, 800, 600 mm), two different bend diameters (53 and 42 mm) and two different locations of test bend in the pipe loop. Six existing bend pressure drop models have been investigated for their prediction accuracy by comparing the predicted versus experimental bend pressure drop values. Based on the conveying data of different products, bend diameters, radius of curvature of bends, a new model for bend loss has been developed, which included physical properties of particles, parameters of gas flow, and the ratio of the radius of curvature of bends to pipe diameter and bend diameter. The new model was validated by using it to predict for bend losses for the experimental data provided in Pan (1992) and by comparing the predicted versus experimental values. The new model was able to predict the pressure loss in bend for fine powder consistently in the range of relative error of 4 to 32%. In another approach, the total pressure drop due to the bend has been divided into its constituent parts, such as due to change in momentum of solids and air through the curvature zone, frictional pressure drop due to solids and air flows through the curvature zone and straight section of pipe (reacceleration zone) after the curvature zone and reacceleration of slowmoving particles at the end of curvature zone to their steady-state velocities. It has been estimated that the average particle velocity at the exit to the curvature zone of bend lies in the range of 73 % to 77% to the steady-state particle velocity values when the flow is fully developed. The pressure losses in the reacceleration zone were typically 4 to 8 times larger than that occurred in the curvature zone. The pressure drop characteristics in the curvature zone show a slightly drooping tendency with an increase in air mass flow rates. However, pressure drop characteristics rose sharply in the reacceleration zone. The product with the largest particle size provided the maximum pressure drop. With an increase in the radius of curvature of bend, the bend pressure drop values got increased. Bend pressure drop was increased sharply with an increase in the conveying air amount, and with a decrease in the pipeline diameter. Bend loss models have also been developed by separately considering the different components of the overall bend loss, such as momentum change of solids and air through the curvature zone, solids-air-wall friction through the curvature and reacceleration zone, and reacceleration of particles in the reacceleration zone. The new models have been evaluated by using it to predict for bend loss for a fly ash sample conveyed through a longer pipeline length, larger pipe diameter, and for larger solids flow rates. The results of validation have shown promising outcomes with the model predicting within 12.2% error, which has been considered to be acceptable.