Experimental Study for the Effect of Inner Spiral Grooves Performance in Horizontal Pipes of Granular Pneumatic Conveying

Pneumatic conveying is a continuous and flexible material handling method which uses positive or negative air pressure to convey materials in pipe. This conveying system is generally divided into two groups of dilute and dense phase.
The purpose of this research was to create spiral grooves inside horizontal pipes which transfer granular materials under dense phase. Also, the performance of these pipes was compared with control pipes. Finally, friction factors obtained in this research were compared to the previous study. To create spiral grooves inside the pipes, a broaching machine was designed and developed. Then, by connecting the broached pipes to a pneumatic conveying test- rig of granular materials, the performance of these pipes was compared with control pipes. The specifications of the broaching machine and test-rig were as follow.
Broaching machine: The machine included chassis, an electromotor with one hp power, a reduction gearbox, a ball screw for converting rotational motion to linear motion, a spiral shaft, a guide with three bolls, broaches and inverter. Cutting operations and creating grooves inside the pipes were done using broaches. These broaches had two angles, attack angle of 15 degrees and a clearance angle of 10 degrees. The spiral angle of broaches was 30 degrees, the spiral pitch was 260 mm, the width of each groove was 1.5 mm, and a number of teeth were 20.
Test- rig: The main components of the test- rig were the air compressor, blow tank, conveying pipes, solid discharge control valve (SDCV), receiving hopper, orifice plate flow meter, pressure transducers, and single point load cell. The compressor was a piston- type, the air flow rate was 405 L min-1 and maximum pressure was 12 bar. For a continuous flow of air and material mixture into conveying pipes, a blow tank was used. To transfer material from blow tank to pipes, a 90-degree bend with a radius of 250 mm and an inner diameter of 40 mm was used. The inner diameter of pips was 40 mm, the thickness was 5 mm and was selected from ABS. In order to measure static pressure of air along the pipes, 10 holes of one mm diameter were drilled on the surroundings of the pipes at intervals of one meter. Then, on each of these holes, a polyethylene bushing was placed. Pressure transducers were threaded on the top of these bushings. A solid discharge control valve was placed at the end of the flow line to control the flow of materials in a dense and continuous phase and to prevent material acceleration. The materials were introduced into the receiving hopper after leaving the valve. To measure the volume flow rate of air, an orifice plate with D and D/2 tapping was used. The pressure transducers were Hogller. For measuring the mass of the materials entering the receiving hopper, a single point load cell (Zemic L6G) was installed under the hopper. A data acquisition system based on ARM microcontroller was used to record output signals from transducers.
The treatments were four levels of groove depth (0, 0.35, 0.55 and 0.9 mm), three levels of air pressure (1, 2 and 3 bar) and three levels of pipe length (3, 6 and 9 m). The transferred material was considered as mung bean. The results of ANOVA showed that the main effects of groove depth, pipe length, and air pressure were significant on the mass flow rate of transmitted mung bean and solid friction factor at 1% probability level. The results indicated that the maximum mass flow rate and minimum friction factor were observed at a pipe length of 3 m, the groove depth of 0.90 mm and air pressure of 3 bar. Minimum mass flow rate and maximum friction factor were observed at pipe length of 9 m, the groove depth of 0 mm (smooth pipe) and air pressure of 1 bar. The results showed that the existence of spiral grooves within horizontal conveying pipes would increase the mass flow rate of the mung bean and reduce the solid friction factor of the mung bean and inner wall of pipes.