Dense phase pneumatic conveying is preferable than dilute phase conveying in many industrial factories, because lower transport velocities are beneficial as they reduce attrition of the particles and reduce erosive impact effects of the particles on the pipe wall and therefore minimise wear. However, dense phase conveying is critically dependent on the physical properties of the materials to be conveyed. For many materials which are either erosive or fragile, they do not exhibit the physical properties required to be conveyed reliably in a low velocity, dense phase flow regime. This is a serious problem in the food, chemical and pharmaceutical industries. One satisfactory approach to this problem which has been widely applied is the use of bypass systems. Bypass pneumatic conveying systems provide the capacity of transporting some materials that are not naturally suited to dense phase flow. Bypass pneumatic conveying systems also provide a passive capability to reduce minimum particulate transport velocities.
In this paper, the operation of internal bypass system was investigated by both experiments and modelling. The bypass pneumatic experimental system was built with a main pipe of 79mm in diameter and an internal bypass pipe with orifice plate flute arrangement. Fly ash and alumina are used in the tests. High speed video camera visualization and differential pressure transmitters were employed to investigate the operation of dense phase bypass pneumatic transport systems and the mechanism of material blockage inhibition provided by this system. Based on the mass conservation, an entire bypass system was numerically modelled using a series of control volumes. An integrated version of the Ideal Gas Equation was applied to evaluate pressures at the central point of each node for air flow in the bypass pipe and the main pipe. A permeability equation was used for air flow through the material plug in the main pipe, while orifice plate theory was employed for flow through an orifice plate located in the flute of the bypass pipe.
The experimental results showed that bypass system was found to consume more energy than conventional system when using the same air mass flow rate due to the increase of friction. The difference in specific energy consumption between two conveying systems reduces with the decrease of conveying velocity. Therefore, the minimum conveying velocity is strongly recommended as the practical conveying velocity in bypass system. The conveying velocity of alumina in bypass system was much lower than that of conventional pipelines, which resulted in much reduced specific energy consumption. In this system, particulate material blockages were inhibited in bypass systems due to the air penetration into the particulate volume, as was reflected in differential pressure transmitter measurement data and flow visualization. It was found that the model was in good agreement with the experimental observations. This model provides the capacity to simulate the mechanism of material blockage inhibition when the gas solids flow progresses along the pipeline. Modelling results showed that pressure drop increased with decreasing the diameter of orifice plates and the spacing of bypass pipe flute. The influences of conveying materials properties on bypass system performance were also discussed.