The flow of particulate bulk solids in an air-assisted gravity conveyor
The programme of work commenced with a thorough survey of the published literature relating to the air-assisted gravity conveying of bulk particulate solids. Aspects of the behaviour of fluidised powder in stationary beds were studied where it seemed possible that observations of such behaviour might be useful in predicting how the same powder would flow in an air-gravity conveyor. As a result, a simple chart was prepared allowing an estimate to be made of the minimum fIuidising velocity and the velocity at which entrainment of fines could occur from a knowledge only of the density and particle size of the powder concerned. The design and performance of air-gravity conveyors was examined in some detail, considerable effort having been made to bring together as much as possible of the published information on this method of bulk solids transport. Practical problems on the design and operation of conveying installations have been highlighted and techniques are suggested to simplify the preliminary stages of design. One of the difficulties facing the designer of air-gravity conveyors has been the lack of a convenient mathematical model that would enable the performance of a given conveyor to be reliably predicted. A number of possible modelling techniques were therefore investigated and a new modelling approach, based on the uniform flow of a fluid in an inclined channel has been proposed. The experimental work was divided into two parts. In the first part various types of porous distributor material were examined and their relative merits discussed, and then a number of powders, having widely differing characteristics, were tested in a small fluidising rig. Significant features of the fluidisation behaviour of the powders in "stationary" (as opposed to "flowing") beds were recorded, notably their minimum fluidising veIocities and bulk densities. The second part of the experiment programme involved the flow of an aerated p.v.c. powder, of about 120 um mean particle size, in an inclined channel. Two types of porous distributor were used in the channel, and in each case the relationships amongst the solids mass flowrate, the channel slope, the superficial velocity of the fluidising air and the depth of the flowing bed were observed. Using the modelling approach proposed previously, the results of the tests on the channel rig were analysed and it was concluded that, although more experimental data was needed, the correlation between the model and the data obtained so far confirmed that further investigation would be justified.