Fluidized motion conveying systems
The categorizing of fluidized motion conveying systems always represents a problem. They are not generally recognized as pneumatic conveying systems because they only use very low pressure air and the material does not flow through a pipeline. They are, however, clearly not in the mechanical conveying group of conveyors. Until recent years their application was relatively limited because the main driving force was gravity, and so they would only operate on a downward incline, although at a very low angle.
The material is conveyed along a channel which has a continuous porous base. Air enters the material through the porous base and fluidizes the material. In this condition the material will behave like a liquid and flow down an inclined channel. The channel is generally closed to keep the system dust tight. In early systems the channel ran with the material only partly filling the channel. The fluidizing air escaped into the space above the flowing material and was ducted to a filtration plant. In a recent development the channel runs full of material and horizontal conveying is possible.
Air-assisted gravity conveyors
In situations where the flow of a material can be downwards, the air-assisted gravity conveyor has a number of advantages over pneumatic conveying systems. Plant cap- ital costs can be much lower, operating costs are significantly lower, and a wide range of materials can be conveyed at a very low velocity. Air-assisted gravity conveyors can be regarded as an extreme form of dense phase conveying.
The conveyor consists essentially of a channel, divided longitudinally by means of a suitable porous membrane on which the material is conveyed. A sketch of such a system is given in Figure 2.16. If a small quantity of low pressure air is fed through the membrane, the inter-particle and particle/wall contact forces will be reduced and the material will behave like a liquid. If a slight slope is imparted to the conveyor, the material will flow.
These conveyors are often referred to as ‘air slides’. They have been in use for over 100 years and are still widely used today for materials such as alumina, cement and fly ash. Air-gravity conveyors, ranging in width from 100 to 600 mm, can convey mater- ials over distances of up to 100 m, and are suitable for material flow rates of up to about 3000 tonne/h. In general, most materials in the mean particle size and density
ranges from 40 to 500 f.Lm and 1400 to 5000 kg/m3, respectively, are the easiest to convey and will flow very well down shallow slopes.
The Geldart classification of fluidization behaviour
These materials correspond to Group B materials in Geldart’s classification of flu- idization behaviour [3] which is presented in Figure 2.17. When the supply of fluidiz- ing air is shut-off with these materials they de-aerate rapidly, hence the bed collapses and flow stops almost instantaneously. This means that they are easy to control and will not flood feed. It is this group of materials, however, which cannot be conveyed in dense phase in conventional conveying systems, because they have little or no air retention capability. The Geldart classification can be used to a limited extent to identify which materials might be capable of dense phase conveying.
Materials of larger particle size and/or high density in Group D can usually be conveyed in a similar manner but the quantity of fluidizing air required tends to become rather high. This group of materials might be considered suitable for dense phase con- veying in plug flow, but this is only the case if they are essentially mono-sized. Materials having a high value of mean particle size, with a wide size distribution, generally have very poor permeability and so are not capable.
Group A includes materials of small particle size and/or low density and these may have a tendency to continue flowing for a time after the air supply has been shut-off because of their air retention properties. It is generally this group of materials that are ideal candidates for dense phase pneumatic conveying in sliding bed flow.
Group C includes cohesive powders that are difficult to fluidize satisfactorily, because of high inter-particulate forces resulting from the very small particle size, and are unsuitable for conveying in this manner, although slightly cohesive materials can usually be conveyed provided that the slope of the channel is great enough. These materials will generally convey well in dense phase provided that they can be fed into the pipeline. Care must be taken with ultra-fine particles in pneumatic conveying systems, however, because they have a tendency to coat the pipeline wall. This issue is considered at various points in the Guide.
It is an essential requirement that the material is sufficiently aerated on the channel membrane for flow to take place. The porous base, therefore, must be of high enough resistance to ensure that when part of it is clear of material the remainder is not starved of air. Material segregation by size and density can occur during transport and can be significant in a long channel. In an extreme case a deposit of coarse particles may con- tinuously build up on the bottom of the channel until the solids flow ceases altogether. The air-gravity conveyor, however, by virtue of its flow mechanism is particularly suit- able for both abrasive and friable materials.
Full channel conveyors
Hanrot [4] describes a pressurized horizontal conveying system developed by Aluminium Pechiney to convey alumina. The alumina was conveyed from a single supply point to more than 100 outlets. Electrolysis pots on a modern aluminium smelter were required to be filled and the distance from the silo to the furthest outlet was about 180 m. Air at a pressure of 0.1 bar is used. A sketch of the system is given in Figure 2.18 and this illustrates the principle of operation.
A conveying channel is employed, as with the air-assisted gravity conveyor, but the channel runs full of material. Balancing columns are positioned on the conveying duct and are used for dedusting. This is not a continuously operating system in the application described. It is a batch type system and its object is to meet the demands of the intermittent filling of the pot hoppers. The system, however, is clearly capable of continuous operation and of significant further development.