Modifying plant components
If the conveying line is already operating under optimum conditions, or if optimization does not achieve the desired increase in material flow rate, it is possible that a modification to one or two of the plant components will result in an increase in capacity. For a given system, the material flow rate is dependent to a large extent upon the conveying line pressure drop available. It is necessary, therefore, to either provide the system with more pressure or to minimize the pressure drop associated with some of the plant components and thereby make more available for the conveying of the material.
The total pressure drop for the conveying system is made up of that due to the feeding device, that due to the conveying line and that due to the filtration plant at the end of the line. If the pressure drop associated with the material feeding and filtration units can be reduced, a greater pressure drop will be available for conveying the material in the pipeline, which will enable more material to be conveyed. Air supply and extraction lines, particularly if they are long, should also be included in this review.
Increasing the pipeline to a larger bore will almost certainly achieve an improvement in performance. An increase to the next standard size, however, may achieve a material flow rate higher than necessary. If the conveying line pressure drop is greater than about 0.8 bar for a positive pressure system, or more than about 0.4 bar for a vacuum system, the possibility of stepping the pipeline to a larger bore part way along its length would be well worthwhile exploring.
This is certainly the case for virtually all materials conveyed in dilute phase and for most materials conveyed in dense phase in sliding bed flow. It is only where materials exhibit a pressure minimum point in their conveying characteristics that the benefits would need to be reviewed carefully. Stepping of the pipeline will reduce the high val- ues of conveying air velocity that can result towards the end of single bore pipelines. Since pressure drop is dependent upon both pipe bore and velocity the stepping of the pipeline to a larger bore will generally help on both accounts. The stepping of pipeline systems was considered in Section 9.4 in relation to conveying air velocities, and in Section 14.8 in relation to conveying performance.
Replacing plant components
If optimizing the conveying conditions and modifying plant items does not result in the desired increase in performance it will probably be necessary to replace one or more of the plant items. One of the easiest items to replace is probably the blower or compressor. It is important to realise, however, that if no other changes are made to the plant the output of the new air mover will only need to be increased in terms of delivery pressure. Only a relatively small increase in volumetric flow rate will be required in order to compensate for the increase in delivery pressure to maintain the necessary conveying line inlet air velocity. If a rotary valve is used in such a system it may also be necessary to take into account the corresponding increase in air leakage that is likely to result.
There are, of course, limits as to what can be achieved by replacing the blower or compressor. For negative pressure, or vacuum systems, the increase in available pressure drop may only be marginal. With positive pressure systems the material feeding device may impose a limit on the maximum supply pressure, particularly with low pressure rotary valves and venturi feeders. An increase in conveying line pressure drop can result in an increase in material flow rate as has been shown with the various material conveying characteristics presented but there is clearly a limit on the improvement that can be achieved.
If a significant improvement in performance is required then an increase in pipeline bore will be required. It is quite likely, however, that a new blower or compressor will be required as well, unless the original one installed was grossly over-rated. The possibility of increasing the speed of an existing machine to deliver a higher air flow rate could be explored, along with the capability of the existing drive motor. The capability of the existing filtration plant would also have to be examined for the higher air flow rate.