System not capable of duty
As with the problem of pipeline blockage, considered in the previous chapter, the inability of a system to achieve the rated duty could result from an error in the system design. Alternatively, it is possible that the problem could be rectified by some simple adjustment to the plant. It is particularly important to determine whether the limitation on material flow rate is due to the material feeding device or to the pipeline and air supply.
The first check to be made is on the conveying line pressure drop. If this is below the capability of the air mover it is probable that insufficient material is being fed into the pipeline. This may be rectified by adjusting the controls on the feeding device. If the maximum output of the feeder does not meet the conveying capability of the pipe- line, however, it will probably be necessary to fit a larger feeder.
In the case of feeders delivering material into positive pressure conveying systems, there will be a leakage of air across the device. It should be checked that the feeder is operating satisfactorily with the material before recommending a larger size. In the case of rotary valves, for example, leakage air can restrict the flow of material into the valve. The leakage air might also aerate the material to such an extent that there is a significant reduction in bulk density. The effectiveness of air vents and the clearances on all moving parts should also be checked.
If the conveying line pressure drop is at the design value, however, it would indicate that it is the pipeline or the air supply that is the main cause of the system not being able to achieve the required material flow rate.
Another check to be made should be on the filtration unit. If this is incorrectly sized for the duty it is possible that the pressure drop across the filter will be unnecessarily high. Filter cloth surface areas are sized primarily on volumetric air flow rate. If it is incorrectly sized an additional unit could be installed, if there is sufficient space. If there is not sufficient room, then the filter unit will probably have to be replaced with a larger unit.
Before going to this length, however, a check should be made that cleaning cycles are satisfactory, cleaning is effective, and that the filter cloths do not need replacing. The pressure drop across a filtration unit should be low and so this element of pressure drop is only likely to be significant on low pressure conveying systems.
Reduce air flow rate
An improvement in system performance will often be obtained by reducing the quantity of air that is used for conveying the material, particularly if the system is over- rated in terms of the volumetric flow rate of air that is supplied. This, however, must be carefully considered before being undertaken. It is essentially a programme of optimization that is required and it must be carefully planned.