System considerations
Being at the end of the conveying process, its importance is often overlooked, but incorrect design and specification can cause endless problems in the conveying system. It is also important that the separation system is not considered in isolation. The influence that the system can have on the filter, and the influence that the filter can have on the system need to be considered in addition.
Blow tank systems
Although with batch systems both reverse air jet filters and mechanically shaken filters can be employed, care must be taken in sizing these units with respect to the volumetric flow rate of air. If, at the end of a conveying cycle the pipeline and blow tank have to be vented through the filter unit, the air flow rate will be considerably greater than the steady air flow rating of the air mover. This is particularly the case if the blow tank operates at a high pressure, for the transient nature of the air flow through the conveying cycle is significantly magnified.
This will result in a considerable increase in the air velocity through the filter, possibly resulting in blinded filters, giving higher filter resistance and subsequent difficulty with cleaning. This is particularly so in the case of mechanically shaken filters. It is essential in these circumstances to reduce the air supply at the end of the conveying cycle in order to keep the total air flow rate to as low a value as possible. To cater for these surges simply by increasing the filter size may be a more expensive solution.
Vacuum conveying systems
In vacuum conveying systems the clean air at outlet from the filter is drawn through an exhauster. Should a filter bag split, or otherwise fail, material will be carried over to the exhauster. Although a turbo blower can tolerate a certain amount of dusty air, pro- vided that it is not abrasive, positive displacement blowers cannot, and so some form of protection must be provided. A cyclone is often used for this purpose, and although its efficiency with respect to fine particles is rather low, it will allow time for the sys- tem to be shut down before serious damage occurs to the blower.
The design parameter for sizing fabric filters is related to the superficial air velocity across the filter fabric. In a positive pressure conveying system the air flow rate that can be used in this evaluation is simply the volumetric rating of the air mover, unless the material conveyed is at a high temperature. In a negative pressure system the filter is under vacuum and this will have to be taken into account.
In comparison with a positive pressure system, employing the same ‘free air’ flow rate, a vacuum system operating under 0.5 bar of vacuum, for example, will need to have a filter approximately twice the size of one required for a positive pressure sys- tem. This is not a fair comparison, of course, since the pipeline bore for the vacuum system will be larger, but it does highlight the need to take account of both pressure and temperature in sizing. The modelling for this is considered in Chapter 9.