CARTRIDGE FILTERS: PF
The “PF” filter is a dust collector cartridges made of polyester (non-shedding) whose efficiency normally reaches 99.92% according to B.I.A. (Berufgenossenschaftliches Institut für Arbeitssicherheit) tests, to which the cartridges are periodically subjected.
OPERATION
The filter works “under depression”, this means that the dusty air is aspirated by the centrifugal fan, placed downstream of the system, which creates the necessary depression to overcome both the pressure drops of the circuit and those caused by the resistance of the filters, once they have reached their operating regime. As is known, each type of filter needs a few days to reach its operating regime to give maximum efficiency after a limited accumulation of dust (cake) has formed on the filter medium, which acts as a prefilter. For this reason, it is not advisable to carry out instrumental measurements of residual dustiness immediately after commissioning. The dusty air, aspirated by the fan, enters the pre-separation and decantation chamber with which each PF filter is equipped, where, by gravity, a slowing down of the entry speed and a first separation between the coarser and finer particles takes place. The pre-separation and decantation chamber also has the function of uniformly distributing the mass of dusty air and guiding it into the lower part of the filter bank, to force it to invest the cartridges from below, lapping them longitudinally and avoiding dangerous mechanical stresses from lateral impact.
FILTRATION
The cartridges are normally arranged vertically in the filter chamber and are crossed by the air from the outside to the inside. The air loses its dust content which deposits on the outer surface of the filter cartridges. The air, after being de-dusted by traveling upward inside the cartridge itself, reaches the “clean chamber” through the hole in which the venturi-type ejector is located. From here, the filtered air exits the filter body, passes through the fan and is expelled, normally, into the atmosphere through the chimney. In some cases it is possible to recirculate the filtered air into the internal environment, with significant economic advantages thanks to the lower heat dispersion.
CLEANING THE CARTRIDGES
The cleaning system is the classic “reverse jet” counter-current system by means of compressed air jets blown inside the cartridges, row by row, in a cycle, according to the program set on the electronic sequencer supplied. The compressed air is blown inside the venturi ejector, with which each cartridge is equipped. The pressure wave caused by the short, but intense, jet of compressed air slightly deforms the “star” shape of the cartridge, exploiting its elasticity. The shock wave causes the detachment of the dust deposited on the outside of the cartridge. Since the cleaning takes place during the normal operation of the suction system, it is found that part of the detached dust, thanks to the venturi effect, is again drawn back onto the cartridge by the fan.
It is necessary to proceed with the “post-cleaning” allowing the “reverse jet” system to continue working, for a short period, even after the centrifugal fan has stopped. The absence of suction facilitates the cleaning of the cartridges from which the dust detaches almost totally to fall into the hopper.
The time required for “post-cleaning” is related to the nature of the dust, its granulometry and concentration. Sophisticated electronic sequencers preside over the cleaning operations, during and after the operation of the suction system, controlling the “ΔP” value, the programmed intervention of the cleaning and “post-cleaning” system and the dust leak detector; any dust leaks whose emission value exceeds the permitted limit are also reported.