The “PF” filter is a dedusting unit with polyester cartridges (that are non-disposable), with efficiency ratings that normally reach 99.92% according to B.I.A. tests (Berufgenossenschaftliches Institut für Arbeitssicherheit - Inter-professional Institute for Workplace Safety) which the cartridges are periodically subjected to.
The filter works with low pressure suction, which means that the dusty air is sucked in by the centrifugal fan downstream. The fan creates the necessary low pressure to overcome the load loss of the circuit and losses caused by filter resistance, once the system is working steadily. It is common knowledge that every filter requires a few days to reach a steady state to perform with maximum efficiency after a cake of dust, which operates as a prefilter, has formed on the filtering fabric. For this reason, it is not good to measure residual dust after starting the machinery.
The dusty air is sucked in by the fan, enters the pre-separation and decanting chamber that every PF filter is built with, and gravity causes the infeed speed to slow down and the first separation of larger particles from finer particles takes place. The pre-separation and decanting chamber also has the function of uniformly distributing the mass of dusty air and piloting it to the lower part of the filter unit, to force it through the cartridges from the bottom. Air passes through the filter unit longitudinally and avoids dangerous mechanical stresses that would be produced by a lateral impact.
Fabric cartridges are usually arranged vertically in the filtering chamber. Air goes through them from the exterior towards the interior. The air loses its dust content, which deposits on the external surfaces of the filter cartridges. The dedusted air flows upwards inside the cartridge, and reaches the "clean room" through the hole where the venturi ejector is located.
From here, the filtered air comes out of the filter body, and is expelled through the fan, usually, into the atmosphere through the chimney. In some cases, it is possible to recirculate the filtered air in the internal environment, with considerable financial advantages due to a lower heat dispersion.
CLEANING THE CARTRIDGES
The cleaning system is the classic "reverse pulse jet" system, in counter current of jets of compressed air blown inside the cartridges, row after row, in cycles, according to the program set on the electronic sequencer provided.
Compressed air is blown inside the venturi ejector, which is fitted on each cartridge. The pressure wave produced by the short but intense compressed air jet mechanically deforms the cartridges star-shaped surface, using elasticity.
The shock wave causes the dust cake that has formed on the exterior of the cartridge to detach.
Since cleaning takes place during the normal operation of the suction system, it has been verified that part of the dust that detached due to the "venturi effect" is sucked into the cartridge again by the fan. It is necessary to go on to the post cleaning step and allow the reverse jet system to continue working, © Bruno Balducci srl – Italy – PF UK Rev.1/2015 for a short time, even after the centrifugal fan is turned off. The absence of suction facilitates the cleaning of the cartridges, and almost all of the dust detaching during this phase falls into hopper. The time necessary for the "post cleaning" phases depends on the nature of the dust, their granulometry and concentration.
Sophisticated electronic sequencers monitor the cleaning operations, during and after the suction system function, checking the "Δp" value, the planned cleaning and "post cleaning" operations and the dust leak detector. Lastly, dust leaks with a level of emission above the allowed limit are highlighted.
Dust that is filtered and separated from the air is collected in specific metal containers and moved towards the exterior with mechanical systems such as rotary valves, screws, etc.
PF filters are designed according to the Client's specifications, in compliance with DPR 152/06 for the external environment and Leg. Decree 81/08 for the improvement of employee health in the workplace.