There are several principles in the distribution of dust inside a pulse jet dust collector that are important to understand.
1) The cleaning frequency is related to the pressure drop across the filter media and filter cake by the square of the pressure drop. This is because the quantity of dust that the media can hold between cleanings is related to the pressure drop by the same relationship.
At a one inch average pressure drop the media can hold 16 times the amount of dust than it can hold at a 4 inch average pressure drop. Operating at an average pressure drop of four inches will require16 times the pulsing frequency to maintain equilibrium.
The dust penetration through a dust collector is related to this pulsing frequency, when there are no leaks and an efficient filter cake is formed. In the example above the penetration at 4 inch operating pressure drop is at least 16 times more than operating at one inch pressure drop.
Collectors should be operated at the minimum pulsing frequency to maintain equilibrium in pressure drops. Over cleaning wastes compressed air, lowers collection efficiency and shortens bag or cartridge life.
Pulsing Frequency is affected by the following:
Porosity of the dust. The higher the porosity the lower the cleaning frequency. Paper dust has low porosity and metallic fumes have high porosity.
Coarseness of the dust. Coarse dusts have lower cleaning frequency than the same load of fine dust.
Dust Load. If the dust load is doubled the pulsing frequency is doubled (Assuming identical particle size distributions)
Liquids. Liquid droplets in the exhaust stream or compressed air supply may radically affect filter cake characteristics. A hard crust may form, i.e. ciment dust.
Other Factors that affect pulsing frequency
• Permeability of the basic filter media
• Ineffective seal design at the dust wall
• Improper filter element installation. In snap band filters not seated properly and insufficient tension in clamped systems.
• Impingement of the cleaning jet against the internal surfaces of the cleaning element. The impingement can be induced by the shape of the bottom of the filter element. Concave bottoms are effective and convex shapes are not.
2) Distribution of dust in the filter compartments.
Fine dust behaves like gas in that it follows the path of least resistance. For long and narrow collectors with inlets on the narrow wall, the fine dust will be drawn to the bags closer to the inlet and there will be very little fine dust furthest from the inlet. Multiple inlets on the long side wall distribute the fine dust among the bags. In some cases of existing collectors, multiple timers are applied to the rows of bags closest to the inlet which are cleaned more often to distribute the flow and (filter wear) in the collector.
Coarse dust with high inertia acts like billiard balls independent of the exhaust gas flow in the collector. In a collector with pyramidal hoppers and the inlet entering the hopper, the high inertial dust will strike the hopper walls and ricochet up to the bottom of the filter elements and cause wear and premature failure of the filter elements. There are many clever designs of baffles to absorb the dust impact and deflect these high inertia particles toward the hopper outlet. These baffles are shaped or constructed to absorb or resist the impact of high inertia, sometimes abrasive dust particles. Some are made of hard abrasive resistant metals and others are coated with rubber to resist the wear. In some instances like in pneumatic conveyors and raw mills in the cement manufacturing process, the inlets are directed down and into the hoppers.
For more information on Dust Behavior ... Dust Collector Selection Guide and Correspondence Course.
