General ConsiderationsThe effects of static electricity on the collection of dry particulate in fabric collectors is rather simple but misunderstood. For the most part, cartridge dust collectors experience the same issues.
First we must consider the cause of static charge build up in a collector. It occurs because the dust being collected is akin to a capacitor in an electronic circuit. In this day of computer chips the designer may not be familiar with this phenomenon. The capacity has two conductive plates separated by a layer of insulating material that has high enough insulation values that the static charge remains for relatively long periods. The charge can be removed by grounding one side of the capacitor. The charges then drain.
In dust collectors where the dust forms in the filter cake, the static charges may enter the collector on the surface of dry particulate dust. If the dust has high dielectric resistance properties, it can accumulate and build up in the filter cake. It can be viewed as many particles each carrying a static charge and acting like a miniature capacitor. The static charge will then build up on the surfaces and may reach a high enough level where a spark can be produced. This spark can trigger the explosion of explosive dusts.
Mechanical Cleaning (Shaker) Dust Collectors
In a fabric collector with a mechanical shaking mechanism to remove the dust, the collector is most vulnerable during the cleaning process. The dust is shaken from the filter bags in the process of shaking the cake, sparks sometimes are produced. Invariably, the dust/ gas mixture passes between the upper and lower explosive limits. A serious explosion may occur.
Usually these collectors will have explosion vents which relieve the high pressures that are generated in an explosion, presumably keeping the housing from being damaged and protecting the operating personnel near the dust collector.
In an attempt to keep this static charge from building to threatening levels, measures are included in an attempt to bleed this charge to ground. These include one or more of the following:
1) Sewing in grounding wires into the filter media.
2) Impregnating carbon or other conductive coating into the filter cloth.
These often give the designer a false sense of security in applying these to dust collectors. As explained above the dust, itself, insulates the charge and it remains in the cake until it reaches a point where a spark is generated. If the dust concentration is above the lower explosive limit and below the upper explosive limit, an explosion can occur. Fortunately, generation of the spark may not occur if the timing of the spark and dust concentration level do not coincide. An explosion does not occur in these cases.
Continuous Cleaning Reverse Jet Pulsed Dust Collectors
When dust, with the same properties described above, is vented in the same operations, using a reverse pulse jet cleaning system, the danger is considerably diminished unless the pulsing is applied in “off line” cleaning mode where the fan is stopped.
These collectors clean the bags by injecting air from the clean air plenum backwards through individual bags as the flow continues through the collector. This cleaning agitates the filter cake so the static charges are dissipated.
The danger of explosion occurs when the dust concentration coming into the collector reaches a level between the lower and upper explosive limit concentrations. This is highly unlikely but we recommend that properly sized explosion vents are installed which normally coincides with the requirements of insurance underwriting firms.
The explosions can occur when there is dust build up in ducts especially when long horizontal runs are encountered. The spark can be generated in ducts and the explosion front can travel down the duct into the dust collector, igniting a secondary explosion as the concentration in the collector housing is driven above the lower explosive limit for that dust. Even with no build up in the ductwork, an upset can occur in the process which generates sufficient dust concentrations.
One method of nullifying the possibilities of danger due to duct build up is to install an automatic booster / duct cleaner device. This booster can serve to automatically clean out any drop out in long horizontal duct runs.
Another phenomenon can affect of dust collector systems, is where the dust has high dielectric properties and the dust, because of static charges, will build up on the outside bend of an elbow. This dust can trigger an explosion if this dust is also flammable and explosive. Some examples of dust where this problem is often a factor are toners for copy machines and electrolyte powder used in alkaline batteries. The solution is to insert a pulsed air jet that agitates the built up dust that dissipates the charge. Some dry powder coating compounds are also subject to static charge build up in powder coating systems.
Use these links for more information:
Booster / Duct Cleaner
Quencher spark arrestor