12 Practical Tips For A Safe Wood Dust Collection System

Ducts shall be galvanized rolled, spiral or snap together pipe. No plastic pipe, since the friction of the wood dust within the wall of the pipe creates a static charge that can cause an explosion. In addition, the elbows and other fittings available in PVC restrict the airflow, which is necessary to convey the wood dust.
The system must be for wood dust, chips and shavings only. No metal grinding, sanding or cutting. Under no circumstances should any operation that generates sparks be part of a wood dust collection system, since the potential is quite high for a spark to ignite the dust and result in an explosion.
Dust must not settle out in the ducts. Insufficient suction caused by dirty filter bags, poorly designed ductwork or the fan rotating backwards (incorrect direction of rotation will reduce airflow to 50%) will allow dust to collect in the bottom of the main duct and become a possible starting point for a fire. The poor transport velocity should be corrected and clean-out/inspection doors should be installed into the duct.
Suction hoods must be at the source of the dust so that the particles are thrown into the hood by the blade, wheel or belt. Proper hooding is necessary to capture the dust or the system will be ineffective. Overhead canopy hoods are not effective since dust does not rise, as does smoke. Also, the updraft will bring the dust into the workers' breathing zone.
The National Fire Protection Association (NFPA) requires that, where conditions permit, the blower be located after the dust collector so that the blower is on the "clean air side". See Figure 1. A nail or other metal object, such as a tool, can be sucked into the duct, strike the fan blade and cause a spark that results in an explosion. There are many inexpensive packaged dust collectors that consist of a blower on the intake side followed by filter bags. The use of "blower into bag" units should be limited to a few such units within a fire area of a facility. It is not the intent of NFPA for a multiple number of "blower into bag" units to be used instead of a central dust collection system with a main duct and dust collector located outdoors.
Production or industrial scale woodworking operations and particularly those that do much sanding, should locate the dust collector outdoors. The dust collector may be located indoors if it is adjacent to an exterior wall and includes an explosion vent(s) ducted to the outdoors or an explosion suppression system that meets the requirements of NFPA 69, Standard on Explosion Prevention Systems.
The filtered air from an outdoor dust collecting system used for production or sanding shall not be recirculated unless it includes a spark detection and suppression system that closes an abort gate so that a fire can not return through the duct back into the building. See Figure 2.
Small scale shops (less than six to eight people) with woodworking operations that are incidental to the principal occupancy, usually do not require spark detection or suppression systems. Explosion vents ion the ductwork and dust collector are required for finely divided wood dust, such as wood flour, and are recommended for all other types of wood dust, where conditions permit. Dust produced from hammermills, sanding or high speed routing of MDF is typically very file and, therefore, has a much greater potential for explosion than general wood dust. The explosion vent can be a hinged door(s) with a special set of latches or a one-time rupture panel. See Figure 3.
Cyclones are suitable for typical wood dust, chips and shavings, but are not very effective for sanding dust. The pull through style is preferred because the blower is on the clean air side. The dust receptacle at the bottom, must be 100% air tight. Any leak at the bottom of a pull through cyclone dust collector will prevent the dust from swirling downward and shoot it out the exhaust. If filter bags follow the cyclone, they will be overloaded with dust, impede the airflow and be a potential source for a fire. Check the flexible connection to the 55-gallon drum, slide gate of a hopper or airlock for any leaks.
The dust collecting system should not be excessively noisy in the shop. The sound level of the air and dust traveling through the ducts should be no greater than 80-85 db. Noisy systems prevent a worker from being heard if he has an emergency situation.
In hand sanding operations it's difficult to capture the dust, since the worker is moving around. Flexible arms are not practical. Either a walk-in booth with dust collector built into the rear wall or special sanders with a high vacuum systems should be used.
Dust collectors that require that one go inside to change the filters or maintain the shaker are classified by OSHA as a confined space and require special procedures for workers o enter. OSHA defines a confined space as any area that is not normally meant for habitation and may pose risk upon entering. Fumes, lack of oxygen, sloped hopper bottoms are some of the risks. Some confined spaces require a permit. Avoid small (15,000 CFM and less) baghouse dust collectors that require entrance for maintenance. Instead use baghouses that have slide out filter removal from either the top or the side. Large systems (15,000 CFM and more) are usually limited to round reverse air flow cleaning baghouses. Although these units require entrance for filter, access is on the clean air side and therefore the worker is exposed to less risk than units that require entrance on the dirty air side.

Insurance companies, fire departments and OSHA have adopted woodworking safety guidelines published by the National Fire Protection Association (NFPA). These guidelines, "Wood Processing and Woodworking Facilities" (No. 664) and "Blower and Exhaust Systems" (No. 91), may be purchased directly from NFPA (800-344-3555). The design of any dust collection system should be reviewed by a property loss prevention expert from your liability insurance company. That person can determine whether your dust collection system is in compliance with current recommended practices. You should not solely depend on the sellers or installers of dust collection equipment for safety and compliance information.

Peter Levitt Peter Levitt, Product Manager Sternvent Co., Inc.