Enclosureless dust collector in woodworking facilities
Enclosureless or unenclosed dust collector are often used in woodworking facilities, especially for light dust producing operations as sawing, edging, scribing, shaping, light planning, tenoning, mortising or (manual) sanding.
DEFINITIONÂ & LEGAL ASPECTS – An enclosureless dust collector is an opened filtration unit that bags or cartridges are directly exposed to working area. According to norm NFPA-664 2007 ed., an enclosureless dust collector is an « air-material separator designed and used to remove dust from the transport air possessing ALL of the following (characteristics):
- the filtration is accomplished by passing dust laden air through filter media, and allowing cleaned air to exit to the surrounding area
- the filter medium is not enclosed in a container
- the filter medium is not mechanically shaken or pressure-pulsed
- the filter medium is under positive pressure
- removal of the collected dust is not continuous or mechanical »
Wood dust is explosive and the use of unenclosed units can reach to fire, smoldering fire, jet of flamme and/or explosion.
Note from BEAULIER – One has to understand that an existing shaker or reverse air bag filters (baghouse) and cartridge type dust collectors can not be considered as an enclosureless unit just by removing the container (enclosure)
LOCATION & CONDITIONS – According to NFPA-664, enclosureless units shall be installed indoors « meeting all of the following criteria:
- the collector is used only for dust pickup from wood processing machinery (i.e. no metal grinder and so forth)
- the collector is not used on sanders or abrasive planers having mechanical material feeds
- each collector is has a maximum air-handling capacity of 2,4 m³/sec [5000 CFM]
- the fan motor is of a totally enclosed, fan cooled design
- the collected dust is removed daily or more frequently if necessary to ensure efficient operation
- the collector is located at least 6,1 m [20 ft] from any means of egress or area routinely occupied by personnel
- multiple collectors in the same room are separated from each other by at least 6,1 m [20 ft]«
Notes from BEAULIER – Enclosureless dust collectors shall not be used for any other industrial application than woodworking; experts from BEAULIER will also recommend:
1-/ to perform a risk analysis prior to installation of an unenclosed unit inside the building, especially for airflows exceeding 0,94 m³/sec [2000 CFM]
2-/ to avoid the use of enclosureless dust collectors for spark or ignition source producing operations (ex.: sawing wood containing nails, staples or metal rods), for dust pickup requiring high static (succion) pressure (ex.: computer numerical control – CNC) or for fine dust producing operations as sanding
3-/ to make sure that the dust collected in bags or barrels, is not exceeding 10 kg/hour per container (which represents about 1 barrel / 8 hours)
FIRE & EXPLOSION PROTECTION – According to NFPA-664 – art. 8.2.2.5.2 & 8.2.2.5.3., enclosureless units of less than 5000 CFM shall be permitted without any additional fire or explosion protection device requirements. According to the experience of engineers from BEAULIER, the following measures shall be implemented in order to control fire and explosion risks:
- Housekeeping – in the enclosureless unit surrounding, floors, walls and surfaces – including top of equipments, footbridges, ducts, cabletrails and pipes – shall be clean of dust accumulations -> as far as possible, dust deposits shall never exceed 3 mm [1/8 in] in order to control explosion risks | note that because of low filtration efficicencies of unenclosed units (usually around 80 to 95% caused by high filtration velocity and dust leakage), fine dust emission rate is higher in these areas
- Jet of flame protection – usually in industrial facilities, the suggested safety distance of 6,1 m [20 ft] can not be respected; in the Province of Quebec (ref. RSST) as in the United Kingdom (ref. HSE – http://www.hse.gov.uk/pubns/wis32.pdf), screens, shields or walls can be installed in a way to protect workers and combustible materials from direct inflammation; usually, one (1) or two (2) directions can be protected and the dust collector shall never be completely enclosed – Deflector plates shall be made of non-combustible material, be strong enough to sustain the flamme pressure applied on it (relatively low pressure) and be designed in a way to divert the jet of flamme (flamme front) to a safe area
- Grounding and bonding – electrostatic discharges is one of the most common ignition source in industry; hoods, ducts, blowers and dust collectors shall be grounded and bonded adequately in a way to prevent static charges to accumulate | note that flexible ducting shall be of static dissipative type (SD) and as short as possible (avoid lengths exceeding 3 to 6 m [10 to 20 ft])
- Electrical classification – Because of possible dust accumulations, electrical devices shall be adequately classified so as not to represent a risk of ignition; if housekeeping is effective and maintained by the owner, fan motor as well as other electrical devices located near the unenclosed dust collector shall be of Class II, div. 2 | TEFC motors and dust tight electrical boxes are adequate
- Fire protection – The minimum protection of an enclosureless dust collector will be provided by a portable fire extinguisher located in the area of the unit; when practical, supplementary fire protection shall consist in automatic sprinklers located just above the filtration bags or cartridges
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