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Air Exhaust / Ventilation Fan Applications in Hazardous Location Use
A hazardous location air exhaust / ventilation fan (sometimes called explosion proof) may be required in any area where the presence of flammable gases, vapors or finely pulverized dust in the atmosphere is sufficient to create a threat of an explosion or fire. It may also be required where easily ignitable fibers or flying's are present. The following information is a representative, but is not an all-inclusive, list of the types of locations and operations that require hazardous location air exhaust / ventilation equipment in at least certain areas. Consult the current National Electric Code® for complete and current information.
Typical Class I Locations Include:
Petroleum refining facilities
Dip tanks containing flammable or combustible liquids
Dry cleaning plants
Organic coating manufacturing plants
Paint spray booths
Spray finishing areas (residue must be considered)
Petroleum dispensing areas
Solvent extraction plants
Plants manufacturing or using pyroxylin
(nitro-cellulose) type and other plastics
(Class II also)
Locations where inhalation anesthetics are used
Utility gas plants and operations involving storage and handling and liquefied petroleum and natural gas
Aircraft hangars and fuel servicing areas
Typical Class II Locations Include:
Grain elevators and bulk handling facilities
Magnesium and aluminum powder manufacture and storage facilities
Starch manufacture and storage facilities
Fireworks manufacture and storage facilities
Flour and feed mills
Pulverized sugar and cocoa packaging and handling areas
Coal preparation and handling facilities
Spice grinding plants
Confectionery manufacturing plants
Typical Class III Locations Include:
Cotton gins and cotton seed mills
Flax producing plants
Explosive Chemical Property Considerations for Hazardous Location Type
Certain chemicals may have characteristics that require safeguards beyond those required for any of the certified atmospheric groups. Carbon disulfide is an example of such a chemical because of its low ignition temperature and the small joint clearance to arrest its flame propagation.
Certain metal dust may have characteristics that require safeguards beyond those required for atmospheres containing the dust of aluminum, magnesium and their commercial alloys. For example, zirconium, thorium and uranium dust have extremely low ignition temperatures and minimal ignition energies lower than any material classified in any of the Class I or Class II groups.
Combustible dust which is electrically nonconductive is produced in the handling and processing of grain, grain products, pulverized sugar, cocoa, dried egg and milk powders, pulverized spices, starches and pastes, potato and wood flour, oil meal from beans and seed, dried hay and other organic materials that may produce combustible dust when processed or handled. Electrically conductive dust is dust with a resistivity less than 105 ohm-centimeter. Dust containing magnesium or aluminum is particularly hazardous and extreme caution is necessary to avoid an ignition and resulting explosion. Explosion severity is the measure of maximum explosion pressure and maximum rate of pressure rise. It is a measure of how violent the ensuing explosion will be. Closely associated with maximum pressure and rate of pressure rise is the length of time pressure is exerted on the surroundings. All of these factors contribute to the total impulse, rather than the force exerted at any one moment, that determines the destructiveness of an explosion. This explains, in part, why dust explosions, which have slower rates of pressure rise, may be more destructive than gas explosions.
Dust that is carbonized or excessively dry is highly susceptible to spontaneous ignition. Equipment and wiring of the type defined in Article 100 of the National Electric Code® as "explosion proof" shall not be required and shall not be acceptable in Class II locations unless approved for such locations.
Complete information relative to the use and/or manufacture of applicable chemicals and their explosive properties as pertaining to air ventilation can be obtained from the National Fire Protection Association.
Spark Proof Construction Considerations for Hazardous Location Fans
The Hazardous Location fans incorporate spark proof construction in its design to minimize the potential for a random spark being the ignition source of an explosion in a hazardous location. All major components are fabricated from materials that will not produce a spark when struck by a direct impact blow i.e. aluminum propeller or blade and explosion proof motors.
As an illustrative example, consider the potential consequence if a tool is dropped and strikes the blade or propeller of an exhaust / ventilation fan in use. With in a non-hazardous atmosphere, the risk for an explosion is minimal. However, if the same tool strikes the exposed, steel surfaces of a fan operating in a hazardous location containing gasoline fumes, the results could be drastically different. The blow produced by the tool could create a random spark. In the case of gasoline fumes, the addition of a proper air and gasoline fume mixture could result in an explosion. Minimizing this potential for a spark ignition is also an important reason to use spark proof tools and related equipment in hazardous locations. By incorporating basic spark proof blade / motor construction, the potential for a spark-ignited explosion caused by a direct blow is greatly minimized. The use of an explosion proof motor alone, or an aluminum blade / propeller, does not qualify the fan as "explosive proof". IMPORTANT! No air exhaust / ventilation fan is totally explosion proof. Only a product utilizing proper design and certification standards can minimize the risk of an explosion in an applicable hazardous atmosphere.