At some point during an engineer's career, he may be confronted with an application which involves an airstream containing one or more potentially explosive components.  Now, since we know that there are electrical enclosures which are rated as explosion-proof, and there are motors which are rated as explosion-proof, it is natural to think that it logically follows that there are also fans which are rated as explosion-proof.  But THERE IS NO SUCH THING AS AN EXPLOSION-PROOF FAN.  There are several reasons for this. 

A potentially explosive airstream is always in contact with both the interior of the fan housing (or fan guard), and with the fan impeller.  An explosion-proof motor is designed such that the interior of the motor is isolated, usually by shaft seals, from contact with the airstream outside.

Since the potentially explosive airstream cannot be isolated from contact with fan components, other methods are necessary to minimize if not eliminate the possibility of igniting the airstream.  

AMCA Standard 99-0401 Classifications of Spark Resistant Construction was developed to address the question of spark resistant construction, and to give fan purchasers and fan manufacturers a number of options to choose from in the construction of such fans. 

Consider the ways in which such ignition might occur as the airstream flows through the fan.  Generally, the source of ignition for a contained, potentially explosive airstream is a spark.  Sparks may originate in one of two ways.

A static discharge can cause ignition.  All that is required is a high enough potential.  AMCA 99-0401 provides recommendations for the prevention and possible elimination of the likelihood of such discharge through the use of devices which collect static build-up and drain it to ground, rendering it harmless. 

A friction spark can cause ignition.  In a fan, the source of the friction spark would be the contact of two components which are capable of producing a friction spark.  Principally, this might be contact between the rotating impeller and some fixed component of the fan.  

There are many ways of reducing the possibility of a friction spark between fan components.  We say "reducing" the possibility of contact because the fan impeller rotates during operation, and there are several ways in which a fixed component of the fan might come into contact with the rotating impeller.  If the impeller has a close running clearance, a severe out of balance condition might result in metal-to-metal contact.  Likewise, a fan housing is generally designed to withstand internal pressure generated by the fan impeller, but the housing is not usually designed to withstand serious external forces resulting from accidental impact, such as might result from a dropped crate or a fork lift accident.  It should be noted that such construction is possible, should the application warrant the additional cost. 

Quite often, a high degree of spark resistance can be obtained through the careful selection of materials used for constructing the fan impeller and housing components.  Since the generation of a spark requires that at least one material contain iron, one approach is to use a material containing no iron.  Such a material might be used for construction of the fixed components of a fan, such as the housing and inlet.  The impeller, however, requires a strong material for structural integrity at high speed.  The use of a material which contains little or no iron might require that the impeller operate at a lower speed than is needed to generate the required pressure, so sometimes a low or no-iron material is not a good choice for the impeller.  Some copper alloys are quite suitable for housing construction and may be "teamed" with a material having the strength suitable for the impeller's mechanical requirements. 

In addition to the trade-off of strength for spark resistance, there are a few surprises:  Some non-ferrous alloys oxidize in service and become capable of generating a spark by friction.  Other non-ferrous materials are prone to build-up and discharge of dangerous static electricity.  The choice of materials is one which should always be made with very great care.