Aaon Fan Engineering White Paper

Aaon has pushed the envelope of packaged rooftop unit design by providing a wide selection of fan types in their systems, including forward curved housed centrifugals, backward inclined un-housed centrifugals (plenum fans) and axial fans. And, for their larger units, they have generally moved towards direct-driven plenum fans for supply air in arrays as best fits the application. This move has not been random, but is based upon solid engineering reasoning based on the inherent stability and efficiency of these types of fans. The newly published Aaon white paper, Value in the Air provides a thorough justification for this emphasis on direct-drive plenum fans. But in doing so, it also provides an excellent primer on general fan engineering topics including overloading vs. non-overloading fan curves, fan stability in single and dual fan applications, fan control for VAV systems, and the overall energy impact of fans on our HVAC systems. It is an excellent resource and one that engineers should review to enhance their understanding of fan systems.

Fan Engineering: Spark Resistance Ratings

Every once in a while we will see a specification for "explosion proof" fans. While this may be a desirable characteristic, "explosion proof" is not a specifiable option, and usually is included because of confusion with electrical component (i.e. motors, disconnects) specifications.



Instead, fans are generally classified by "spark resistance". AMCA has created a standard that defines three different levels of spark resistance, classes A, B and C.

These classes, listed in decreasing order of assurance, are generally concerned with the prevention of sparks caused by the rubbing together of spark-producing metallic components (generally ferrous materials). These classes only address spark risks due to an explosive airstream, and do not address explosive conditions outside the fan. A summary of the different levels of protection is found in this helpful engineering paper from Twin City Fans.

Type C: The fan is designed so that if the impeller or shaft comes loose and shifts during operation, two ferrous parts will not come into contact.

Type B: In general, this requires a nonferrous impeller and a nonferrous rubbing ring around the shaft hole. Also, extra locking systems are required to prevent the fan impeller, shaft, and bearings from shifting.

Type A: This requires a nonferrous airstream. Also, the extra locking systems are required as in Type B.

As with any engineering decision, the correct level of spark resistance to specify depends strongly upon the particulars of the project: The gasses or substances expected, the concentration of these contaminants, the location of the air-moving device, etc.