Bifurcated fan

Unsuitable for flammable and explosive gases and vapors unless a bifurcated fan is used... 

Bifurcated fan An axial flow fan that directs the airstream around the motor, which is enclosed in a protective casing. It is used for handling corrosive, high-temperature, and explosive dusts, vapors and gases. 

When extracting hot gases (e.g. from above the inlet and outlet ends of oxidation ovens), bifurcated fans should be used to give some protection to the fan motor. Impellors must not be made of aluminum alloys, since these are readily corroded. Other fan systems can be incorporated directly in the duct using axial fans, which for better control, can be fitted with adjustable pitch blades. 

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