Flexible joint allowed in smoke extract fans?

[capaula]

Is it allowed to use flexible joints at all connecting the fan to the ducts in smoke extraction systems?
Usually those fans are rated for 300 °C and 2 hours (UL listed in the US)

My question is if they are allowed at all, even if the joints are rated the same.

Thank you

Claudia

 

[lilliput1]

See Duro-Dyne Thermafab fire rated duct flexible connection.