The BF is a wider circular flange, with two sealing lips, entering deeper into the duct for more flexibility.
The duct ends do not need to be precisely cut.
Can act as a telesope fitting.
Assembly thanks to SR Joining Clips.
Manufactured in Germany.
Screws are less recommended, as they protrude inside the duct, hindering the air-flow and the cleaning operations, and could be the cause of leakages. Spot-welding should not be used.
The metal lip of the flange automatically seals the connection. When correctly installed the air-tightness class C (DIN EN 12237) or ATC 3 (DIN EN 16798-3) are reached. The two sealing lips do not necessarily mean that the BF flange is air-tighter. The BF flange has comparable leakage rates as the AF flange. See also ZI-702.
To reach the air-tightness class C no additional measures, such as the use of adhesive tapes, sealants or mastic, are necessary. The necessary SR Joining Clips must be ordered with gasket.
The flanges stiffen the ducts and help maintaining their round shape. They also improve the resistance of the ducts under negative pressures. Ducts should nevertheless be strong enough by themselves and resist the required pressures (+ or -) without taking the flanges into account.
The flange is automatically centered in the duct. No gaps are formed between the two ends of the BF flange, since these are welded together.
More expensive than the other flange types.
Unsuitable for fittings with short ends.
The flanges can be installed on ducts with stiffening ribs. In these cases, notches cannot be used. It is recommended to flatten the stiffening beads at the ends of the ducts and to fix the flanges with rivets and/or screws. Sealing measures (sealants, mastic) are required to reach the air-tightness class C.