The RBA are insulation damper 100% tight by air barrier, which is recommended when there is an absolute necessity to avoid any passage of gas from upstream to downstream.
When the butterflies close, one air barrier fan injects air in the space between shell and the two butterflies. The overpressure obtained in that space avoids any passage of gas.
Air barrier can, on specific demand, be heated to avoid acid condensation.
RBA M dampers with hollowed butterfly were especially designed to reduce distance between flanges.
This type of damper is especially used for insulation of cogeneration engine or turbine, of boiler and of fan.