Why AHU manufacturers moved to plug fans, and when a housed fan is still the better answer.
Plug fan (RLM, plenum fan)
A backward curved impeller on a direct-drive motor, mounted on an inlet cone and support frame without a scroll. Air discharges radially into the unit plenum. Advantages: short installation length, no belt drive, easy fan-wall arrays with EC motors for redundancy and turndown, simple maintenance. Efficiency depends on the plenum design; keep the inlet cone free and the discharge unobstructed.
Housed centrifugal fan
Impeller in a scroll housing with a defined outlet. Recovers velocity pressure in the scroll, so it is the better choice when the fan discharges directly into a duct at high pressure or when the plenum is too small for a plug fan. Belt-driven housed fans (AT, ADH, RDH series) remain standard in large commercial units.
Selection notes
Plug fans are specified by static pressure at the unit outlet; housed fans by total or static pressure at the duct connection. The categories on this site — Plug Fans and Commercial Centrifugal Blowers — carry the matching curves.