Bellmouth Return sizing (ceiling return air system)

[RMERMP]

can anyone teach me how to size the bellmouth if i have 20000cfm supply air? i read the size of bellmouth should be double the size of the main return duct so that the return air will have low velocity in the ceiling. thanks

 

[ZDR1985]

Air or any fluid inlet are normally designed with bell mouth to minimize aerodynamic drag or in this case a reverse nozzle effect.