How to Size a Mixing Box/Plenum?

[CuriouslyGeorge]

I require 525 cfm OA to mix with 2350 cfm RA before entering the AHU. I assume I need a plenum to make this possible where the OA is coming in perpendicular to the RA. Should the OA and RA ducts meet in the same plane, or is it best to let one enter the plenum first and then the other lower down...as the return duct drops down from the ceiling to head into the AHU inlet running horizontally 1.5 fan diameters before entering the AHU, correct?

Thanks for your help!
- C.George

 

[imok2]

George, If I could I would connect OSA and Return air duct on the same plane but the most important thing to consider is that you get a proper mixed air temperature AND keep the pressure drop to a minimum total return air drop at no more then .08 to 1.0 per 100 ft

 

[KenRad]

imok2 is correct about the importance of mixing.

Especially if you are using cold outside air (<40F) for free cooling in the winter, it is important to have proper mixing before the air hits a coil. I don't install this kind of air handler anymore without some kind of mixing device (air blender, etc.) between the plenum and the filter bank. Otherwise, you will either have to deal with frequent trips of the freeze protection device, or a frozen coil due to stratification.

It also helps to use parallel dampers for OA and RA, and orient them in a way that promotes mixing in the plenum.

---KenRad