Air Change Rate with Low Air Circulation

[Cusereef]

I figured an example is the easiest way to explain my question. A 500-sq.ft laboratory with 9-ft ceiling includes an HVAC system delivering enough air volume for an air change (AC) rate of 10 AC/hr. However, both the air intake and exhaust are located in the ceiling, 18-ft apart. In this closed room with no additional air circulation air monitoring indicates that the air change rate is less closer to the floor (0-2 ft). Is there a way to calculate the air change rate at different room heights that includes air supply and exhaust located in the ceiling? Assume the laboratory is empty with standard temp, pressure, etc.

 

[russ_watters]

It isn't possible to calculate air change rate with height. There are just too many variables. Temperature of supply air, shape of the diffusers/registers - and the profile wouldn't be continuous throughout the room (there will always be dead spots). Even just walking around in the lab will stir-up the air. Plus, in the summer, the air coming out of the diffusers is cold and drops, while in the winter, air coming out may be warm and stay at the ceiling.

Supplying low, returning high is one potential solution. Fume hoods are another.

 

[engineroom]

Assumeing you have a standard configered grille replace with a directional grille on the outlet pointed towards the closet wall, this should get the air to do a general loop about the room. It is the cheepest solution that should improve the situation. For a few hundred bucks or less is worth a try.