The placement and location of ceiling registers (diffusers) for the introduction of replacement air can have a direct impact on the proper operation of the exhaust hood. Diffusers that have been designed for commercial spaces, such as “four way diffusers” were developed to have a high induction capability. That means they discharge at a relatively high velocity to entrain and “mix” room air with supply air creating a uniform temperature throughout the space. While this approach is ideal for most occupied spaces, it can and does have a detrimental impact on the performance of an exhaust hood.
The reason for that is that if placed near the face of an exhaust hood, this high velocity discharge will have a “waterfall” effect on the face of the hood. This air will then tend to “pull” the smoke and heat, which is rising, back down with it and spill into the space.
To prevent this from happening, traditional four way ceiling registers (diffusers) or other high velocity induction registers should not be used in proximity to the hood. There are a number of alternatives that have been developed suitable for supply air introduction near exhaust hoods. The key is not to exceed 70 FPM at the face of the hood. Look at the velocity profile of any selected supply air device and confirm it does not exceed this threshold based on the installation distance from the hood.
There are perforated supply plenums, specially designed 2×2 and 2×4 ceiling registers (diffusers) as well, along with fabric “sock” type that may be suitable for installation.