by / Norm Grusnick, P. Eng
Unlike typical office spaces where diffusers are spaced to promote even mixing and a uniform temperature profile throughout the room, laboratories need to supply large volumes of air at low velocities to disperse impurities and make up for exhaust air volumes without interfering with fume hood function. The airflow pattern is designed to produce a uniform pattern to prevent dead spots where contaminants can linger. Projecting into the space is necessary for the high degree of control offered by this configuration.
The most serious application problem to be considered is a high sensitivity to inlet conditions. These diffusers employ a deep back pan and internal baffles to minimize this effect. Also the radial face allows for a true radial pattern. This is required if entrainment is to be avoided. In the past, all available flush types of diffusers have a multi-jet, rather than a radial displacement, pattern. This results in induction, potential contaminant injection, and uneven room air velocities and temperatures.
All diffusers (including radial flow diffusers) have operational limitations. Radial flow diffusers are limited in terms of room-diffuser delta-T and max/min air flow rates. The radial flow diffuser with the curved blades and round shape has the broadest range of performance in terms of both flow and delta-T and its ability to maintain a radial flow pattern over a range of flows. Flush face designs are often more limited due to the compromises made in attempting to achieve radial flow from a flat surface.
The interaction of supply and induced airflows at the surface of a perforated metal face is very complex. A jet of air emerging from a hole in a flat surface tends to entrain air from the surrounding space. Once the jet becomes even slightly less than perpendicular to the surface, it tends to follow along the surface. This is due to the fact that any small orifice jet has negative static pressure. When near a surface, it tends to adhere to that surface. The radial flow diffuser maintains many jets, all perpendicular to the curved surface.
Why is radial flow important? Again, the goal is to minimize induction. When there is induction, the jet entrains contaminants from surrounding areas and changes in temperature. A true radial pattern will have nearly constant temperatures and will not contain any particles from surrounding spaces.
In conclusion, the radial flow diffuser offers the least compromise, most stable air delivery, and lowest entrained contaminants of any design assuring optimal performance for laboratories with VAV hoods and other critical environments. The radial flow ceiling diffuser system allows greater flexibility during installation by being the least sensitive design to inlet conditions.
