This section offers some general criteria for the selection of supply-air diffusers. One application has a ceiling diffuser supplied from a branch duct by a short segment of flex duct. The best designs incorporate a volume damper into the takeoff from the branch duct to balance the airflow. A less attractive solution is to use the adjustable-volume damper integrated in the diffuser itself. This solution results in more noise, however.
It’s helpful to have a short segment of duct off the main branch, before the diffuser, with a properly-sized duct to help limit the airflow to the diffuser. This modulates the airflow more efficiently, is quieter than volume dampers, and makes the entire system easier to balance.
The supply diffusers are placed to provide a uniform flow of air in a space. In large spaces with glass exposure, it’s good practice to situate a diffuser near the glass surface, to prevent the cascading of air across the surface and to limit the convection from the glass itself. The manufacturer’s data sheets are an excellent source of information on the airflow characteristics of diffusers.
The principal criterion is air quality at or under the design NC (noise criteria) rating. Limiting the selection process to the NC-30 level, diffusers can be selected to provide the necessary throw distance from the source. This will ensure thorough distribution of air in the space.
The single constant to the selection process is to maintain an airflow rate of 25 to 70 ft/min, averaging about 50 ft/min. Any lesser volume flow will be uncomfortable to the occupants and should be avoided. Keeping this in mind, and knowing the number of diffusers needed to achieve a uniform air distribution, an outlet type can be chosen. Architectural and space considerations will also affect the selection process. Many types of diffusers are available on the market today, with a wide range of performance characteristics.
Exhaust and return grilles should be located away from the supply inlets, so the air is not taken from the space before it has a chance to circulate. The return-air path can even be used to accentuate the movement of air in the space or to draw the air toward a stagnant zone.
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Excerpt from Energy Conservation Projects.