Selection of Diffusers and Grilles

• Type and style
• Function
• Air-volume requirement
• Throw requirement
• Pressure requirement
• Sound requirement

Generally, air volumes for internal zones of building spaces vary from 1 to 3 cfm per square foot of floor area. Exterior zones require higher air volumes of 2.5 to 4 cfm per square foot. In some cases, only the heating or cooling load of the exterior wall panel or glass surface is to be carried by the distribution center. Then, the air volume per linear foot of diffuser or grille will vary from 20 to 200 cfm, depending on heat-transfer coefficient, wall height, and infiltration rate.

Throw and occupied-area air location are closely related. Both could be considered in the analysis of specific area requirements. The minimum-maximum throw for a given condition of aeration is based upon a terminal velocity at that distance from the diffuser. The residual room velocity is a function of throw to terminal velocity. Throw values are based on terminal velocities ranging from 75 to 150 fpm with corresponding residual room velocities of 75 to 150 fpm. The diffuser or grille location, together with the air pattern selected, should generally direct the air path above the occupied zone. The air path then induces room air along its throw as it expands in cross-section. This equalizes temperature and velocity within the stream. With the throw terminating in a partition or wall surface, the mixed air path further dissipates energy.

Ceiling-mounted grilles and diffusers are recommended for vertical down patterns. Some locations in the room may need to be cooler than others. Also, some room locations may be harder to condition because of air-flow problems. These grilles are used in areas adjacent to perimeter wall locations that require localized spot conditioning. Ceiling heights of 12 feet or greater are needed. The throw for vertical projection is greatly affected by supply air temperature and proximity of wall surfaces.

Sidewall-mounted diffusers and grilles have horizontal values based on a ceiling height of 8 to l0 feet. The diffuser or grille is mounted approximately 1 foot below the ceiling. For a given throw, the room air motion will increase or decrease inversely with the ceiling height. For a given air-pattern setting and room air motion, the listed minimum-maximum throw value can be decreased by l foot for each 1-foot increase in ceiling height above 10 feet. Throw values are furnished by the manufacturer. When sidewall grilles are installed remote from the ceiling (more than 3 feet away), reduce rated throw values by 20 percent.

Sill-mounted diffusers or grilles have throw values based on an 8- to 10-foot ceiling height with the outlet installed in the top of a 30-inch-high sill. For a given throw, the room air motion will change with the ceiling height. For a given air-pattern setting and room air motion, the listed minimum-maximum throw value can be decreased by 2 feet for each 1-foot increase in ceiling height above 10 feet. Decrease 1 foot for each 1-foot decrease in sill height.

The minimum throw results in a room air motion higher than that obtained when utilizing the maximum throw. Thus, 50 fpm rather than 35 fpm is the air motion. The listed minimum throw indicates the minimum distance recommended. The minimum distance is from the diffuser to a wall or major obstruction, such as a structural beam. The listed maximum throw is the recommended maximum distance to a wall or major obstruction. Throw values and the occupied area velocity for sidewall grilles and ceiling diffusers are based on flush ceiling construction, providing an unobstructed air-stream path. The listed maximum throw times 1.3 is the complete throw of the air stream where the terminal velocity equals the room air velocity. Rated occupied-area velocities range from 25 to 35 fpm for maximum listed throws and 35 to 50 fpm for minimum listed throws.

Cooled air drop or heated-air rise are of practical significance when supplying heated or cooled air from a sidewall grille. If the throw is such that the air stream prematurely enters the occupied zone, considerable draft may be experienced. This is due to incomplete mixing. The total airdrop must be considered when the wall grille is located a distance from the ceiling. Cooled air drop is controlled by spacing the wall grille from the ceiling and adjusting the grilles upward 15 inches. Heated-air rise contributes significantly to temperature stratification in the upper part of the room.

The minimum separation between grille and ceiling must be 2 feet or more. The minimum mounting separation must also be 2 feet or more. The minimum mounting height should be 7 feet. The diffuser or grille minimum pressure for a given air volume is reflected in ultimate system fan horsepower requirements. A diffuser or grille with a lower pressure rating requires less total energy than a unit with a higher pressure rating for a given air volume and effective area. Diffusers and grilles of a given size with lower pressure ratings usually have a lower sound level rating at a specified air volume.