Engineering Guide Air Distribution

Air Distribution guide

Proper selection of air diffusion devices requires basic knowledge of the mechanics of room air distribution. Figures 1 and 2 illustrate the interactions of the major factors influencing room air distribution.

Primary Air
Primary air is defined as the conditioned air discharged by the supply outlet. This air provides the motive force for room air motion.

Total Air
Total air is defined as the mixture of primary air and entrained room air which is under the influence of supply outlet conditions. This is commonly considered to be the air within an envelope of 50 fpm [0.25 m/s] (or
greater) velocity. The temperature difference between the total air and the room air creates buoyant effects which cause cold supply air to drop and warm air to rise.

Throw is the distance from the center of the outlet face to a point where the velocity of the air stream is reduced to a specified velocity, usually 150 [0.75], 100 [0.50] or 50 fpm [0.25 m/s] (Figure 3).
These velocities are referred to as terminal velocity and therefore indicated as T150 [T0.75], T100 [T0.50],
T50 [T0.25] respectively. Throw is primarily a function of mass flow and outlet velocity and therefore can be
reduced by decreasing either of these values.

The drop of cool total air, as shown in Figure 1, is the result of vertical spread of the air stream due to entrainment of room air, and the buoyancy effect due to the density differences between the total air package
and the surrounding primary room air. The term density is very important as drop is primarily dependent upon the mass flow of the total air. Drop can be minimized by spreading air uniformly over the ceiling surface, thus reducing the mass flow per unit surface area.

The spread of an outlet is defined as the divergence of the air stream in a horizontal or vertical plane and is a function of the outlet geometry (Figure 3).

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