ASHRAE Journal - September 2011 - 44

Tc = average (control) room drybulb temperature, °F Vx = local airstream centerline velocity, fpm By determining the value of T50 from a manufacturer’s catalog, and measuring the characteristic length L from the project’s plans, the ratio can be determined and the predicted ADPI value can be estimated from Table 2.

Terminal Device High Sidewall Grilles

Room Load, T50/L for Maximum For ADPI Range of T50/L Btu/h·ft2 Maximum ADPI ADPI Greater Than 80 60 40 20 <10 80 60 40 20 80 60 40 20 80 60 40 20 80 60 40 20 60 40 20 11 to 50 1.8 1.8 1.6 1.5 1.4 0.8 0.8 0.8 0.8 1.7 1.7 1.3 0.9 0.7 0.7 0.7 0.7 0.3 0.3 0.3 0.3 2.5 1.0 1.0 2.0 68 72 78 85 90 76 83 88 93 61 72 86 95 94 94 94 94 85 88 91 92 86 92 95 96 – 70 70 80 80 70 80 80 90 60 70 80 90 90 80 – – 80 80 80 80 80 90 90 90 80 – 1.5 to 2.2 1.2 to 2.3 1.0 to 1.9 0.7 to 2.1 0.7 to 1.3 0.7 to 1.2 0.5 to 1.5 0.7 to 1.3 1.5 to 1.7 1.4 to 1.7 1.2 to 1.8 0.8 to 1.3 0.6 to 1.5 0.6 to 1.7 – – 0.3 to 0.7 0.3 to 0.8 0.3 to 1.1 0.3 to 1.5 <3.8 <3.0 <4.5 1.4 to 2.7 1.0 to 3.4

Circular Ceiling Diffusers

Location of Return/Exhaust Grilles
ASHRAE Handbook—HVAC Applications, Chapter 57, Room Air Distribution, recommends that “A return inlet affects room air motion only in its immediate vicinity. The intake should be located in the stagnant zone to return the warmest room air during cooling or the coolest room air during heating. The importance of the location depends on the relative size of the stagnant zone, which depends on the type of outlet.” Also, the return or exhaust grille does not short circuit the supply jet. In fact, if a supply jet is at or above 150 fpm (0.76 m/s) and directed over a return, the jet will either pull air out to the return or exhaust or create a “blanking effect” over the return or exhaust where no air is taken out of the space. If the return or exhaust is placed at the end of the supply jet where the velocity is 150 fpm (0.76 m/s) or less, the air being returned is at or near room temperature and the amount of air being returned is close to 5% of the total air motion in the space.
Colliding Airstream Horizontal Throw

Sill Grille, Straight Vanes

Sill Grille, Spread Vanes Ceiling Slot Diffusers (For T100/L) Light Troffer Diffusers Perforated, louvered ceiling diffusers

Table 2: Air diffusion performance index (ADPI) selection guide (from Table 4, Chapter 57 of the 2011 ASHRAE Handbook—Applications).

High Velocities in Occupied Zone
A system can be designed with proper capacities and air changes but still result in discomfort due to air greater than 50 fpm (0.25 m/s) entering the occupied zone. Designing to avoid the air-distribution problems shown in Figures 5 and 6 will greatly improve the comfort of a space.

Examples Using Throw Data
The following examples show how the throw data can be used to predict the resulting air motion in a space. In this example, the space is 20 ft × 20 ft with 9 ft (6 m × 6 m with 3 m) ceilings and the required airflow is 950 cfm (448 L/s).
Throw Distance of Jet = Horizontal + Vertical

Throw Distance of Jet = Horizontal + Vertical

Obstruction in Path of Discharge Jet Horizontal Throw

Vertical Throw

Vertical Throw

Figure 5 (left): Colliding airstreams can project a high velocity airstream into the occupied zone. This can be predicted by mapping the catalog throw data or the T50/L ratio. Figure 6 (right): Obstructions in the airstream such as architectural details and support beams can force the supply jet down into the occupied zone causing occupant discomfort.
44 ASHRAE Journal ashrae.org September 2011
ASHRAE Journal - September 2011

Table of Contents for the Digital Edition of ASHRAE Journal - September 2011
