Outdoor Air Intakes
AIR DISTRIBUTION DESIGN
1. The selection and placement of supply air outlets are critical to the comfort in the space. Select the type of air distribution outlets and their location to maximize “fully mixed” air patterns to obtain temperature equalization in the served space. Three air distribution variables must be considered:
Throw is the distance from the center of the outlet face to a point where the velocity of the supply air stream is reduced to a specified velocity, usually 100 fpm for slot outlets or 50 fpm for diffusers and grilles.
Drop of a cool supply air steam 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.
Spread is defined as the divergence of the supply air stream in a horizontal or vertical plane and is a function of outlet geometry.
Supply air distribution for each space must be configured to maximize the supply air pattern “coverage” considering these three variables.
2. If air is to be used for both heating and cooling, a separate perimeter air system with vertical upward discharge would be ideal. However, if both heating and cooling is provided by overhead air diffusers (more typical), there are several precautions in order:
a. Supply outlets should be located only a few feet from the outside wall and the discharge should be directed toward the wall.
b. Outlets should be selected to reach the wall when heating, but not so great a velocity provided to cause excessive drafts when cooling.
c. Several outlets, especially space linear or slot diffusers, should be used to allow even distribution of warm air over the wall.
d. The heating supply air temperature should not exceed 95°F in order reduce the buoyant force of the air and ensure airflow to the floor.
e. The maximum recommended distance between linear slot diffusers is three diffuser lengths. The maximum recommended distance for diffusers from an exterior wall, with parallel flow to the wall is two diffuser lengths. Avoid placing supply air linear slot diffusers that allow airflows to collide at right angles.
3. Select supply outlets for an Air Diffusion Performance Index (ADPI) of at least 80 at maximum design airflow. For this duty, the following types of air outlets are recommended:
Location / Type / Required Typical Outlet Throw(feet)a / Location / Type / Required Typical Outlet Throw
(feet)a
Sidewallb / Adjustable (airfoil) blade / 1.6 x L / Overhead / Round or square louver face / 0.8 x L (round)
2.0 x L (square)
Linear bar / 1.6 x L / Round or square perforated facec / 2.0 x L
Nozzle (eye ball or drum louver) / 2.0 x L / Plaque face / 0.8 x L
Floor/Sill / Linear bar / 1.3 x L / Linear slot / 1.0 x L
Swirl / 0.7 x L / Light troffer / 1.0 x L
Air dispersion duct / 2.0 x L
a. Where L is the “space characteristic length” defined as the distance between a supply outlet and the most remote wall, ceiling, or floor (as applicable) or half the distance between supply devices. Increase L for overhead outlets by 1 foot for each foot of ceiling height above 9 feet.
b. Sidewall supply outlets should be mounted in the sidewall within 2 feet of the ceiling to take advantage of Coanda Effect.
c. Internal adjustable pattern blades are required.
4. For VAV applications, it is important to analyze the ADPI at both maximum and minimum airflows. For most air outlets, as flow is reduced, throw is reduced. If throw is reduced too much, ADPI is compromised and air distribution becomes poorer, maybe resulting in stagnant areas, “dumping,” etc.
Outlets with a wide T/L range are more applicable to VAV systems as they can maintain a high ADPI even when turned down to low air volume. Light troffer diffusers have the largest T/L range of all outlets, making them an excellent choice for VAV applications. Plaque face, louver face, and perforated face diffusers have T/L ranges lower than light troffer diffusers, but are still sufficiently large enough for most VAV applications. Sidewall grilles, nozzles, and drum louvers have fairly small T/L ranges and should not be applied for VAV distribution.
5. Select supply outlets for low noise levels: approximately RC 30 for normal applications, RC 20 for hospital patient rooms and other "quiet" locations, and RC 40 for industrial or non-critical applications.
6. Return air inlet location has very little effect on room air diffusion. However, return air inlets should be located a sufficient distance from the supply outlet so that short-circuiting of supply air does not occur. As feasible, locate the returns in the stagnant zone to remove unwanted warm or cool air (for cooling, a high sidewall or ceiling return will remove warm air from the space; for heating, a low sidewall return will remove
warm stagnant air).
Inlet devices for return or exhaust air duty should be selected to match adjacent air supply devices on the basis of minimum pressure loss and noise. Eggcrate and perforated face inlet devices are preferred.
7. Avoid use of fire-rated ceiling-floor or ceiling-roof assemblies. These assemblies complicate the design by requiring radiation dampers at each device and, ultimately, lead to both poor air distribution and noise problems. Over time, their effectiveness degrades as ceiling tiles are removed for maintenance and not replaced in accordance with their UL listing.
8. All balancing dampers must be located as far upstream of air distribution devices as possible and never be installed in the throat of the device.
HVAC Design Guidelines
AIR DISTRIBUTION DESIGN 2