Chapter 11: Space Air DiffusionConditioned air is normally supplied to air outlets at velocities much higher than would be acceptable in the occupied space.

•How the supply air is delivered to or returned from a conditioned space through air outlets or return grilles.•To assure temperature, humidities, and air velocities (noise level) within the occupied space are held at acceptable conditions.•To assure that there is reasonable uniformity of temperature throughout the occupied space. •The fans/ducts that distribute conditioned air is covered by Chapter 12. This chapter emphasizes the air outlets (diffusers) or grilles as well as the mixing of the conditioned air with the room air.

 Behavior of Jets

A jet having a higher temperature than the space temperature is being discharged into the space. Room air is entrained by the jet and mixed with the jet.

 Behavior of Jets

Behavior of Jets

• Buoyant forces cause the jet to rise when the air is warmer than the space air.•Buoyant forces cause the jet to drop when the air is cooler than the space air.•The throw is the distance from the outlet to where the maximum velocity in the jet has decreased to some specified value such as 50, 100, or 150 ft/min (0.25, 0.5, or 0.75 m/s).•The buoyant forces result in shorter throw for jet velocities less than 150 ft/min.•A surface less than about one foot from the jet may increase throw for outlets and decrease the drop for horizontal jets.

Room Air Motion

Basic Air Motion

Basic Air Motion

Group C: Good for heating applications, but not as good for cooling due to the low throw of the spreading jet.

Basic Air Motion

Group B: Good for cooling due to the large throw of the non-spreading jet, but not as good for heating due to the too large throw for heating that causes a larger stratified zone in the occupied space.

Basic Air Motion

Group A: Good for cooling, very popular in commercial applications due to large entrainment rate, but not as good for heating. The jet has difficulty to come down and mix with the room air.

Jet issued from high side wall.

Jet issued from the ceiling with horizontal spreading .

Basic Air Motion

Group D: Good for special applications such as displacement ventilation, which is often used to remove contaminants from a space.

Basic Air Motion

Group E: Jet issued from the ceiling vertically without spreading. Good for special applications such as cooling large glass areas.

Noise- Noise Criterion (NC)

In general, levels below a NC of 30 are considered to be quiet, whereas levels above a NC of 50 are considered noisy.

Air-Distribution System Design

thq Heat transfer from a human body is affected by both temperature difference and air velocity as air velocity will affect heat transfer coefficient h significantly. When the temperature or velocity is significantly not uniform within a conditioned space, an acceptable average temperature or velocity may not be acceptable locally.

Air-Distribution System Design

Air Distribution Performance Index (ADPI)

•Research indicates that a high percentage of people in sedentary occupations are comfortable where the effective draft temperature is between -3F (-1.7 C) and +2 F(1.1 C) and the air velocity is less than 70 ft/min (0.36 m/s).

•The ADPI is defined as the percentage of measurement taken at many locations in the occupied zone of a space that meet the -3 F to 2 F effective draft temperature criteria.

Air-Distribution System Design

General design Procedure

Characteristic Room Length 

Louvered ceiling diffuser Troffer diffuser

ADPI Selection Guide

X50 is normally considered to be the throw.

Performance Data for a Typical Linear Diffuser

For the throw, Min., Mid., and Max correspond respectively to X150 , X100, and X50

Performance Data for a Typical Round Ceiling Diffuser

Performance Data for an Adjustable-Type, High Sidewall Diffuser

Adjustable-Type, High Sidewall Diffuser (continued)

Adjustable-Type, High Sidewall Diffuser (continued)

Performance Data for one type of Return Grille

Performance Data for one type of Return Grille (continued)

Example 11.1

Wall perpendicular to the jet

Example 11.2

Example 11.3

Variable air-volume air-distribution system

Variable air-volume (VAV) air-distribution systems usually involve the use of linear or T-bar diffusers and a thermostat-controlled metering device, referred to as a VAV terminal box .

Variable Volume, Fan Powered Terminals

Because of the very low flow rates that may occur in variable volume systems, fan powered terminals are often used to maintain adequate ventilation air to the space.

Example 11.4