Ask The HvacMan
Air Grilles
Air Diffusers
Air Quailty
All Air Systems
All Water Systems
Building .Managament Systems ..BMS
Cooling Towers
Cooling Load Calculation
Energy Saving
Duct ,Smacna
Dampers ,Air
Dust Collection
Fire Dampers
Glass Selection
Heat Exchangers,water
Heat Recovery
Heat Tracing Systems
Hepa Filters
Hvac Applications
Humidifiers / Dehumidifiers
Insulation , Duct
Insulation , Pipe
Insulation , Sound
Nano Tech.,In Building
Occupancy Sensors
Pneumatic Conveying
Pool Ventilation
Process Piping
Radiant Heating
Refrigerant Systems
Solar Collectors
Steam Generation
Tables & Charts Gnr.
VAV Sytems
VRV Systems
Solar Collectors
Flat Plate Collectors
Evacuated Tube Collectors
Concentrating Collectors
Transpired Collectors
Solar Control Systems
Standalone Systems
Grid Connected Systems
Hybrid Systems
Back-up Systems
Solar Cells
Solar Arrays
Change Controller
Hybrid Systems
Grid Systems
Water Pumping
Using Wind Energy
Enviromental Aspects
Buyer's Guide
Save Energy
Solar Water Heating
Solar Electric Systems
Wind Turbines
Passive Solar Heating
Passive Solar Cooling
Building Material
Water Conservation
Ground Source Heat-Pumps
Green Hotels

Glass &Windows Selection

Air Diffuser Selection

Surface Effect

An airstream moving adjacent to, or in contact with, a wall or ceiling surface creates a low-pressure area immediately adjacent to that surface, causing the air to remain in contact with the surface substantially throughout the length of throw. This surface effect, commonly referred to as the Coanda effect, counteracts the drop of a horizontally projected cool airstream.

Ceiling diffusers exhibit a surface effect to a high degree because a circular air pattern blankets the entire ceiling area surrounding each outlet. This effect diminishes with a directional discharge that does not blanket the full ceiling surface surrounding the outlet. Linear diffusers, which discharge the airstream in a single direction across the ceiling, exhibit less surface effect than radial pattern discharge; however, the effect is greater with longer diffusers and with diffusers that have spread accessories at the outlet face. Sidewall grilles exhibit varying degrees of surface effect, depending on the spread of the particular air pattern and the proximity and angle of airstream approach to the ceiling.

In many installations, the outlets must be mounted on an exposed duct and discharge the airstream into free space. In this case, the airstream entrains air on both its upper and lower surfaces. As a result, a higher rate of entrainment is obtained and the throw is shortened by 30% of the equivalent throw along a surface. Because there is no surface effect with diffusers installed on exposed ducts, with a cooling differential the supply air drops more rapidly toward the floor unless the outlet surface provides an upward deflection to the discharge stream. Conical and louver face diffusers with radial patterns normally exhibit this upward deflection. Linear and flush perforated face diffusers normally do not possess this characteristic.

Temperature Differential

Heated, horizontally projected air rises and then falls as it cools. Downward projection of cooling air or upward projection of heated air increases with an increase in the temperature difference. Similarly, downward projection of heated air and upward projection of cool air decreases with an increase in the temperature difference.

Low-temperature supply air (i.e., in the range of 38 to 45°F) requires special attention to the environment in which the diffusing and distribution equipment is operating. Cold starts in a saturated environment will cause condensation in all but specially treated equipment. Ramp starts, with a gradual decrease of the relative humidity of the indoor environment, avoid condensation except with an unusually high internal load or high infiltration.

Sound Level

The sound level from an outlet is a function of its discharge velocity and the transmission of system noise. For a given air capacity, a larger outlet has a lower discharge velocity and corresponding lower generated sound. A larger outlet also allows a higher level of sound to pass through the outlet, which may appear as outlet-generated sound. High-frequency sound can be the result of excessive outlet velocity but may also be generated in the duct by the moving airstream. Low-pitched sounds are generally mechanical equipment sound and/or terminal box or balancing damper sound transmitted through the duct and outlet to the room.

The cause of the sound can usually be pinpointed as outlet or system sounds by removing the outlet core during operation. If the sound remains essentially unchanged, the system is at fault. If the sound is significantly reduced, it may be caused by a highly irregular velocity profile at the entrance to the diffuser. The velocity profile should be measured. If the velocity varies less than 10% in the air outlet entrance neck, the outlet is causing the noise. If the velocity profile at the entrance indicates peak velocities significantly higher than average, check the manufacturer’s data for the sound at
the peak velocity. If this rating approximates the observed sound, the velocity profile in the duct must be corrected to achieve design performance. Note that a high-velocity free stream jet does not cause a high sound level until the jet impinges against an interfering surface or edge.


Smudging is the deposition of dirt particles on the air outlet or surface that is contiguous with the outlet. Dirt particles may be either in the room air that is entrained in the discharge or in the air supply to the outlet. Smudging is more prevalent with ceiling diffusers and linear diffusers that discharge the air parallel to the mounting surface than with grilles that discharge air perpendicular to the surface.

Dirt from room air is deposited most frequently at the edge of the stream, where the entrained air comes in contact with the surface, rather than at the center of the stream, which tends to wipe the surface with clean supply air. Edges of the stream occur at interruptions in the discharge stream, such as at a blank section of a linear diffuser or at the corner of a directional rectangular diffuser.



  Discuss on the Message Board
Legionnare Disease
Energy Saving
Control Software
Hotel Design Books

Hotel Design