![]() ![]() The sound level from an outlet is largely a function of its discharge velocity and transmission of system noise. More information on jet behavior can be found in Chapter 20 of the 2017 ASHRAE Handbook-Fundamentals. As a result, the rate of entrainment is higher and the throw is shortened. When outlets are mounted such that they discharge into a free space not near an adjacent surface (e.g., exposed duct-mounted grille applications), the airstream entrains air around the entire perimeter of the jet. Sidewall outlets exhibit varying degrees of Coanda effect, depending on the spread of the particular air pattern and the proximity and angle of airstream approach to the surface. This effect diminishes with a directional discharge that does not blanket the full ceiling surface surrounding the outlet. Round and four-way horizontal-throw ceiling outlets exhibit a high Coanda effect because the discharge air pattern blankets the ceiling area surrounding each outlet. This Coanda effect, also referred to as the surface or ceiling effect, counteracts the drop of a horizontally projected cool airstream. Some general characteristics include the following:Īn airstream moving adjacent to or in contact with a wall or ceiling 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. Outlet locations and patterns also affect a jet’s throw, entrainment, and temperature equalization capabilities. Outlets should be sized to project air so that its velocity and temperature reach acceptable levels before entering the occupied zone. As the two air masses mix, the temperature of the jet approaches the room air temperature (Rock and Zhu 2002). The momentum of the jet remains constant, so velocity decreases as the mass increases. This entrained air increases the total air in the jet stream. Supply airflow from an outlet entrains room air into the jet. Accessories may also be necessary for proper air distribution in a space, so they must be selected and used according to the manufacturers’ recommendations. For example, an outlet cannot discharge air properly and uniformly unless the air enters it in a straight and uniform manner. In fully mixed systems, supply air outlets, properly sized and located, control the air pattern to obtain proper air mixing and temperature equalization in the space.Īccessories used with an outlet regulate the volume of supply air and control its flow pattern. Review the following publications when selecting systems and equipment for room air distribution: Additional design guidance is also provided in Bauman (2003).įigure 2 illustrates the spectrum between the two extremes (fully mixed and fully stratified) of room air distribution strategies. Because task/ambient distribution requires a high level of individual control, it is not covered in this chapter, but is discussed in Chapter 20 of the 2017 ASHRAE Handbook-Fundamentals. Examples of task/ambient systems are personally controlled desk outlets and spot-conditioning systems. Task/ambient air distribution focuses on conditioning only a portion of the space for thermal comfort and/or process control. Most underfloor air distribution designs are examples of this type of system. Partially mixed systems provide some mixing of air in the occupied and/or process space while creating stratified conditions in the volume above. Thermal displacement ventilation is an example of this type of system. Overhead air distribution is an example of this type of system.įully stratified systems have little or no mixing of air within the occupied and/or process space. Fully mixed systems have little or no thermal stratification of air within the occupied and/or process space. ![]()
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