User:Gfuertes/Displacement Ventilation

Displacement ventilation is an air delivery strategy that uses thermal stratification by supplying cool and clean air at floor level and extracting the air above the occupied zone, usually at ceiling height.

System Design
A typical displacement ventilation system, such as one in an office space, supplies conditioned cool air from an air handling unit (AHU) through a diffuser that is placed on a wall at floor level. The cool air spreads through the floor of the space and then rises as the air warms due to heat exchange with other heat sources in the space (e.g., occupants, computers, lights). The warmer air has a lower density than the cool air, and thus creates upward convective flows known as thermal plumes. The warm air then exits the zone via an exhaust fan placed at the ceiling height of the room.

Diffuser types vary by application. Diffusers can be located against a wall (“wall-mounted”), at the corner of a room (“corner-mounted”), or above the floor but not against a wall (“free-standing”).

Displacement ventilation can be coupled with other cooling and heating sources, such as radiant chilled ceilings or baseboard heating.

History
Displacement ventilation was first applied in an industrial building in Scandinavia in 1978, and has frequently been used in similar applications, as well as office spaces, throughout Scandinavia since that time. By 1989, it was estimated that displacement ventilation comprised the 50% in industrial applications and 25% in offices within Nordic countries.

Applications in the United States have not been as widespread as in Scandinavia. Some research has been done to assess the practicality of this application in U.S. markets due to different typical space designs and application in hot and humid climates, as well as research to assess the potential indoor environmental quality and energy-saving benefits of this strategy in the U.S. and elsewhere.

Applications
Displacement ventilation is used in the Suvarnabhumi International Airport in Bangkok, Thailand and the NASA Jet Propulsion Laboratory Flight Projects Center building, among other applications.

Benefits
Indoor Air Quality: One repeatedly cited benefit of displacement ventilation is the superior indoor air quality achieved with exhausting contaminated air out of the room. Unlike mixing ventilation, displacement ventilation provides clean air to a room and removes contaminants created by heat sources in a room, resulting in an improved air quality.

Energy Savings: Studies have demonstrated that displacement ventilation may save energy as compared to standard mixing ventilation, depending on the use type of the building, design/massing/orientation, and other factors. Research regarding this topic is ongoing.

Limitations
Space Limitations: Displacement ventilation is best suited for taller spaces (higher than 3 meters [10 feet]) in which large airflows are required for air quality purposes. . Standard mixing ventilation may be better suited for smaller spaces where air quality is not as great a concern, such as single-occupant offices, and where the room height is not tall (e.g., lower than 2.3 meters [7.5 feet]).

Conditioning type: Due to the unique properties of thermal stratification, displacement ventilation is typically used for cooling rather than for heating. In many cases, a separate heating source, such as a radiator or baseboard, is used in during heating periods.

Thermal comfort: Displacement ventilation can be a cause of discomfort due to large vertical temperature differences and drafts. There is a tradeoff inherent in these two issues: by increasing the flow rate (and the ability to remove greater thermal loads), the vertical temperature gradient can be reduced, but this could increase the risk of drafts. Pairing displacement ventilation with radiant chilled ceilings is an effort to mitigate this problem.

Contaminants: While a benefit of displacement ventilation is that air quality improves because contaminants in the air are able to leave the room, this assumes that all contaminants are produced by heat sources. However, when chilled ceilings are used, there is a risk of higher contaminant concentration in the occupied zone because of possible down flow driven by walls or panels.

Design Guidelines
Different guidelines have been published to provide guidance on designing displacement ventilation systems, including:
 * REHVA. (2002) Displacement Ventilation in Non-Industrial Premises. Federation of European Heating and Air-conditioning Associations.
 * Chen, Q. and Glicksman, L. (1999) Performance Evaluation and Development of Design Guidelines for Displacement Ventilation. Building Technology Program, Massachusetts Institute of Technology. Cambridge, MA.
 * Skistad, H. 1994. Displacement ventilation. Research Studies Press, John Wiley & Sons, Ltd., west Sussex. UK.

Research
A number of researchers have studied the effects of displacement ventilation in spaces. Mundt has focused on air quality, contaminants and convection plumes in displacement ventilation scenarios. Nielsen has studied the temperature gradients and distribution in displacement ventilation applications. Livchak and Nall have studied the possibility of displacement ventilation in hot and humid climates. Loveday et al have researched the issue of combining displacement ventilation systems with chilled ceilings. Melikov et al have conducted a field evaluation of displacement ventilation.