Displacement ventilation

See also: Underfloor air distribution

Displacement ventilation is an room air distribution strategy where conditioned outdoor air is supplied at floor level and extracted above the occupied zone, usually at ceiling height.^[1]

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 low induction diffuser. The cool air spreads through the floor of the space and then rises as the air warms due to heat exchange with 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 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”).^[2] Displacement ventilation can be coupled with other cooling and heating sources, such as radiant chilled ceilings^[3] or baseboard heating^[1].

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.^[1] By 1989, it was estimated that displacement ventilation comprised the 50% in industrial applications and 25% in offices within Nordic countries.^[4] 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^[1] and application in hot and humid climates^[5], 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 has been applied in the Suvarnabhumi International Airport in Bangkok, Thailand^[6] and the NASA Jet Propulsion Laboratory Flight Projects Center building^[7], 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.^[1][2]

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.^[1] 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.^[2]. 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]).^[2]

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.^[1]

Thermal comfort: Displacement ventilation can be a cause of discomfort due to large vertical temperature differences and drafts^[8]. 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.^[1] Pairing displacement ventilation with radiant chilled ceilings is an effort to mitigate this problem.^[9]

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.^[10]

Design guidelines

Different guidelines have been published to provide guidance on designing displacement ventilation systems, including:

• Skistad H., Mundt E., Nielsen P.V., Hagstrom K., Railo J. (2002). Displacement Ventilation in Non-Industrial Premises. Federation of European Heating and Air-conditioning Associations.^[2]
• Chen, Q. and Glicksman, L. (2003). Performance Evaluation and Development of Design Guidelines for Displacement Ventilation. Atlanta: ASHRAE.^[1]
• 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.^[11][12][13] Nielsen has studied the temperature gradients and distribution in displacement ventilation applications.^[14][15] Livchak and Nall have studied the possibility of displacement ventilation in hot and humid climates.^[5] Loveday et al have researched the issue of combining displacement ventilation systems with chilled ceilings.^[9] Melikov et al have conducted a field evaluation of displacement ventilation.^[16]

References

1. ^ ^{a b c d e f g h i} 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.
2. ^ ^{a b c d e} REHVA. (2002). Displacement Ventilation in Non-Industrial Premises. Federation of European Heating and Air-conditioning Associations.
3. ^ Schiavon, Stefano; Bauman F, Tully B, and Rimmer J. (2010). "Room air stratification in combined chilled ceiling and displacement ventilation systems". Proceeding of the 7th International Conference IAQVEC2010. http://www.escholarship.org/uc/item/40r2b5xq. Retrieved 9 December 2010. 
4. ^ Svensson, A.G.L. (1989). Nordic Experiences of Displacement Ventilation Systems ASHRAE Transactions, 95(2).
5. ^ ^{a b} Livchak, A. and Nall, D. (2001). Displacement Ventilation – Application for Hot and Humid Climate. Clima 2000/Napoli 2001 World Congress - Napoli (I), 15–18 September 2001.
6. ^ "Using a Constant Volume Displacement Ventilation System to Create a Micro Climate in a Large Airport Terminal in Bangkok". repository.tamu.edu. http://repository.tamu.edu/handle/1969.1/6690. Retrieved 9 December 2010. 
7. ^ "NASA’s Out of this World Green Building - Web Exclusives - EDC Magazine". edcmag.com. http://www.edcmag.com/Articles/Web_Exclusive/BNP_GUID_9-5-2006_A_10000000000000933412. Retrieved 9 December 2010. 
8. ^ See ANSI/ASHRAE Standard 55, Thermal Environmental Conditions for Human Occupancy for further discussion of thermal comfort limits
9. ^ ^{a b} Loveday, D.L., et al (2002). "Displacement ventilation environments with chilled ceilings: thermal comfort design within the context of the BS EN ISO7730 versus adaptive debate." Energy and Buildings 34 (2002) 573–579
10. ^ Li, Y., Sandburg, M., and Fuchs, L. (1992) Vertical Temperature Profiles in Rooms by Displacement: Full-Scale Measurement and Nodal Modelling. Indoor Air: 2 (1992) 225-243.
11. ^ Mundt, E (1994). "Contamination Distribution in Displacement Ventilation -- Influence of Disturbances". Building and Environment 29 (3): 311–317. doi:10.1016/0360-1323(94)90028-0. 
12. ^ Mundt, E. (1995). "Displacement Ventilation Systems -- Convection Flows and Temperature Gradients." Building and Environment. Vol. 30, No. I, pp. 129-133, 1995
13. ^ Mundt, E. (2001). "Non-buoyant pollutant sources and particles in displacement ventilation." Building and Environment 36 (2001) 829–836
14. ^ Nielsen, P. (1995). "Vertical Temperature Distribution in a Room with Displacement Ventilation." Indoor Environmental Technology Paper No. 48, Instituttet for Bygningsteknik, Aalborg Universitet.
15. ^ Nielsen, P. (1996). "Temperature Distribution in a Displacement Ventilated Room." Indoor Environmental Technology Paper No. 67, Instituttet for Bygningsteknik, Aalborg Universitet.
16. ^ Melikov, Arsen; G. Pitchurov, K. Naydenov, G. Langkilde (June 2005). "Field study of occupants’thermal comfort in rooms with displacement ventilation". Indoor Air 15 (3): 205–214. doi:10.1111/j.1600-0668.2005.00337.x.