Improving air quality through stratified air ventilation
October 5, 2010
Courtesy of NRC-CNRC
Supplying fresh air directly into a room, at a temperature slightly lower than room temperature, can save energy and improve air quality. These are key reasons why stratified air ventilation systems are being installed in buildings across Canada.
Stratified ventilation systems take a fundamentally different approach from those found in the majority of non-residential buildings, which currently use a fully mixed and dilution approach to ventilation. Stratified air is introduced to a space close to floor level at a lower temperature than the set point. The air is then heated by occupants and equipment and the upward movement of the warming air removes contaminants from the breathing zone and exhausts them at ceiling level. This creates a non-uniform environment in terms of temperature and pollutant distribution, but acceptable conditions at the breathing zone.
Previous research has shown that this type of system works well for regions where buildings require year-round cooling. There are also a growing number of buildings using this approach in Canada, where buildings require heating during winter months. Providing supplemental heating can destroy the stratified conditions causing the ventilation air to rise to ceiling level, which affects the conditions experienced by the occupants.
To evaluate the effect of supplemental heating, the NRC Institute for Research in Construction is conducting field studies in existing buildings with stratified ventilation systems, along with detailed studies in the Indoor Environment Research Facility or IERF.
Two field studies have been completed to date. The results show that the measured contaminant removal efficiency is better than that predicted in previous studies for heating conditions. In addition, key predictors of thermal comfort (the measured vertical air temperature difference and draft rating at ankle and head height) are also generally within limits set by ASHRAE standards. (ASHRAE develops standards for both its members and others professionally concerned with the design and maintenance of indoor environments.) However, the design of these systems varies from existing best practice guidelines of ASHRAE and REHVA (Federation of European Heating and Air-conditioning Associations) and the full benefits of a stratified ventilation system may not be fully realized. Control issues as well as localized thermal discomfort have been identified in the field.
To better understand the control of stratified ventilation systems and the interaction between core areas near the middle of the building and perimeter areas next to windows, the IERF has been retrofitted with a stratified ventilation system. This will allow researchers to examine the air flow patterns and thermal environment in detail over the 2010/11 heating season. Once the physical experiments are completed, human studies experiments will be conducted to evaluate the response of occupants to the stratified ventilation system compared to an overhead system.
For more information, contact Iain Macdonald at 613-993-9676 or iain.macdonald@nrc-cnrc.gc.ca.
*This story originally appeared in the September issue of Construction Innovation and is reprinted with permission from the NRC Institute for Research in Construction. For a free subscription to the newsletter, register at www.nrc-cnrc.gc.ca/irc/ci
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