Adsorbent Air Cleaning: A New Way To Think About Ventilation 

Business management of buildings

Adsorbent air cleaning technology controls two of the most common contaminants in buildings: carbon dioxide and volatile organic compounds.


In recent years, facility managers have begun to rethink their approach to building ventilation. A number of factors are driving this trend: First, the building community is paying more attention to the energy costs of ventilating with large amounts of outdoor air. Second, there is growing recognition that alternatives to a high outdoor air approach may be more reliable at keeping contaminants within safe levels.

Techniques that filter or neutralize contaminants can reduce energy costs and provide equivalent or even better air quality. However, integrating these technologies into buildings can be challenging, and careful attention must be paid to how a technology interacts with a building’s HVAC system to be effective.

Adsorbent Air Purification (AAC) is a technology that controls two of the most common contaminants in buildings: carbon dioxide and volatile organic compounds (VOCs). Adsorbent air purifiers place highly porous materials, similar to activated carbon, in an air stream. As air flows through it, the contaminating molecules “stick” to the porous surfaces by electrostatic forces, while downstream clean air comes out. The system filters the air while the building is occupied. At night, the air purifiers close themselves off from the rest of the ductwork, while internal heaters raise the temperature of the adsorbent media. This process, called regeneration, loosens the contaminants from the media. Finally, an exhaust fan removes the polluted air through a special duct, making the regenerated air purifiers ready for the next day.

AAC systems are modular, which makes them easy to adapt to the size of an HVAC system: if you need to filter more air, just install more modules. The AAC modules are installed in the highly polluted return air stream. After the filtered air has passed through the modules, it mixes with the outside air and passes through the rest of the air handling unit as usual. By removing CO2 and VOCs, operators can significantly reduce the flow of outside air – sometimes as much as 70 percent. In retrofit applications, this reduction leads to energy savings. When building additional projects, AAC modules are used to replace a larger outdoor volume, which offsets the need to install additional HVAC system capacity and can reduce a project’s investment costs.

While AAC technology can deliver significant benefits, success requires careful integration with a building’s HVAC system and a detailed understanding of how system design and operation determine energy consumption. The technology is based on the ability to accurately monitor and control the flow of outside air, which can be compromised by improperly installed or calibrated airflow monitors, stuck dampers or incorrect fan control.

Design features of some HVAC systems may limit the ability to reduce outside air; For example, buildings with large volumes of local exhaust without a dedicated make-up air system require significant outside air flows to avoid building pressure problems. A system can be operated in a way that already limits the demand for cooling or heating, leaving little scope for further reductions. For example, a building can keep its supply air temperature low in winter if it still has a significant cooling load, eliminating the need to preheat the incoming outside air. When considering air cleaning for a building, a facilities manager or designer should confirm that the HVAC system is in good working order and that the design conditions are appropriate for the success of the AAC system.

With the attention that has been paid to Covid-19 over the past year, some consideration should be given to the role AAC plays in protecting against disease transmission. While AAC does not directly remove or neutralize pathogens, it can play an indirect role by improving indoor humidity. A relative humidity between 40 and 60 percent limits the reproduction of bacteria and fungi and reduces the mechanisms of viral transmission. For example, AAC can play a supporting role in the infection control of the ventilation system.

Adsorbent air purification represents a new approach to ventilation, filtering out CO2 and VOCs instead of diluting them with large amounts of outside air. By reducing reliance on outside air, air purification saves energy for cooling and heating, while maintaining safe concentrations of contaminants. To realize the benefits, operators and designers must ensure that the HVAC system is compatible with reducing outside air and reducing cooling and/or heating demand. When properly applied, it joins a growing list of alternative options to the old outdoor-only paradigm.

Drew Morrison is an energy engineer with: slipstream. He works with building owners, architects and engineers to design buildings that achieve a high level of energy performance combined with a high level of health and comfort for the occupants.

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