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Effective Air Cleaning Technologies and Methods

Jul 1, 2024 | Public | 0 comments

A significant result of the recent pandemic is a heightened awareness of the need for better control of indoor air quality (IAQ) to include the mitigation of pathogens, gas phase pollutants and particulates from the supply air stream. The industry-accepted minimum standard for ventilation in commercial buildings is ASHRAE Standard 62.1. The standard prescribes the amount of outside air ventilation, recirculated airflow and filtration necessary to maintain acceptable indoor air quality based on building use and occupancy. Outside air ventilation is an effective air cleaning method because it dilutes all items of concern in the airstream – microbes, dust particles, pollen spores, volatile organic compounds (VOC) – and exhausts them to the outside environment. It should be noted that carbon dioxide is not considered a pollutant, but instead, an indication of ventilation effectiveness.

The most common and straightforward method provided in ASHRAE 62.1 for determining the minimum amount of outside air (OA) is the Ventilation Rate Procedure (VRP). The VRP determines the necessary amount of OA based on the type of space, floor area of the zone and the design occupancy. Most locations have outdoor air that is sufficient to be used directly with nominal filtration to protect the system components from large particles.

However, there are times in some locations for which the outside air is not appropriate for ventilation of occupied spaces. An example of this would be in areas susceptible to wildfires. Another consideration in using outside air is the energy penalty for bringing in large amounts of air that must be significantly conditioned before being mixed with the supply or return air stream.

ASHRAE 62.1 offers an alternate engineering method for determining the appropriate amount of OA. This method is known as the Indoor Air Quality Procedure (IAQP). Rather than specifying a minimum amount of OA, this procedure focuses on removing contaminants of interest from the air inside the building. This provides opportunities to apply technologies targeted at specific contaminants. The potential benefit of using the IAQP is to reduce the amount of outside air that must be brought into the building, thereby reducing the total conditioning load on the ERV or DOAS. The method allows for substituting treated indoor air for some or all of the OA. The air can be treated using one or several techniques discussed later in this paper.

The primary drawback is that it requires expert engineering skills to do properly because it is a complex process that requires detailed knowledge of the pollutants, including gas phase pollutants (formaldehyde, benzene and others) present in the building. It also requires verifiable air cleaning data from the manufacturers of the cleaning devices. A particular challenge with the IAQP is the requirement to poll building occupants regarding perceived air quality post-build. If more than 20 percent of occupants report the space to be offensive, design changes are required.

Because of the complexity and notable risks, very few jobs benefit from this level of engineering effort. Caution should be used when evaluating a decision to downsize equipment because of reduced OA requirements. It locks the site into adopting the specific technologies used in the IAQP analyses to remain compliant. If the air treatment system is offline or removed, there would be a high probability the system would not have the capacity to use more common methods to maintain 62.1 by bringing in additional OA. It may be better to design the system for full capacity with the minimum OA derived via the VRP and benefit from higher part- load system efficiencies.

Consider the three main types of air pollutants and the typical ways they are removed from the air stream:

Particulates and Dust

Traditional ventilation with properly selected filters is the primary method used for removing particulates from the air. The filter type, MERV rating and configuration (pre-filter plus final filter or a single stage) are determined by the application. Commercial and educational spaces will typically employ MERV 8 prefilters with MERV 11 or MERV 13 final filters, while industrial and medical locations may require higher MERV ratings or even HEPA filters. Electrostatic filters may help to reduce fan power in some situations but can add significant component first costs in addition to increasing air handler length and increasing maintenance costs. New, lower pressure drop static filters are being offered to the market, such that the fan power differences are not significant.

Gas Phase Pollutants (VOCs and Others)

The most convenient way to remove gas phase pollutants is outside air ventilation. This is the recommended method for most installations.

Another way to remove gas-phase pollutants is to employ gas-phase adsorption devices. These devices capture molecular contaminants from the air in the building versus diluting the air by bringing in cold or hot outside air. Note that these technologies act on gas-phase molecules but do not filter the air, which is still done with traditional filters. This approach may reduce the amount of heating and/or cooling required to comply with 62.1. In some cases, capital savings are claimed by reducing the size of heating / cooling equipment necessary to condition OA. To do so, compliance with ASHRAE 62.1 with these devices requires the use of the aforementioned IAQP.

Airborne Pathogens

The traditional methods employed to mitigate airborne pathogens are:

  • Ventilation
  • Filtration
  • UV-C systems

These technologies enable facility managers to conform with the most up- to-date CDC guidelines, ASHRAE Standard 62.1 and the newly released ASHRAE Standard 241.

It is important to differentiate between outside air ventilation as prescribed in ASHRAE Standard 62.1 and the Effective Clean Airflow referenced in the CDC guidelines and ASHRAE Standard 241. The Effective Clean Airflow is pathogen-free air, which can be outside air, filtered return air or disinfected air.

Adsorption devices are not effective at mitigating pathogens.

CDC guidelines are 5 ACH of Effective Clean Air for pathogens.

There are many IAQ technologies available in the HVAC marketplace – some known for decades and others quite new, so facility managers have choices.

Selecting the best technology for the application can be a difficult endeavor and facility managers must look for appropriate solutions that are competitive, reliable and measurable. The recommendations made in this article are consistent with ASHRAE recommendations and industry best practices.

Jon Douglas is the Director, Healthy Building Systems, US at Johnson Controls and Jim Bogart is the Senior Product Manager, Airside Advanced Technology, at Johnson Controls. Johnson Controls, the global leader for smart, healthy and sustainable buildings.

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