Birds, Sticks and Stones – Filtering Outdoor Air Used for Ventilation

I reviewed and excellent article in the April 2022 ASHRAE Journal about ventilation and filtration (# ventilation, #filtration, #Indoor air quality). Technical details are described in the article subtitled – Filtering Ventilation Air for Sustainable Airflow Measurement – Ross Montgomery, P.E. (https://www.ashrae.org/technical-resources/ashrae-journal) The article offers many straightforward issues that warrant discussion. Some of these issues are both controversial and timely, such as the wholesale policy endorsement by the author and ASHRAE for adding more outdoor air for ventilation to commercial and public spaces as a given. This is because of the US Government National COVID-19 plan. -National COVID-19 Preparedness Plan – https://tinyurl.com/2p8em3s , studies showing increased performance because of increased ventilation in U.S. schools balanced with increased energy consumption when additional outdoor air. (US EPA, Fisk). Further, adding additional outdoor air without creating side effects such as unwanted humidity or pollutants is often at great expense and expertise in balancing mechanical systems.

My take on the article is its focus on a great amount of detail that outdoor air addition has to be evaluated and also needs filtration to ensure “sensitive measurement devices” are protected as a given, but exceptions are given in the article in the form of case studies that many times this is not the case. That said, ‘sensitive measurement devices’ could be substituted with ‘coils,’ ‘fire dampers’ or ‘smoke detectors’ for example. I say this because based on my personal experience, I’ve witnessed many cases of ignored damages to these because of uncontrolled and unfiltered outdoor air. When evaluating outdoor air (O.A.) intakes and ducting, their location and conditions encountered are often harsh and allow contaminants, smoke, and pollutants into them unheeded. This is common in my experience. Traditional terms of the initial filtration entry point for O.A. intakes were often called “bird screens.” This used to be an obvious term to stop birds and bats from nesting in the intakes. Picture a screen attached to a duct entry point with openings designed to stop most birds, large particles, and debris (Birds, Sticks and Stones). Frequently, I would find that outdoor air intakes and ducting would go uninspected and would expose the air handler to roosting creatures such as bats, smoke from adjacent kitchen exhausts and pollutants such as carbon monoxide. As the article ascribes, this condition has to be addressed and even if ambient outdoor air conditions have improved, especially with the reduction of primary air pollutants there are still many other conditions. One could also add to the list other local contaminants – dust, clay, pollen, insects, water (liquid or vapor), wildfire smoke and vehicle emissions can all enter OA ducts in climate environments.

The article describes the rules for assuring filtration including from O.A. have changed over time by mechanical means (ANSI/ASHRAE 52.2-2017 -Rev) method of Testing General Ventilation Air Cleaning Devices for Removal Efficiency by Particle Size, ISO 2016:”ISO 16890 – air Filters for General Ventilation – International Organization for Standards. Both these standards lead to further discussion about what an air cleaner is comprised of instead of a simple ‘screen.’

Back to outdoor air filtration. My field experience as an Industrial Hygienist is to assure a visual assessment of the system as well as gathering basic measurements with air flow devices (#CFM, #pressure drop). This is task uses a hot wire anemometer or rotating vane anemometer and checking the outside air intake with a measurement of the duct size. This information is a decent estimate as it collects the flow per minute and duct dimension to give a total airflow of cubic feet per minute (CFM). While this is not a the same “basis of design” comparison that a contractor, testing adjusting and balance (TAB) professional and commissioning agent provide in the installation, inspection and functional performance it is an compliment to their role for accuracy and compliance with the design. In order to check this, flows are made on each side of the filter bank in the outdoor air if so equipped. We also report on the effectiveness of the filtration and flow amounts for ventilation compliance.

In Florida where I live the primary concern is that a HVAC System has to have code compliant outdoor air supplied in a hot and humid ambient environment. Typically, this represents problems across the design, construction, and commissioning process, along with constant operational concerns. The ASHRAE article provides a case study in a school HVAC system with multiple built-up air handling units. The discussion was very revealing. In summary, the commissioning was delayed, and the TAB professional left several units without any supplied outdoor air at all. In addition, to compensate for the lack of outdoor airflow changes to the return air flow led to air balance errors over / under pressurization, increased maintenance costs and increased risk to occupant meaning student health, illness or air contaminant issues because of the ventilation inaccuracies, inadequacies, and deficiencies. It bears out that the issue needs a lot of discussion and I’m more than happy to review what I can provide to facilitate this with interested stakeholders.

References

1.?????US Government National COVID-19 plan. -National COVID-19 Preparedness Plan – https://tinyurl.com/2p8em3s

2.?????Fisk, W. 2017. “The Ventilation Problem In Schools: Literature review.” Energy Analysis and Environmental Impacts Division, Lawrence Berkeley National Laboratory. https://tinyurl.com/yvhbsw8h

3.?????U.S. EPA. 2012. “Student Health and Academic Performance: Quick reference Guide.” U.S. Environmental Protection Agency. https://tinyurl/yc83dx3e