HEPA Filtration

There's been a lot more conversation in the last number of weeks around ventilation and filtration systems, focused on schools in particular. The decisions around how to move forward with the school year is weighing on the hearts and minds of Ontario parents, teachers, students, administrators and policy makers. I work with GPA Inc., we are a leading provider of ventilation solutions to the Ontario marketplace. We are partnered with globally recognized manufacturers like Swegon, VTS, AnnexAir, TMI, Multistack/AuraGreen. Some readers may not be familiar with these companies, but they each have remarkable capabilities in HVAC systems design and manufacturing, they are each leaders. I am writing this post to provide an important note to consider when searching for a Portable (or otherwise) HEPA filtration solution. A quick spot about me; my experience in HVAC systems spans 15 years, mostly focused on geothermal heat pumps, hydronics and the water side of HVAC systems. Prior to this experience I was educated as a Mechanical Engineering Technologist. Though I am writing this article to discuss primarily HEPA filtration, it should be noted that Ventilation and Outdoor Air are key components to a complete risk mitigation strategy, I just won't be commenting on them in this article.

We are by now all aware that adding portable filtration units, or upgrading filters in central and distributed HVAC systems are not 100% solutions. These are additive strategies to help minimize potential for exposure within indoor spaces. Coupled with all the other recommended practices, they collectively add up to our best shot at a safe indoor environment while living with COVID-19. In the last couple months conversations around filtration and HVAC systems has ramped up as we started to see definitive evidence that COVID-19 can spread via aerosol. This means transmission through ventilation systems is possible. It also means spaces with poor ventilation/minimal air changes per hour (ACH), represent a potential hot spot for transmission, as aerosols can linger in the air for some time.

HEPA (High Efficient Particulate Air) filtration is a strategy to help mitigate some risk in a conditioned space. Covid-19 is shown to be between 0.10-1.0 micron in size. When released in aerosol, most small viruses like this will attach themselves to a surrogate virus carrier such as a small dust particle or vapor of a larger size. HEPA filters can capture 99.99% of particles down to 0.3 microns in size. As such, there is a good opportunity for effectiveness when applied appropriately. Unfortunately, the term HEPA can be thrown around too loosely in marketing materials and prospective buyers should be aware of what to look for. When selecting these systems, one must be sure to verify the components meet the highest of standards for what they are designed to do. I have seen examples of products that claim “HEPA Grade” filtration, but cannot support claims with evidence of testing to a reliable standard. Manufacturing a filter with HEPA media, does not mean that the filter itself meets true HEPA efficiency requirements. Filter leakage contributes to the HEPA designation. Even tiny leaks in the media or disruption of the seal between the media pack and the filter frame can diminish the efficiency of the filter such that it does not meet HEPA standards. Buyers need to verify to what testing standard the filter has been tested. The IEST (Institute of Environmental Sciences and Technology) has developed standards for testing of filter media, and filter leakage for HEPA. The manufacturer of the filter should have the performance data related to this testing readily available.

How HEPA is applied as a system, will also greatly impact overall effectiveness. Since I am addressing only portable filtration systems in this short article, I'll provide just one note with respect to existing central and distributed HVAC equipment: For larger buildings, an engineering consultant should review the system before simply swapping one filter for another. Residentially, a contractor can take a look to verify the application. Higher efficiency filters typically have higher pressure drops, that may negatively impact the HVAC system as a whole. Applying a portable HEPA filtration system in a zone (like a classroom), can be very simple. Roll it in, and plug it in. What you do need to know first, is the volume of air in the room, and the desired ACH. Multiply the room volume by desired ACH, divide by 60 and you'll get the required CFM for the filtration unit. Advice may vary, but at time of writing, the recommended ACH for a classroom (for example) is anywhere between 4-12. To give an idea, the minimum number of air changes required for recirculating non-ducted filtration units for infectious disease areas is 12 ACH. The higher the required ACH, the greater the CFM required, the larger the filtration unit and the greater the cost.

Verifying the performance of the HEPA filter installed in your portable unit, and confirming the ACH required, will go a long way to ensuring you get the right portable filter for your application, that actually performs the way you need it to. As always, if buying the cheapest product for the job, you might not get what you are looking for...

• August 18, 2020
• Jeff Hunter