National Academies Press: OpenBook

Air Quality in Transit Buses (2023)

Chapter: Day 1 Session 3

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Suggested Citation:"Day 1 Session 3." National Academies of Sciences, Engineering, and Medicine. 2023. Air Quality in Transit Buses. Washington, DC: The National Academies Press. doi: 10.17226/27033.
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Session 3

Updating and Improving Filtration

Nathan Edwards, MITRE (formerly), U.S. Partnership for Assured Electronics (USPAE) (current), Moderator

Presenters

Charles Franz, Bay Area Rapid Transit

Daniel Cheng, Bay Area Rapid Transit

Ariel Piedmont, Sound Transit

Tim Wagner, Sound Transit

Charles Franz started with an overview of the ventilation for the Bay Area Rapid Transit (BART) system railcars. The supply air is pulled through the HVAC units underneath the car, filtered and conditioned (either cold or warm), and then is sent up through the walls, entering the cabin from above, or from under the window, depending on the different generations of cars. The circulated air is pulled through the return ducts through the HVAC unit; while 30% fresh air is pulled through the air intakes on the sides of the cars, the fresh and circulated air is mixed, and the process continues. All the units are mounted underneath the car body. There are three generations of cars: AB-cars, C-cars, and the new generation DNE cars. The C-cars have two HVACs per car, for a total cooling capacity of 14 tons and an airflow of 4,000 cubic feet per minute (CFM). The other legacy AB-cars have different types of HVAC units and different arrangements. The new fleet of the future DNE class car has a better cooling capacity, with two HVAC units per car with a cooling capacity of 16 tons and total airflow of 4,300 CFM. The supply ducts are on the ceiling for better airflow and better cooling for standees, and there is an improved design against refrigerant leakage and a switch from an ozone-depleting refrigerant to a new, environmentally friendly refrigerant. The onboard controller also communicates the status to the train operator, so issues with the HVACs can be addressed as they happen. A team in the central control office sorts those faults and dispatches people in real time to address them. In terms of airflow and the change-out rate, calculations show a total turnover every 70 seconds. The air in the cars is about 75% filtered, with the remaining 25% drawn in from the outside, although that amount is a little less on the legacy trains. Mixed air comes into the unit through the filters and is passed through the filters drawn in from the evaporator fan. As the mixed air passes through the filter, particles are trapped and the supply to the passenger cabin through the air ducts is improved.

Franz continued to explain that in response to the pandemic and to improve ridership, BART took immediate steps to upgrade filtration. One step was an initiative to upgrade the air filters in the HVAC systems to improve the filtration in the cars and also look into UV-C lamps in the HVAC unit. Before the COVID-19 pandemic, BART was using MERV

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Suggested Citation:"Day 1 Session 3." National Academies of Sciences, Engineering, and Medicine. 2023. Air Quality in Transit Buses. Washington, DC: The National Academies Press. doi: 10.17226/27033.
×

8 filters in the HVAC systems, with the recommendation at the time being a MERV 13 filter or better. BART’s target was to get to MERV 14 filters with a minipleat; this is a tightly pleated filter that increased surface area, reduced accumulation in any one spot, and resulted in a better pressure drop than the larger pleats. The filters get changed out approximately every 3 months of service. BART used different types of cars in the fleet in 2020 to test the pressure drop and found that it did not have an impact on the HVAC system and that there was no significant reduction of supply air flow.

Franz explained that BART is also looking into UV-C light to kill pathogens. BART conducted a pilot program in 2020 to install a UV-C lamp, which survived the wear and tear of the vehicles and the environment. The pilot program will next move to outfit 20 vehicles of the new fleet as old cars are retired.

Ariel Piedmont discussed how Sound Transit in the Seattle, WA, area has upgraded its bus ventilation system from MERV 4 filters to MERV 11 filters. The particle removal efficiency of different MERV filters is not a straight line; for example, a MERV 16 filter captures 99% of a certain size particle but only captures about 70% of the 0.3-micron particles. Respiratory aerosols that are potentially infectious are somewhere in the 0.3 micron to 1 micron range, and dilution ventilation is a key to protecting bus operators, partners, and riders. Dilution ventilation is, specifically, the amount of outside air added to the clean fraction of that recirculated air. For instance, at 10 ACH through a MERV 4 filter, there is still almost zero clean filtered air, but if those same 10 ACH were put through a MERV 8 filter, 20% of the air would be considered clean. With a MERV 11 filter, which is somewhere in that 0.3-to-1 micron range, about 40% would be clean air. Sound Transit conducted carbon dioxide tracer gas sampling to measure how much outside air was being changed over in the vehicles and found that the bus fleet had low outside ACH and low filtration, with the result that the total dilution ventilation rate was low.

Tim Wagner explained that, without freezing up evaporator coils due to excessively restricted airflow, or going outside the 6,500-mile service life, or voiding any warranties, Sound Transit wanted the best MERV rating filter possible. One of the bus series that was first tested was a 2019 vehicle, one of Sound Transit’s newer buses. Sound Transit started by talking to both the original equipment manufacturer (OEM) for the HVAC systems and the OEM for the bus manufacturer. Sound Transit found that the use of higher MERV rating filters had not been tested and that what they would do to the buses had not been researched. The agency came up with a test apparatus and looked at the MERV 4 filters from the OEM side. Sound Transit buses were using a horsehair type filter media that is flexible and looked at going as high or higher than a MERV 13 filter, but the MERV 13 filters are not as flexible and cannot be easily dropped into place.

Piedmont noted that when Sound Transit started the test process, MERV 9 filters were the only option available. Sound Transit gained access to the variable fan speed controller so that it could always set the fan speed to 75%. After putting in the test filter

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Suggested Citation:"Day 1 Session 3." National Academies of Sciences, Engineering, and Medicine. 2023. Air Quality in Transit Buses. Washington, DC: The National Academies Press. doi: 10.17226/27033.
×

media, it proceeded with a static pressure drop test. At that fan speed, a differential manometer, which measures the difference in pressure between two places, measured the pressure on the other side. The airflow through the filter was measured with thermal anemometers, which measure the flow rate by using one or more simple temperature sensors to monitor the amount of heat removed from a surface. The shape of this filter has a lot of complex curves, and there was not sufficient depth for the MERV 13 filter area to hit the airflow rates that were needed to not exceed the appropriate static pressure drop. Sound Transit tested MERV 7, 8, and 9 filters across different buses, checking the filters every week for the first month and a half. The agency found that the numbers were not much different—around 3,000 CFM was running through the system at 75% fan speed. The results varied somewhat across different buses but were not tested long enough to see the filters get visibly dirty. None of the filters restricted airflow below 25 CFM per passenger, which was one of the contract specifications in the bus design. Across the three filters tested—MERV 7, 8, and 9—no effects were seen that would cause adverse effects to the buses.

Piedmont and Wagner summarized other findings:

  • For measuring static pressure drop, the HVAC manufacturer advised staying below 0.6 inches of water column.
  • MERV 9 or MERV 11 saw good flow-through, and these two filters showed the lowest drawdown times. MERV 11 was approved for the double-decker buses.
  • There were not any significant airflow drops, which indicated that the filters were continuing to work about as well as the MERV 4, that is, slightly lower.
  • These data showed that a larger rollout to a bigger scale was possible.
  • Upgrading the bus fleet to MERV 11 has brought the original 0–4 ACH up to at least 15 ACH.
  • The higher ACH will help prevent large accumulations of respiratory aerosols and provide enough air to prevent buildup behind an operator barrier.
  • Performance to date: no damaged equipment, no broken compressors, no complaints from the agency’s maintainers.
  • With regard to wildfire smoke, work with King County Metro to test MERV 11 by running the fan on high with 40 ACH produced a 50% reduction in smoke levels inside the bus versus outside the bus.
  • There was a greater percentage reduction for other indoor-generated pollutants, such as cigarette smoke, whereas the outside air is considered clean.
  • Some of these filters are treated with an antimicrobial to prevent mold growth, and those may have a limited shelf life and may be worth changing out more frequently.

In response to a question regarding the cost differences experienced with the upgrades at BART, Daniel Cheng replied that it depended on the filter. In general, MERV 8 filters

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Suggested Citation:"Day 1 Session 3." National Academies of Sciences, Engineering, and Medicine. 2023. Air Quality in Transit Buses. Washington, DC: The National Academies Press. doi: 10.17226/27033.
×

cost around $10 each, but MERV 14 filters are about $30 to $40 each. Wagner noted a $1 to $2 difference per square foot for the media itself.

In response to the question of how much opening doors and windows affects the testing within transit vehicles, Piedmont responded that they used stationary buses with windows closed during testing, but that many of their buses do not have windows that open. Also, most of Sound Transit testing was done with the doors closed. Piedmont noted, however, that in testing vehicles with doors open, it depends on how the ventilation system is set up. For the buses, Sound Transit found that they generally do not have any significant positive pressure. There is a relatively small amount of fresh air that is forced in mechanically, but it depended largely on how much wind was blowing. In testing at a bus stop that was open to a field where there was clean air and the wind was blowing on that side, the bus received decent air exchange. The testing turned out to be highly variable, and not very predictable, because it was not driven by a mechanical system and was environmentally dependent. Cheng added that during testing, BART chose those cars that went through tunnels on their routes, which is the worst-case scenario. After 12 weeks of testing, BART collected the MERV 14 filters and sent them out to a contractor for pressure drop testing.

An audience member asked if the statically charged MERV 13 filters lose their charge and efficacy, and at what rate. Piedmont responded that a filter may be passively treated where it is initially charged, but then the charge can wear off. There are also some active systems for electrostatic filters that continuously maintain charge and will likely work longer. Regarding supply chain issues, Piedmont noted that the filters themselves have not been the issue, but the items sourced to customize the fit of the filters have been. Wagner added that sometimes the minimum quantity allowed from a manufacturer is still a large quantity for an individual agency to order. Agencies in their region have been working together to procure the minimum amount at one time.

In this session, a chat comment by Kevin Carmody noted that UV solutions are very dependent on the intensity and strength of the UV light, the distance to the treated surface/air, and the duration of exposure (speed of airflow), and asked how effective these solutions are and what maintenance (e.g., UV bulb cleaning) is needed to keep them effective (to which an estimated cleaning rate of once per year with alcohol was supplied anonymously). Sam Brauer responded that UV-B light (280–315 nanometers) “will also destroy organic molecules, including proteins (the outer coat of SARS CoV-2), as well as nucleic acids (DNA and RNA)” and noted that an advantage of using UV-C light (200–280 nanometers) is that it is “less damaging to human skin and possibly the polymers used in bus cabins.” [The 2005 International Agency for Research on Cancer report, Exposure to Artificial UV Radiation and Skin Cancer, notes that, while “The stratosphere stops almost all UV radiation <290 nm (UVC)” from reaching the Earth’s surface,” the National Toxicology Program’s 2002 10th Report on Carcinogens includes UV-A, UV-B and UV-C radiation in its list of “known carcinogens to humans.” The U.S.

Page 25
Suggested Citation:"Day 1 Session 3." National Academies of Sciences, Engineering, and Medicine. 2023. Air Quality in Transit Buses. Washington, DC: The National Academies Press. doi: 10.17226/27033.
×

Food and Drug Administration notes that although UV-C light has a very low ability to penetrate skin, it can efficiently damage the eye and generate cytotoxic ozone gas; see https://www.fda.gov/radiation-emitting-products/tanning/ultraviolet-uv-radiation.]

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Suggested Citation:"Day 1 Session 3." National Academies of Sciences, Engineering, and Medicine. 2023. Air Quality in Transit Buses. Washington, DC: The National Academies Press. doi: 10.17226/27033.
×
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Suggested Citation:"Day 1 Session 3." National Academies of Sciences, Engineering, and Medicine. 2023. Air Quality in Transit Buses. Washington, DC: The National Academies Press. doi: 10.17226/27033.
×
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Suggested Citation:"Day 1 Session 3." National Academies of Sciences, Engineering, and Medicine. 2023. Air Quality in Transit Buses. Washington, DC: The National Academies Press. doi: 10.17226/27033.
×
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Suggested Citation:"Day 1 Session 3." National Academies of Sciences, Engineering, and Medicine. 2023. Air Quality in Transit Buses. Washington, DC: The National Academies Press. doi: 10.17226/27033.
×
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Suggested Citation:"Day 1 Session 3." National Academies of Sciences, Engineering, and Medicine. 2023. Air Quality in Transit Buses. Washington, DC: The National Academies Press. doi: 10.17226/27033.
×
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Next: Day 2 Session 1 »
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 Air Quality in Transit Buses
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With a major drop in U.S. transit ridership since the start of the COVID-19 pandemic, an increased understanding of infectious disease in confined spaces and the role of droplets and particles in transmission has been increasingly important to the bus industry. A combination of experiments, models, and simulations in fluid dynamics has been employed to understand how aerosols move in spaces containing people.

TRB's Transportation Insights 2: Air Quality in Transit Buses provides a summary of a June 2022 in-person TRB Transit Cooperative Research Program (TCRP) Insight Event.

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