Making terminals safer through HVAC design

Airport terminals are designed to accommodate large numbers of people, which mean they have very complex HVAC systems with sophisticated controls. In addition, a terminal building has different occupancy classifications, which require different levels of air exchanges, temperature, and humidity.

To ensure your terminal is operating at peak efficiency, and that sensors and control devices are properly calibrated, consider retro-commissioning your facility. This will ensure the HVAC system and controls are operating as intended. Review over-ride and occupancy settings to make sure there are no conditions present that could help spread the virus or other contaminants.

Because virus particles can cling to dust and spread more easily, while doing your retro-commissioning, thoroughly clean the entire HVAC system components, especially those more susceptible to dust accumulation, such as air dampers, ducts and coils, including inside the ducts to the outside grills.

To note, adjusting the settings also may help. For example, instead of shutting down overnight or on weekends, the HVAC system could run additional occupied hours after passengers disperse to increase the replacement of air and minimize stale air. Well managed control sequences/setpoints (temperature and humidity control) help minimize virus propagation.

While studies still are ongoing about how the coronavirus spreads via air, evidence suggests that measures to change indoor airflow patterns could play a role in reducing transmission.

APM-3 station at Tampa Airport – Transportation Terminal Airflow Management, positively pressurize the platform area to push air towards the Automated People Mover doors.

Airport terminals typically have very high ceilings, in which air distribution is neither appropriate nor effective. To improve airflow, introduce supply air into the breathing zone (less than 12 feet above the floor) and place the exhaust air return grills at the ceiling to allow stratification of hot air, and reduce the overall cooling load as well as prevent entrainment of aerosols.

Changing airflow patterns to create laminar vertical airflow – air moving in the same speed and in a straight path – may effectively prevent the airborne transmission of coronavirus particles. This principle already is used to prevent the spread of particles in clean rooms and hospital operating rooms.

Building pressurization is key in mitigating infiltration of dirty, humid air. From arrivals to departures—doors being open simultaneously, and shuttle car doors with dozens of passengers – maintaining humidity control inside the building is the best way to reduce the risk of COVID-19 for passengers.

Ensuring there is more Outside Air than Exhaust Air will mitigate unwanted infiltration, especially humidity. The upcoming American Society of Heating, Refrigeration and Air-Conditioning Engineers (ASHRAE) Airport Terminal chapter will address recommendations for pressurization, to push air out of each open gate during the boarding or deplaning process.

The World Health Organization (WHO) states that COVID-19 transmission risk increases in areas of high pollution because Particulate Matter PM level 2.5 and higher has small droplets or fine particles that act as a carrier.

Given that COVID-19 is a person-to-person transmission, it is important to upgrade filtration devices as close to the source as possible. Commercial HVAC systems should be able to accept MERV 14 filters, while packaged or light duty equipment may be limited to MERV 11. It is important to distinguish these limitations early on when deciding on a retrofit or new design.

For example, HVAC systems that serve multiple spaces with a common air handler have an advantage over in-room recirculating units because the media capturing the contaminants is remote from the room.

The US’s National Institute of Standards and Technology (NIST) has software called fate of transport of indoor microbiological aerosols (FaTIMA), which allows us to determine the fate of COVID and other airborne contaminants associated with ventilation, filtration, and depositive and inactivation mechanisms. The FaTIMA illustrates a representation of single well-mixed mechanically served zone and incorporates particle source and removal system.

The following graphs show the results of an infected passenger in the space for one hour. Under the current conditions, the virus will remain in the air for over an hour after the person leaves the room, but by increasing the filtration from Merv 13 to Merv 15 the airborne concentrations are eliminated 20% faster and 33% faster than if a 500 cfm HEPA air filter is added to the space.

Effects of single infected person sneezing once in the space starting at t=30 min (person enters space).

ASHRAE recommends that any increase level of MERV rating over the code minimums is a direct benefit. So, whether it is designing a retrofit unit—or a new system—pay particular attention to the highest level of filter efficiency for the sake of calculating the fan horsepower.

Further simulations found that using a Merv 15 filter is the most cost effective filtration, and optionally, the use of a MERV 13 filter paired with photocatalytic oxidation (PCO) is the most effective air cleaning solution found to limit the spread of the virus. It is suggested by ASHRAE to size the system for minimum 2 air changes per hour.

Devices such as Ultra-Violet (UV-C) and Bi-Polar ionisation are certainly popular in the industry. Be mindful of what uniform testing has been made available to ASHRAE for consensus of measuring performance and controlling the device. Many of these bolt on devices are good for localised disinfection, such as using UV-C on cooling coils.

Design of UV-C can be enhanced for airstream sterilisation of Coronavirus and Influenza A, by increasing the dosing upwards of 4000 micro-watt-sec/cm2 to provide enough UV-C intensity for the brief moment the air passes over the lamps. The topic of Bi-Polar ionisation (BPI) can be very lengthy, and detailed, due to the varied technologies, limited testing, and studies.

The overall take-away here should be that for BPI to be effective, it requires air changes, not just a once through, and to read the studies of possible con’s to the use of negative ions on health and wellness, to better understand the applicability of this technique for your buildings.

In short, sterilisation of the air is a component of indoor environment sterilisation, not the entire solution, because cleaning the air alone is not enough of a buffer between spreading viruses from person to person.

Engineering the next generation of airport terminals. As we learn to live in a post-pandemic world, our approach to protecting passengers will need to change as people become more aware of the importance of indoor air quality.

Each airport terminal is individualistic to that site, so planning your HVAC system alongside of the architectural design of passenger travel and flow is key to develop early on. Knowing what works, and what does not, requires engineers with good experience in aviation and specialty system designs.

The aviation industry has the opportunity to lead the way by leveraging technology in the fight against COVID-19 and any future viral threats.