Title: Post-Pandemic HVAC Systems Strategies for High-Rise Office Buildings

ctbuh.org/papers

Title: Post-Pandemic HVAC Systems Strategies For High-Rise Office Buildings

Authors: Mehdi Jalayerian, PE, Managing Director, Environmental Systems Design, Inc. Tyler Jensen, PE, Studio Leader, Environmental Systems Design, Inc. Kenneth Griffin, MBS, Senior Building Analyst, Environmental Systems Design, Inc.

Subjects: COVID MEP Sustainability/Green/Energy

Keywords: Air Filtration HVAC MEP Office Public Health Sustainability Systems

Publication Date: 2020

Original Publication: CTBUH Journal 2020 Issue IV

Paper Type: 1. Book chapter/Part chapter 2. Journal paper 3. Conference proceeding 4. Unpublished conference paper 5. Magazine article 6. Unpublished

Abstract The impact of the COVID-19 pandemic on building Tyler Jensen will present “How Do Air design and strategy will be as revolutionary as the Distribution Methods and Air rise of the first skyscraper. COVID-19 and potential Exchange Rates Impact Viral Spread in High-Rise Offices?” future pandemics have forever changed the design at the CTBUH 2020 virtual approach and methodology for high-rise office conference, on 17 November. Mehdi Jalayerian buildings. Heating, ventilation and air-conditioning (HVAC) operational changes since the onset of the pandemic have been important to improve wellness and increase occupant comfort. These include moves recommended by ASHRAE (such as more outside air, better filters), along with air-cleaning technologies that can readily be added to existing systems, such as Ultraviolet Germicidal Irradiation (UVGI) and Bipolar Ionization (BPI). Tyler Jensen Kenneth Griffin But what if future tall buildings were designed to better respond to a pandemic

Authors from the start? Moving forward, high-performing buildings should be configured Mehdi Jalayerian, PE, Managing Director with mechanical systems that minimize or eliminate air mixing between floors. Tyler Jensen, PE, Studio Leader Kenneth Griffin, MBS, Senior Building Analyst They should optimize ventilation effectiveness within the space. In the increasingly Environmental Systems Design, Inc. 233 South Wacker Drive, Suite 5300 connected world, intelligent sensors can provide air quality data that is useful for Chicago, Illinois both operators and occupants. With forward-thinking transparency, the data can United States 60606 t: +1 312 456 2268 be compiled into meaningful metrics and shared with occupants to give them e: [email protected] esdglobal.com insight into building operations and performance.

Mehdi Jalayerian is Managing Director at ESD. His Keywords: COVID-19, HVAC, IAQ, Intelligent Buildings, Pandemics, Ventilation expertise spans three decades of work on iconic tall, public assembly and institutional buildings that include the world’s first positive-energy large-scale Introduction Though it’s too late to change an existing building and the world’s next tallest building. He is a frequent speaker on integrated building systems building’s HVAC design, now is the time to design, and is a major contributing author to the As 2020 dawned, imagining the possibility of rethink how future buildings will deal with ASHRAE Design Guide for Tall, Supertall and Megatall Building Systems. He holds a Master’s degree in a pandemic was not on the radar of future pandemics. Though hardly anyone mechanical engineering from the University of Kansas developers, engineers, architects, or anyone today was alive during the previous and is a licensed professional engineer in 14 states. else in the tall-building community. Today, it’s pandemic that hit the world in 1918, there is Tyler Jensen is a Studio Leader in ESD’s High Topic Number One. To respond to the current no excuse not to be prepared for the next Performance Buildings group. He has broad experience across a variety of markets, with a focus pandemic, building owners and operators are one, with all we have learned. on new construction, tall buildings, infrastructure, using the tools available to them and large commercial interiors, and repositioning projects. He holds a Bachelor of Science and Master of Science adopting operational changes that go Mechanical Engineering degrees from Washington hand-in-hand with the requirements of the HVAC Guidelines University in St. Louis, and is a licensed professional engineer in Illinois. pandemic. Stretching the limits of existing HVAC systems, owners and operators are Primary disease mitigation strategies today Kenneth Griffin is an architect and Master of Building Science through his education, with 13 increasing the use of outside air when consist of social distancing, wearing masks, years of professional work experience in building conditions allow, running the systems for keeping sick people out of buildings, energy performance analysis and building science. He has worked on numerous analytical projects, longer periods to flush the building before cleaning interior spaces, and maintaining including envelope optimization, daylighting analysis, and after occupancy, and upgrading good hygiene. Many proactive building parametric analysis, HVAC optimization, thermal comfort analysis, net-zero buildings, existing building mechanical filters when possible. In the owners and operators are seeking ways to retrofits, building integrated renewable technology, longer term, however, system configurations further help mitigate the chance of climate analysis, code compliance, LEED energy modeling, and urban-scale energy analysis. and other operations are poised to change transmission of the virus, and more and more drastically in future high-rises. are turning to their HVAC systems. This is

48 | MEP CTBUH Journal | 2020 Issue IV especially important with mounting and water heaters will be used less, which Wholesale HVAC system reconfiguration is evidence that airborne transmission of can prompt stagnant water issues. not feasible, but upgraded filters and COVID-19 is possible, especially in spaces advanced air cleaning systems can readily with poor ventilation and filtration. Perhaps the most significant risk during be retrofitted to existing systems. heating season: minor problems that linger The American Society of Heating, on unoccupied floors may develop into MERV Filters Refrigeration and Air-Conditioning major problems without a tenant present to MERV ratings define the effectiveness of air Engineers (ASHRAE), the predominant HVAC notify management. These are all crucial filters, with higher-rated filters able to capture professional association, has published considerations that building operators a greater percentage of particles, and able to guidelines for the pandemic. HVAC systems cannot ignore. capture smaller particles. Recent ASHRAE test can change operation by diluting the indoor data confirmed that properly-designed air air with more fresh ventilation air and filters are highly effective at removing cleaning the indoor air via filtration media. Retrofit Opportunities airborne viruses. MERV 5 filters, typically used More specifically, ASHRAE guidance for for residential applications, captured office buildings (ASHRAE 2020) includes In addition to the ASHRAE-recommended approximately 30 percent of airborne advice such as: operational changes, what else can be particles in the test, while MERV 13 filters done in existing tall office buildings to captured approximately 90 percent of • When outdoor temperatures allow, improve air quality and provide comfort to airborne particles (see Figure 1). increase the overall ventilation by going occupants as they return to the office? to partial air-economizer mode to increase outside air delivery and reduce recirculation. 100 • Disable any demand-control ventilation (DCV) sequences that would otherwise 80

reduce ventilation air for increased energy Viral Filtration Efficiency efficiency. 60 E1 - Initial

• Increase hours of operation of systems E2 - Initial 40 E - Initial before and after typical occupancy to E ciency (%) 3 flush the space, and to provide additional time for ventilation dilution. 20 • Upgrade and improve existing filtration 0 media. Minimum Efficiency Reporting A B C D Value (MERV) 13 minimum is MERV 5 MERV 12 MERV 13 MERV 14 recommended, and MERV 14 or 15 is even Filter Type better. High-Efficiency Particulate Air Figure 1. The level of viral filtration efficiency of filters with various Minimum Efficiency Reporting Value (MERV) ratings. (HEPA) filtration is not feasible in typical Source: ASHRAE, Zhang et al. (2020); redrawn by CTBUH commercial air handling systems, and isn’t necessary for tall office buildings.

Beyond disease mitigation, many building operators have yet to focus on the Properly-designed air filters are highly challenges of having low occupancy in buildings, especially during the cold winter effective“ at removing airborne viruses. MERV months, as many office workers continue to work from home. 5 filters, typically used for residential

With low occupancy, there’s not as much applications, captured approximately 30 demand on building systems, and operators accustomed to operating systems in a percent of airborne particles in the test, while certain way could face obstacles. Technology MERV 13 filters captured approximately 90 equipment and plug loads will be reduced because the majority of the workforce is at percent of airborne particles. home. Toilets will not be flushed as often, ”

CTBUH Journal | 2020 Issue IV MEP | 49 MERV 13 is the minimum suggested for airborne exposure and may reduce disease UVGI also only disinfects what it “sees” (e.g., high-performing office space, with the transmission, though neither should be surfaces in direct exposure to the light potential for MERV 14 or MERV 15 for even considered a “silver bullet” that is 100 beams), so systems installed in air-handling better virus removal (Zhang et al. 2020). percent effective. units only neutralize viruses in the return air But higher performance comes with an stream that have made their way back to the additional pressure drop for the supply fans Both UVGI and BPI may be appropriate for unit. UVGI lamps can also be installed directly and higher energy consumption. Depending new construction and retrofit applications in the office space—either at the ceiling on the existing system capabilities, supply-fan to improve indoor air quality and mitigate level directed above the occupied zone or and motor upgrades may be needed. airborne infectious diseases. Both activated after-hours when the building is in technologies can be reasonably cost- unoccupied mode only—but those solutions HEPA filters, typically used for hospital and effectively retrofitted to existing air- are cost-prohibitive and aesthetically clean room applications, are deemed handling systems. Careful consideration challenging for high-rise office buildings. impractical for permanent commercial can help determine which technology is applications due to the high pressure drop best for the application, evaluated on a BPI Systems that would be required. Typical fan case-by-case basis. BPI systems work by generating and horsepower would need to increase by more distributing charged ions into the space. The than 50 percent just to accommodate HEPA UVGI Systems ions attack surface and airborne pathogens filters. However, portable and packaged HEPA UVGI is an established technology that is and break them down to kill or inactivate filtration units can be used for high-traffic proven effective at reducing pathogens on them. The charged ions also agglomerate common spaces and for localized air cleaning surfaces and in the airstream. It is specifically airborne contaminants into larger masses after an event. recommended by ASHRAE in its COVID-19 that can either drop out of the airstream or response guidance (ASHRAE 2020). It can more easily be filtered by the air-handling Advanced air cleaning systems can provide easily be retrofitted into existing air handling unit (AHU). further benefit. Two such air-cleaning systems, but exposure to ultraviolet rays is technologies that can readily be added to harmful to humans, so additional safeties BPI can provide many of the same benefits as existing systems include Ultraviolet and interlocks are required for safe UVGI, with the added benefits of odor Germicidal Irradiation (UVGI) and Bipolar operation, especially with the high-intensity control and Volatile Organic Compound Ionization (BPI). These can help reduce lamps required for airstream disinfection. (VOC) and particulate matter reduction.

Exhaust Air Exhaust Air Fresh Fresh Air Air Recirculated Air HU

Recirculated Air Exhaust Recirculated Air Air Fresh Exhaust Air HU Supply Air Air HU Supply Air Supply Air Return Air

HU HU

eenrale r Hanln eenrale r Hanln Cenrale r Hanln se Cenral eae use r se oal r Inae an aus

Figure 2. Three typical configurations for air-handling systems in commercial office buildings. © Environmental Systems Design, Inc., redrawn by CTBUH

50 | MEP CTBUH Journal | 2020 Issue IV Recirculated Air

Recirculated Air Fresh Air AHU

Fresh Air

Supply Supply Air Air

Exhaust Air

Figure 3. Floor-by-floor air-handling units (AHUs) with central dedicated outside air (DOAS) and central exhaust systems. Left: Full floor-wide system view; Right: Detail at intake. © Environmental Systems Design, Inc., redrawn by CTBUH

Although typically installed in the air 2). At the zone level, these arrangements They should consider air delivery schemes handling unit, BPIs can also kill pathogens in can be coupled with conventional overhead that reduce local air mixing within the space. the air and on surfaces in the space, not just or underfloor air-distribution systems. at the AHU. Two configurations look to have the best Traditionally, many tall office buildings have odds of achieving success. The decentralized However, some BPI products generate utilized central air-handling systems with floor-by-floor AHUs eliminate air recirculation harmful ozone, and there is a lack of scientific dedicated mechanical floors incorporating between floors and still enable air data and testing protocols with which to large, built-up systems that serve groups of economizer/purge capability. They also evaluate and compare technologies. These 20 to 30 floors. This approach reduces the provide the most rentable square footage risks must be evaluated, but can be mitigated mechanical footprint on tenant floors, compared to other options. Underfloor air to some extent. If BPI is used, proper centralizes maintenance, and delivers good distribution increases the number of space specification to ozone-free Underwriters energy performance with a cost-efficient air changes per hour (ACH), while enabling Laboratory (UL) standards is critical. Field HVAC system. High-performing buildings of air economizer/purge capability during performance testing and validation to the future should be designed to address greater number of hours per year; confirm the efficacy and proper operation of shifting demands—with the flexibility to significantly increases space ventilation the installed product is also essential. efficiently respond to future pandemics, and effectiveness, with a single-pass airflow from with HVAC systems that prioritize occupant floor to ceiling; and minimizes local air health and wellness. (Of course, building mixing compared to the traditional overhead Changes Ahead for Air-Handling Systems codes differ across the world and have mixing system. limitations, so the following information may A high-rise office building typically includes not represent “one-size-fits-all” solutions.) Other options are not as effective, but have either a centralized or decentralized their benefits. In the traditional centralized arrangement of its air handling systems. A To achieve these goals, high-performing AHU configuration, air is filtered prior to centralized system provides ventilation and buildings should be configured with redistribution to the space, but it is recirculated air conditioning to multiple floors. A mechanical systems that minimize or between all floors in the office stack that the decentralized system arrangement will eliminate air mixing between floors. They air handling unit serves. With dedicated include dedicated or direct outside air, as should optimize ventilation effectiveness outside air system (DOAS) and floor-by-floor well as a relief/exhaust air system, and limits within the space, maximize fresh air volume systems, air recirculation is minimized to the air recirculation to each floor (see Figure and provide economizer/purge capability. individual floors only (see Figure 3).

CTBUH Journal | 2020 Issue IV MEP | 51 A centralized DOAS can be replaced with a localized (each-floor) fresh-air intake and relief/exhaust that can completely isolate duct distribution between floors (see figures Supply 4 and 5). Air Exhaust Air Localized fresh-air intakes and relief/ AHU exhaust require coordination and louver Supply Air integration with the exterior façade at each Outside Air level. Floor-by-floor fan rooms (see Figure 6) can either be located at the perimeter for Return Air easy duct connections to the louvers, or within the core, to maximize views and leasable floor area. Core-based floor-by- floor fan rooms will require coordination of large intakes and relief/exhaust duct connections from the core fan rooms to the perimeter louvers.

Flexibility is also increasingly important Figure 4. Decentralized floor-by-floor AHUs, with local outside air and exhaust with perimeter fan room. © Environmental Systems Design, Inc., redrawn by CTBUH with so much uncertainty in the world. Floor-by-floor systems provide the best flexibility for single tenants and single floors to operate their systems independently. Ventilation, filtration, and advanced air Recirculated Air cleaning enhancements can be implemented flexibly, depending on individual tenant needs. And floor-by-floor Supply systems will be best-positioned to respond Air and adapt to unknown future requirements. AHU

An array of potential systems and Supply Air Recirculated Air configurations for tall office building HVAC is provided in Table 1.

Improved Space Airflow

Most tall office buildings rely on overhead air distribution to supply conditioned air Figure 5. Decentralized floor-by-floor AHUs with local outside air and exhaust, paired with core-based fan room. to the occupied space for ventilation and © Environmental Systems Design, Inc., redrawn by CTBUH cooling. There are many different system types and variations, but all overhead supply systems use air diffusion and air Tall office buildings with overhead supply air pressurized raised floor air plenum and mixing to perform properly. Primary create a fully mixed space, which is effective low-velocity swirl diffusers to supply cool air cooling supply air is around 55°F (12.7°C), for evenly distributing cooling and ventilation at the floor level, typically 65–68°F (18–20°C) which would be too cold and drafty to air (see Figure 7). However, another air (see figures 8 and 9). The system works best blow directly on occupants, so ceiling distribution method can provide improved with a separate dedicated perimeter system diffusers are designed to maximize air indoor air quality (IAQ) and pathogen for envelope heating and cooling, so that mixing. The cold jet of primary air from removal, without fully-mixed room air. the UFAD system operates at a consistent, the diffuser induces room air to move stable pressure to accommodate the towards the diffuser, mix, and transfer Underfloor air distribution (UFAD) minimizes interior cooling and ventilation only. heat from the space. supply air ductwork and instead utilizes a A combined system may work in milder

52 | MEP CTBUH Journal | 2020 Issue IV DOAS + Decentralized Centralized AHUs UFAD Floor-by-floor AHUs Floor-by-floor AHUs Depends on system No recirculation between No recirculation between configuration floors floors Air Mixing Air recirculates between Typically some Between Floors floors Air recirculation limited Air recirculation limited recirculation, but less to individual floors only. to individual floors only than centralized AHU system. Single air pass through Typical overhead Typical overhead system Typical overhead system occupied zone Air Mixing Within system uses fully-mixed uses fully mixed occupied uses fully mixed occupied Space Heat and pathogens occupied space space space accumulate in stratified zone Typical overhead system Typical overhead system Typical overhead system Ev of 1.2 per ASHRAE Ventilation has Ev of 1.0 per ASHRAE has Ev of 1.0 per ASHRAE has Ev of 1.0 per ASHRAE 62.1 (20% better than

Indoor Quality Air Effectiveness (Ev) 62.1 62.1 62.1 overhead) 100% fresh air purge Economizer 100% fresh air purge Reduced fresh air purge 100% fresh air purge capability Fresh Air Purge capability capability capability Capability Extended economizer operation Space Air Changes per 4-6 ACH at design 4-6 ACH at design 4-6 ACH at design 6-8 ACH at design Hour (ACH) Overall building cost efficient Cost-efficient Higher first cost Higher first cost Higher core and shell First Cost Large centralized Distributed equipment Distributed equipment first cost equipment Lower tenant buildout cost

Cost Excellent energy Good energy Great energy performance Good energy performance performance Energy performance Full economizer Reasonable economizer Full economizer Performance capability Full economizer capability capability capability Reduced fan energy Reduced fan energy Reduced fan energy Efficient zone distribution Figure 6. View of a typical floor-by-floor AHU. Good system turndown Great system turndown © Environmental Systems Design, Inc. Low performing system Simple, efficient, cost Simple, efficient, cost Adequate system turndown effective afterhours Operation effective afterhours turndown Need to operate multiple operation Flexibility operation Depends on system floors to serve a single Need to run central DOAS Fully independent from configuration floor afterhours to serve a single floor other floors afterhours Enables individual floor Enables individual floor filtration and air cleaning filtration and air cleaning Overhead system Plug-and-play floor Renovation enhancements enhancements requires rework of FPBs, supply diffusers can easily Operation Flexibility ductwork, and diffusers. Overhead system Overhead system be relocated requires rework of FPBs, requires rework of FPBs, ductwork, and diffusers ductwork, and diffusers Centralized equipment Distributed maintenance Distributed maintenance simplifies maintenance on all office floors on all office floors Depends on system Maintenance and access Smaller AHUs can be Smaller AHUs can be configuration Large fans can be difficult simpler to maintain simpler to maintain to maintain and replace Main noise-generating AHUs are located on AHUs are located on equipment typically Noise generating occupied office floors occupied office floors centralized on Acoustics equipment centralized on close to occupants close to occupants mechanical floor mechanical floor Smaller fan equipment Smaller fan equipment Minimal noise-generating generates less noise generates less noise equipment required within the office space

Occupant Comfort Occupant Series flow FPBs provide Series flow FPBs provide Series flow FPBs provide Excellent thermal comfort Figure 7. Three-dimensional view of an overhead Thermal Comfort good thermal comfort good thermal comfort good thermal comfort and local control supply air system, showing air flow and temperature Reduced AHU space performance in a compartmented office space. Large AHU space Reduced AHU space No AHU space required at required at mechanical required at mechanical required at mechanical © Environmental Systems Design, Inc. mechanical floors floors floors floors AHU room required at Small air column unit No AHU space required at AHU room required at each typical office floor closets required at typical typical office floors each typical office floor Floor Space office floor No vertical duct shafts Requirement Large vertical duct shafts Reduced vertical duct climates, but is especially problematic in required Reduced vertical duct required shafts required shafts required harsh climates with high envelope heating Overall space Overall space Overall space requirement roughly Overall space requirement roughly requirement roughly and cooling requirements. ImpactArchitectural equivalent requirement roughly equivalent equivalent equivalent Centralized at mechanical Centralized at mechanical Centralized at mechanical Louver Locations At each office floor Office heat sources such as occupants and floors only floors only floors only computers generate a heat plume that is naturally buoyant and rises towards the Key Terms: ACH - Air changes per hour DOAS - Dedicated outdoor air system ceiling, drawing the low-velocity supply air in ASHRAE - American Society of Heating, Refrigeration and Air-Conditioning Engineers FPB - Fan-powered box its wake and creating a stratified zone near AHU - Air-handling unit UFAD - Underfloor air distribution the ceiling. Heat as well as airborne Table 1. Air Handling Systems Comparison. © Environmental Systems Design, Inc.

CTBUH Journal | 2020 Issue IV MEP | 53 Return Air Plenum Ceiling

Primary Air Damper

Induced Return Typical Floor Swirl Diffuser Typical Return Air Primary Air Return Riser Air Column Mixing Box

Supply Fan Primary Air Supply Riser

Perimeter Changeover Heating & Cooling

Figure 8. Diagram of an underfloor air-distribution (UFAD) system with a ceiling-based return air plenum. © Environmental Systems Design, Inc., redrawn by CTBUH

employed in office buildings, but predominantly for owner-occupied buildings. While overall installation cost is typically equal Rethinking the tenant improvement or even less than traditional approaches, allowance“ allocation could make UFAD systems UFAD systems push much of the first cost to the base building rather than the tenant feasible even in tall core-and-shell office buildout, which can be challenging from a leasing standpoint. buildings, and the IAQ benefits are clear. Rethinking the tenant improvement ” allowance allocation could make UFAD systems feasible even in tall core-and-shell contaminants and pathogens accumulate office buildings, and the IAQ benefits are clear. above the occupants, limiting potential exposure duration. Intelligent IAQ Monitoring With UFAD systems, particulates and pathogens travel much more directly out of The COVID-19 pandemic has highlighted the the space compared to traditional overhead importance of HVAC systems and indoor air systems, where the air swirls and mixes as it quality as something that is front-of-mind for leaves the space. CFD analysis shows the air all occupants in tall office buildings. The way path of potential pathogens exhaled by an people live and work may forever be occupant for both systems, with clear changed as a result of the pandemic, and performance improvements for the UFAD office buildings will need to adapt in order system (see figures 10 and 11). to prioritize the health, safety, and wellness of tenants. Raised-floor air distribution systems have Figure 9. Three-dimensional view of an underfloor air been used since the 1950s in computer Even after the pandemic subsides, occupants distribution (UFAD) system, showing air flow and temperature performance in a compartmented office space. rooms to accommodate wiring distribution will be more finely attuned to a building’s © Environmental Systems Design, Inc. as well as cooling. The system has also been IAQ, and will demand higher-performing

54 | MEP CTBUH Journal | 2020 Issue IV systems as well as insights into their office building HVAC systems will be References environment. Intelligent air quality sensors rethought and reconfigured to best respond. ASHRAE (2020). “Commercial Building Readiness Guide.” can continuously measure, verify, and Commercial Technical Resources. ASHRAE, August 13, 2020. https://ashrae.org/technical-resources/commercial. provide visibility into a tall building’s air No one in the tall building community quality status. foresaw the pandemic. The good news is, all of Zhang et al. (2020). “Study of Viral Filtration Performance us can play a role in our long-term response: of Residential HVAC Filters.” ASHRAE Journal, August. In the increasingly connected world, the improved buildings of tomorrow.  intelligent sensors can provide air quality data that is useful for both operators and occupants. With forward-thinking transparency, the data can be compiled into meaningful metrics and shared with occupants to give them insight into building operations and performance. By continuously optimizing IAQ and providing full transparency to the occupants, this approach will raise the standard for commercial office buildings and push tall building design to a higher level that can meet and exceed the demands of a post-pandemic workforce.

Tall Office Buildings of the Future

Lessons learned from the current pandemic can help guide design for tall buildings of the future. Buildings that prioritize health, wellness, and IAQ without overlooking sustainability will be well-positioned to attract and retain premier office tenants. Those Figure 10. Airborne particle travel paths, using an overhead system. © Environmental Systems Design, Inc. designed with flexibility integrated into the base systems will best respond to future pandemics and unknown needs. And those that harness data and provide transparency into the occupied environment will provide comfort and confidence for tenants to return to, and remain in the office space.

Decentralized floor-by-floor AHUs eliminate air mixing between office floors and provide flexibility for individual tenant operations and air-quality enhancements. UFAD improves pathogen removal and air quality in the office space. Both system approaches are primed for use in the next generation of high-performing tall office buildings.

The current pandemic will eventually subside, but some impacts will be longstanding. Tall office building tenants will forever be more finely attuned to IAQ and their environment. Forward-thinking tall Figure 11. Airborne particle travel paths, using a UFAD system. © Environmental Systems Design, Inc.

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