introduction - airah...propagation of droplets in an hvac system by simon witts, m.airah, principal,...

Propagation of droplets in an HVAC System

By Simon Witts, M.AIRAH, principal, LCI; and Sam Coleman, mechanical engineer, LCI

INTRODUCTIONThis paper studies the propagation and distribution of water droplets typically generated by talking or more forcefully

from a cough or a sneeze through a typical office and ventilation system. The specific purpose of the paper is to establish

through the use of computational fluid dynamics (CFD) the likelihood of droplets making their way from one room

to another via a typical HVAC system.

The current pandemic has forced the HVAC community to look closely at the systems we use in buildings.

There have been numerous studies and academic papers that look at the distribution of droplets in a room from

a cough or sneeze, but few have gone on to study the likelihood of further distribution via the HVAC systems.

Respiratory diseases such as SARS 2002–2004 and the 1918 Spanish flu (H1N1) are spread by airborne transmission,1, 2

direct and indirect contact. There is a growing body of evidence that SARS-CoV-2 (COVID-19) is the same.3

When a person coughs,4 shouts or sings, the generated particles come from deep within the lungs and the droplets carried in

their breath are small, around 0.5µm in diameter, 1/200th the width of a human hair.

At this size, the interaction between the particle and air means that gravity has little effect and they remain suspended

in the air stream, known as an aerosol. These aerosol particles can remain in suspension for significant periods before

eventually evaporating or bumping into a surface and coming to rest. In an indoor environment, this significantly

increases the indirect potential for infection beyond the widely quoted 1.5m physical distancing advice.

An aerosol is a multi-phase fluid i.e., a mixture of different materials in different phase states, here gas and liquid.

The modelling of multi-phase fluids is complex and time-consuming. Because of this we see most studies worldwide

concentrating on the gaseous behaviour of the aerosol. Modelling only as a gas ignores the fact that aerosols are

suspended droplets with distinct separate physical properties from the gas medium that will come to rest on a

surface given the opportunity.

The focus of this study is to ask, “How effective are HVAC systems at distributing droplets that can harbour infections,

and where do they end up?”

COMPUTATIONAL FLUID DYNAMICS OVERVIEWCFD is a form of numerical modelling where a system, component – or as in this case, a room – can be analysed to understand the flow patterns and fluid behaviour.

In the example discussed, ANSYS CFX has been used to understand the flow patterns in the room, while also accounting for the operational HVAC system and cough droplets of varying diameters. It is fundamental to the analysis that the HVAC system be modelled accurately i.e., with ceiling ductwork and air handling units, as all of the above play a part in imparting and reducing momentum to the air.

From a modelling viewpoint, the accuracy of the simulation is largely dependent on two items: the mesh and the timestep used. An analogy to the mesh in a CFD simulation is the “resolution” of a digital image: a higher resolution with more pixels (i.e., more mesh elements) will yield better quality and more detailed results. In the same way, the smaller the timestep used to capture the transient nature of the cough and its dispersal, the better the evolution of the flow in the analysis.

Special consideration has also been made to the particle dynamics of cough/sneeze droplets. In order to represent reality, the CFD simulation accounts for varying particle sizes, typical to a sneeze or a cough, buoyancy effects i.e., gravity and full interaction (i.e., two-way fully coupled) with the air streams around the room and within the HVAC system.

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COUGHS AND SNEEZESWe often think of coughing and sneezing as very similar, but when investigated they are starkly different. Both are reflexes of the body; how they are generated has a significant difference in the size and quantum of droplets formed.

What is in a sneeze?

To sneeze (verb) “make a sudden involuntary expulsion of air from the nose and mouth due to irritation of one’s nostrils”.

The droplets from a sneeze are generated in the nose and mouth and range in size and diameter anywhere from 100µm to 1200µm, with the average size average size about 386μm.1 These are relatively large droplets, and some are visible to the human eye.

Droplet simulation parameters used for a sneezing source

Sneeze parameter modelled variable units

Minimum Ø 100 µm

Maximum Ø 1,000 µm

Mean Ø 368 µm

std. Deviation 864 µm

Figure 1. Modelled sneeze parameters

After roughly 5s the droplets have run out of forward momentum from the sneeze, and, in still air under the influence of gravity, droplets fall within a small radius of the person sneezing.

What is in a cough?

To Cough (verb) “expel air from the lungs with a sharp sound”.

In contrast to a sneeze, the volume and velocity of air expelled in a cough is considerably greater. While there is debate in research as to the exact number and size distribution of droplets, they are considered to be smaller than those produced by a sneeze. Some sources give larger diameters for droplets; 5, 6 however, Zayas et al 4 gives a strong argument for using smaller diameter droplets of between 0.3µm and 0.7µm, with the average size of about 0.5µm.4 As coughs exit the mouth some larger droplets are generated up to 55µm in diameter, however these larger particles account for < 0.4% of the droplets produced.

Importantly, Zayas 4 identified high and low emitters, with one high emitter giving a cough fluid mass over 14 times larger than the average. Very large outliers exist in the general population that due to fitness, health or other factors have different cough droplet distributions.

Figure 3. Simulation cough and sneeze in still air

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For the purposes of this study a “standard cough” is based on the following droplet distribution profile 4 used in the simulation. Note that a cough with larger droplets was also run for comparison in Simulation 4.

Droplet simulation parameters used for a coughing source

Cough parameter modelled variable units

Minimum Ø 0.3 µm

Maximum Ø 55 µm

Mean Ø 0.5 µm

std. Deviation 0.1 µm

Figure 2. Modelled cough parameters

Once the initial momentum of the air coughed up dissipates, the aerosol particles remain in suspension in the pocket of air into which they were expelled. The particles then move with the general air movement before eventually evaporating, or bumping into a surface and coming to rest, carrying beyond the 1.5m physical distancing circle.

AIR CONDITIONING SYSTEMSThe purpose of an air conditioning system is to ventilate and condition the space that it serves. To do this HVAC design engineers spend a lot of time and effort in designing systems to mix the air in a building as effectively as possible. This prevents discomfort from uneven temperature gradients or stale spots in rooms.

Once aerosol particles are injected into the air stream, they move with the air circulated by the HVAC system. Where the air changes direction quickly, the droplets’ momentum causes them to drift away from the airflow. If this drift causes them to bump into a surface they will come to rest. The modelling tracks the aerosol movement and the points at which the droplets come out of suspension.

What happens in a single room?The following sequence shows the propagation of the aerosol from a single cough in an 8m x 5m meeting room, with a balanced ventilation system providing seven air changes per hour. The room has a single four-way diffuser and two extract points in the ceiling at opposite ends. There are three distinct stages of aerosol dispersal following the initial coughed injection.

Stage 1 – 0 to 30s: The aerosolised droplets stay with the air expelled from the lungs.

Stage 2 – 30 to 240s: The aerosol is dispersed in the room but remains in locally concentrated clumps, while being mixed by the ventilation system.

Stage 3 – 240s+: The aerosol is evenly dispersed throughout the room.

The aerosol remains fully mixed with the room air and is gradually being drawn out by the extract system or colliding with surfaces. In stage 3 we see the room after 20 minutes; roughly 10% of the original aerosol is still in suspension in the room.

Approximately 40% of the original aerosol has left suspension and has come to rest on the room surfaces, around the air conditioning grilles, on the walls, and the sides, top and underside of the table.

Figure 4. Stages of aerosol dispersion

Figure 5. Stages of mixing and aerosol settling points on surfaces at 1,200s.


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The remaining 50% of the particles have been extracted out of the space. In a perfect world, to remove all risk of recirculation of aerosol, the extracted air from a building should be exhausted to outside, eliminating the possibility of recirculation via the HVAC system. Commercial air conditioning systems are not often designed to supply 100% outside air under design conditions, as this significantly increases energy use. In most commercial systems vitiated air is mixed with a proportion of fresh air and re-circulated back through the air handling system.

HOW FAR CAN THE AEROSOL CIRCULATE THROUGH THE HVAC SYSTEM?Two basic systems were modelled:

1. A meeting room/open-plan office connected by ductwork.

2. A more complex floor layout into which two aerosol injections were introduced, simulating a person moving around the floor.

Base system set‑up – distribution through the ductworkThe analysis simulates an office, comprising an individual meeting room and an open-plan area linked by an air handling unit. To reach the open-plan area the aerosol must pass thought the return air plenum, return air ductwork, air handling unit, supply ductwork and supply diffusers.

In the initial simulation a system has been replicated in which there is a medium-grade filter, such as an

AS1324.1-2001 7 grade F5 (AHSRAE 52.2-2017 Table E-1 8 grade MERV 9A). These filter grades offer little resistance to particles at sub 1-micron levels, and this grade of filtration is not uncommon in commercial systems.

System Parameters

Distribution by air movement on a floor plateVentilation systems move air in two ways: distribution through ductwork systems and by the deliberate creation of air imbalance on a floor, resulting in air movement from one zone to another.

This analysis simulates an office suite, comprising offices/meeting rooms, an open-plan area, circulation space and a toilet/washroom area (amenities). The simulation looks at the aerosol penetration around a floor plate, ignoring any distribution through the ductwork system itself. The model is set up with the air balance regime set so that air moves from offices to a circulation zone. The main office space is divided into perimeter and centre zones, with perimeters set to have slightly more supply air (positively balanced) than the centre zone. The amenities are negatively balanced with more extract than supply, as would normally be the case in a typical floor plate.

Figure 7 System ventilation parameters

Figure 6 Base system setup


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ANALYSIS AND DISCUSSIONSimulation 1 – ceiling plenum system, coarse filtration.Figure 9 shows collection points for the aerosol generated from a single cough.The modelling indicates the aerosols are capable of being passed through an HVAC system from one space to another.

For simulation purposes the filter efficiency is taken to be 0%. However, there is likely to be a reduction due to interaction with the internal surfaces in the air handling unit. This will be the subject of further study.

At 720s (12 min) the locations of particles are summarised in Section 8.4. After 720s, 96.7% of the aerosol particles have come to rest on surfaces. The remaining 3.26% are still in suspension in various locations in the system. The adjoining office has 0.64% of the simulated cough in suspension and 0.21% collected on surfaces, which could constitute a risk to inhabitants.5

Simulation 2 – ducted return system, coarse filtration.

As with the base case system, aerosol particles can pass into the adjoining room via the HVAC system. The ceiling void is not included

in the simulation domain.

At 720s (12 min) the locations of the particles are summarised in Section 8.4. After 720s 96.52% of the aerosol particles have come to rest on surfaces. The remaining 3.46% are still in suspension in various locations in the system. The adjoining office has 0.92% of the original simulated cough in suspension and 0.47% collected on surfaces, more than simulation 1.

Simulation 3 – ceiling plenum system, upgraded filtration in the AHUThe final simulation uses simulation 1, but includes upgraded filtration in the main air handling unit to an AS1324.1-2001 7

Figure 9. Aerosol settling pattern at 720s after a single cough in the meeting room

Figure 8. Full office floor plate air flow


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grade F9 (AHSRAE 52.2-2017 Table E-1 8 grade MERV 15A). A filter of this grade will arrest >80% of particulates in the 0.3 to 1µm range. For simulation purposes the filter efficiency is taken to be 80%, the reduction is likely to be higher due to the interaction with the surfaces in the air handling unit. This will be the subject of further study.

The aerosol is almost totally blocked from entering the adjacent space, at 720s (12 min) the locations of the particles are summarised in Section 8.4. After 720s 93.28% of the aerosol particles have come to rest on surfaces; 4.76% have been removed by the filter and AHU. The remaining 1.96% are still in suspension in various locations in the system.

The adjoining office has 0.1% of the simulated cough in suspension and 0.04% collected on surfaces, significantly less than either simulation 1 or 2.

Simulation 4 – modelling heavier particle distribution To establish if similar distribution patterns would be observed with different sized droplets (for example Morawska2), simulation 4 was run using the same input parameters as simulation 1 but with larger particle diameters.

Cough parameter Modelled Variable Units

Minimum Ø 0.3 μm

Maximum Ø 55 μm

Mean Ø 2 μm

Std. Deviation 1.2 μm

Figure 11. Simulation 5 cough particle distribution

The final location of particles was similar to that in simulation 1, as can be observed in Figure 12. One change was more particles stayed in the meeting room than passed on to areas further downstream. This did not stop a portion of particles from making it through to the open office area.

Simulation summaryThe results of simulations 1 through to 4 are summarised below.

Volume Location Sim. 1 Sim. 2 Sim. 3 Sim. 4

Meeting room

Suspended 1.05% 0.71% 0.79% 0.81%

Surfaces 50.51% 54.95% 49.60% 59.11%

Open ceiling

Suspended 0.96% N/A 39.28% 1.59%

Surfaces 39.61% N/A 1.06% 27.36%

RA ductwork

Suspended <0.01% 0.02% 0.01% 0.01%

Surfaces 2.76% 29.81% 2.68% 2.96%

SA ductwork

Suspended <0.01% <0.01% <0.01% <0.01%

Surfaces 3.45% 11.29% 1.68% 6.29%

Open-plan office

Suspended 0.64% 0.92% 0.10% 0.85%

Surfaces 0.21% 0.47% 0.04% 0.31%

Exhausted/Filtered

0.80% 1.86% 4.76% 0.93%

Figure 12: Results summary

Figure 10. Aerosol settling pattern at 720s after a single cough in the meeting room


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Settling pattern for circulated aerosols in the simulated adjacent roomA more detailed model of the open-plan office was simulated to illustrate the effectiveness of aerosol distribution, based on inputs from simulation 2. The heat map sensitivity of Figure 13 is roughly 1/100th of that in previous figures.

The concentration on surfaces is reduced to 1/1000th of shown in earlier figures when F9 filtration is installed.

Simulation 5 – distribution by the HVAC system on a floor plate.

A key take-away from this study is that while it is possible for particles to traverse an HVAC system, a far higher risk of room contamination comes

from localised sources in the spaces.

To illustrate this, a scenario was created with a single occupant coughing twice in a typical office layout. This floor plate included five single offices and two amenities rooms connected to a large open-plan area. The initial cough was inserted into office 1 to the top of Figure 14, then two minutes later a second cough in the open area was introduced. These two areas were connected by a door grille through which approximately half the single-office return air flowed (half returned locally through a ceiling return-air grille).

Particles pass from the initial cough into the open area, but most are collected on surfaces in office 1 or removed via the extract system. In the open-plan area, the majority of particles on surfaces and suspended in the air come from the second cough. An occupant in the open-plan area should therefore be more concerned about someone coughing in an adjoining space,

and even more wary of someone sick in their own area than a cough spreading through a common HVAC system.

In the simulation 14% of the aerosol injected from the second cough passed into the amenities.

OBSERVATIONS FROM THE MODELLINGModelling indicates droplet nuclei from a simulated cough in the 0.4 to 55µm range can traverse through an HVAC system.

Observation 1System type and the availability of surfaces for collision significantly impact the effectiveness of the HVAC system to distribute the droplets.

Observation 2The supply diffusers are designed to mix the air in a room to provide good even temperature and a good turnover of air in the space (air change effectiveness). This is also perfect for the mixing and distribution of aerosols.

In test models the aerosol from the simulated cough was totally mixed in the source room in five to eight minutes. The clearance time for the aerosol to be removed in the source room was more than 20 minutes in all models. This aligns with the known effectiveness of systems to remove gaseous contaminants, where an expected time of 35–46 minutes would be required to achieve a removal of 99% efficiency for 7.5 air changes per hour.9 Modelling indicates the clearance rate is quicker for aerosols as particles are both extracted and adhere to room surfaces.

Observation 3The aerosols were able to reach all supply outlets in the HVAC system, including original source room.

Observation 4In the fully ducted system (scenario 2), the droplet nuclei were able to reach the air handling unit more effectively than in the plenum return system (scenario 1), see table in section 8.5

Observation 5The droplet nuclei are carried by the air streams created by the ventilation system. Where the air pattern is very turbulent

Figure 13. Settling pattern for aerosols in the adjacent room served by the coarse filtered (MERV 9 ) HVAC system


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or changes direction quickly and collides with obstacles, droplets are deposited on the surfaces. The more complex the HVAC system and the greater the number of obstacles in the path of the aerosol the greater the chance that the droplet nuclei will be stopped from circulating.

This is a subject for further analysis.

Observation 6F9 filtration 7 in the air handling unit capable of arresting particles in the sub-micron range has a significant effect on the volume of droplet nuclei passing through the AHU. However, it does not reduce the volume passed in the supply system to zero in modelling.

Observation 7Modelling shows the importance of reducing the initial amount of aerosol inserted into a building. The more aerosol in the system, the greater the chance of distribution via HVAC, either by air patterns on floor plates or via ductwork. This validates the importance of PPE such as masks at helping to control spread and invites the question of whether better filtration is necessary if sick people can be encouraged to use masks or stay home.

Observation 8Aerosol distribution on the floor plate by the normal air patterns and regimes is a more effective transmission mechanism than distribution through ductwork. There is a potential for

high concentrations of aerosol to settle in areas that have more exhaust or return air than supply (negatively balanced). For example, in simulation 5 the cough was injected into the meeting room and the open plan area, yet the amenities show significant aerosol deposits.

LIMITATIONS/FURTHER STUDYStudy limitationsThis simulation is restricted to the variables as given in the report; however, at the level of detail investigated here airflows are complicated systems. Effects from variables such as humidity, local air temperature imbalances, material hygroscopy and even occupant movement will affect results. There is a large body of research that exists on these topics and should be read in conjunction with the findings here.

The absorption rate of droplets onto surfaces was set at 100%, so that all particles encountering a surface would adhere. In a normal office environment, a wide range of materials with varying degrees of adhesion strength will be present, leading to the potential for particles to return to the airflow after a collision. This will increase the number of particles flowing further downstream in the model. Further research and modelling are required to validate if this increase in mass is consequential.

No lifetime limit was placed on particles. Research suggests that some droplets will evaporate more quickly than the simulated time; however, there is also a body of evidence that

Figure 14. Initial cough in red, second cough in blue


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aerosolised droplets and virus particles may remain in the atmosphere well in excess of an hour.10

Further studySeveral simplifications were completed on the air handling system to allow for clearer modelling. It has been determined through this analysis that any sharp changes of air flow, particularly around obstructions, cause particles to deposit. With this knowledge, the addition of more detailed fittings such as volume control dampers, turning vanes, grilles, etc., will decrease the number of particles through the model. This is left for future research, whether as for entire systems or individual parts.

CONCLUSIONThis study demonstrates that a percentage of droplets can pass through an air handling system; however, the viability of a disease to also complete this journey must be established on a case-by-case basis. This percentage rapidly decreases as particles travel further downstream an HVAC system, to a point that only around 1% of the original particles made it to the second room.

This simulation looked at the droplet nuclei from a single simulated cough. Transmission of disease via aerosols through the system therefore makes this a percentage game. The bigger the system, and the more people “connected” to that system, the greater the aerosol load and the greater the chance of droplet nuclei containing infectious particles reaching new hosts. People breath out aerosol droplets when talking, shouting, singing, sneezing, or coughing.

The study demonstrates that good filtration (AS1324.1-2001 7 grade F9 (AHSRAE 52.2-2017 Table E-1 8 grade MERV 15A)) in the HVAC system can significantly reduce the ability of viral laden aerosols to complete the journey from one person to another via the ventilation system, but does not totally eliminate the possibility.

The study shows that aerosols settle on HVAC surfaces in significant numbers. Maintenance of systems serving known infected people therefore should be carried out with the same level of infection control as working in a room with an infected person.

General air-movement patterns on a typical building floorplate are very effective at aerosol distribution, and this movement of aerosol has the potential to be

a significantly greater issue than distribution of aerosol via the ductwork system itself. One person moving through a building will distribute aerosols throughout every space they enter. In each space the HVAC system will also then collect and re-distribute the aerosol, either via general air-movement patterns on the floor plate or to a lesser extent via the ductwork system.

Ventilation system designers have spent years perfecting HVAC systems to create consistent and comfortable environments, which inadvertently are the perfect vehicle for the distribution of aerosols.

References1. ‘Characterizations of particle size distribution of the

exhaled by sneeze’. Han ZY, Weng WG, Huang QY. 20130560, s.l. : J R Soc Interface, 2013, Vol. 10.

2. ‘Droplet fate in indoor environments, or can we prevent the spreadof infection?’ L., Morawska. 16, s.l. : Indoor Air, 2006. doi:10.1111/j.1600-0668.2006.00432.x.

3. Azimi, Parham, et al. Mechanistic Transmission Modeling of COVID-19 on the Diamond Princess Cruise Ship Demonstrates the Importance of Aerosol Transmission. [Document] s.l. : medRxiv, medRxiv, 2020. doi: https://doi.org/10.1101/2020.07.13.20153049.

4. ‘Cough aerosol in healthy participants: fundamental knowledge to optimize droplet-spread infectious respiratory disease management’. Zayas, Gustavo, et al. 11, s.l. : BioMed Central, 2012, Vol. 12. http://www.biomedcentral.com/1471-2466/12/11.

5. ‘Modelling aerosol transport and virus exposure with numerical simulations in relation to SARS-CoV-2 transmission by inhalation indoors’. Vuorinen, Ville, et al. 104866, s.l. : Safety Science, 2020, Vol. 130 (2020). doi.org/10.1016/j.ssci.2020.104866.

6. ‘Characterization of expiration air jets and droplet size distributions immediately at the mouth opening’. C.Y.H. Chao. M.P. Wan, L.Morawska, G.R. Johnson, Z.D. Ristovski, M. Hargreaves, K. Mengersen, S. Corbett, Y. Li, X. Xie, D. Katoshevski. 40, s.l. : Journal of Aerosol Science, 2009.

7. Australia, Standards. Air FIlters for use in general ventilation and airconditioning. Part 1: Application, performance and construction. Australian Standard. s.l. : Australian Standard, 2001. AS 1324.1- 2001.

8. ASHRAE. Method of testing general ventialtion air-cleaning devices for removal efficiency by particle size. s.l. : American Society of Heating, Refrigerating & Air Conditioning Engineers, 2017. ASHRAE 52.2 - 2017.

9. Centers for Disease Control and Prevention. Guidelines for Environmental Infection Control in Health-Care Facilities (2003) - Appendix B. cdc.gov. [Online] 22 July 2019. [Cited: 08 August 2020.] https://www.cdc.gov/infectioncontrol/guidelines/environmental/appendix/air.html#tableb1.

10. Begley, Sharon. Statnews. Statnews.com. [Online] 16 03 2020. [Cited: 04 09 2020.] https://www.statnews.com/2020/03/16/coronavirus-can-become-aerosol-doesnt-mean-doomed/.

11. Transport and removal of expiratory droplets in hospital ward environment. Chao CTH, Wan MP, Sze To GN. s.l. : Aerosol Sci. Technol., 2008, Aerosol Sci. Technol 42, pp. 377-394.

For information about the modelling processes used, please contact the authors directly.

ABOUT THE AUTHORSSimon Witts, M.AIRAHA principal at LCI, Simon Witt, M.AIRAH, is a Chartered Engineer with 34 years’ international experience in the fields of health building design and project management.

Witts has co-authored the new Engineering Guidelines for Healthcare Facilities, released in June 2020 by the Victorian Health and Human Services Building Authority (VHHSBA). Witts’ systems design experience includes heating, ventilation, medical gasses, air conditioning systems, steam and HTHW systems, domestic services, and above-ground drainage. He also has specialist design experience in air movement and contamination control.

Sam ColemanSam Coleman is a mechanical engineer with four years’ experience specialising in energy modelling and analysis, and overall HVAC design. His focus is on modelling software to better develop and manage ESD principles and alternatives.

ACKNOWLEDGEMENTSThis research was supported by LEAPAustralia who provided both Ansys software for simulations and technical expertise with CFD models. We specifically wish to thank our colleagues Joel Thakker and Lewis Clark for their time spent in assisting us.

We also wish to thank LCI Consultants for the opportunity to complete this research and for their technical expertise with ventilation systems used in modelling.

introduction - airah...propagation of droplets in an hvac system by simon witts, m.airah, principal,...

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