On 27 December 2021, seven people dining at four tables in the restaurant Moon Palace (望月樓) at Festival Walk in Kowloon Tong were infected with the Omicron variant. The source was a 44-year-old male Cathay Pacific Airway cabin attendant who was dining at Moon Palace with his father. Subsequent investigation by Professor Yuen Kwok Yung from The University of Hong Kong and several government officials from the Electrical and Mechanical Services Department (EMSD), Food and Environmental Hygiene Department (FEHD) and Centre for Health Protection (CHP) revealed that the air ventilation was non-uniform with airflow essentially being stagnant at the corners of the restaurant. This happened even though the overall ventilation rate was high and with six air cleaning units.

One of those infected was a 66-year-old woman who was also dining at Moon Palace at that time with eight relatives, in a room located at least 20 to 30 m from the index case. The father of the index case was also infected. According to Professor Gabriel Leung, one of the expert advisors to the HKSAR Government, the infection pathway is untraceable but certainly the infection must have occurred through air dispersion. This case reveals the problem of poor air ventilation in the restaurant, causing cross contamination over short and long distances. Further, it renews concerns about overall poor air ventilation and indoor air quality in restaurants in Hong Kong. This paper presents the pitfalls of conventional air ventilation systems in restaurants and proposes a non-conventional ventilation system to mitigate the spread of COVID-19 and improve indoor air quality in restaurants and similar indoor environments.

Existing ventilation systems in restaurants

In a typical and conventional air ventilation system using an Air Handling Unit (AHU), as shown in Figures 1a and 1b, the supply air flows from the ceiling level through an array of evenly distributed supply air grilles over the occupied area. Meanwhile the return air is also extracted at the ceiling level from the side edges of the occupied zone. This overhead (OH) ventilation system forces air at high velocities to achieve a complete mixing of the air to maintain the desired temperature in the occupied zone. Exhaled air from patrons at each table is mixed with the incoming air and spreads horizontally to neighbouring tables. In the event of an index case at one table, diners at neighbouring tables within short distances are likely to be infected. Furthermore, the exhaled air from the index case may be extracted through the return air grilles and be carried to tables further away through AHU and supply air ducts.

Figure 1a: Existing indoor ventilation system using Air Handling Units (plan view)

Figure 1b: Air flow in existing indoor ventilation system using Air Handling Units (section view)

In another conventional system using Fan Coil Units, the supply air flows through an array of supply air grilles at one end while the return air is extracted through return air grilles two to three metres away, as shown in Figures 2a and 2b. This mode of ventilation provides more localised circulation. While cross contamination over long distances is less likely, neighbouring patrons are more susceptible to be infected by a nearby index case.

Figure 2a : Existing indoor ventilation using fan coil units (plan view)

Figure 2b: Air flow in existing indoor ventilation system using fan coil units (section view)

A detailed study of an infected case in a crowded and poorly ventilated restaurant in Guangzhou on 24 January 2020 has shown that the recirculation zone above the tables was mixed with supplied and returned air, and the infection was localized. The restaurant used a split type air conditioning system to produce the airflow ¹.

It is believed that in the Hong Kong Moon Palace case, the 66-year-old woman was dining with her relatives at over 20 m away from the index case and therefore must have breathed in virus-laden aerosols exhaled from the index case through the return air grilles. Subsequently, the conditioned air was resupplied through the air supply grilles to the affected room. The distance of separation between the index case and the affected room became immaterial as the transmission pathway was through the air duct system.

Proposed ventilation system

In order to minimise cross contamination caused by mixing of the air supply and exhaled air in a confined space, such as a restaurant, the ventilation rate must be sufficiently high to dilute the exhaled air from the index case. American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) standard 62.1-2019 recommends a minimum ventilation rate in restaurants/dining rooms to be 5.1 L/s per person. However, this is not normally achieved by restaurants/dining rooms due to cost saving reasons. For example, the ventilation rate in the above-mentioned Guangzhou restaurant had a ventilation rate of only about 0.5 L/s per person, which is far below the ASHRAE recommended rate.

High ventilation rates and clean air units do not guarantee non-spreading of virus-laden aerosols, as revealed in the Moon Palace case, since the ventilation rate may not be uniform in the dining hall, especially near the corners and clean air units work within a localised air space. An alternative and non-conventional design of the ventilation system involves using an Underfloor Air Distribution System (UFAD) to extract the air from each table vertically upward through a return air grille. This design uses thermal convection induced by the body heat of patrons seated around the tables and from the extraction fan above. The return air is subsequently cooled, filtered and UV sterilized before resupplying using a pressurised booster through supply air ducts to a raised floor. Here the conditioned air is supplied to an array of diffusers located at low levels. The preferred height is below the table level and a short distance of approximately 100 mm above the floor of the occupied zone. Figure 3 shows the proposed UV sterilized UFAD ventilation system.

The main advantages of this non-conventional mode of ventilation for restaurants include ^2,3,4:

1. layout flexibility, reduced life cycle costs and minimal noise;
2. minimising cross contamination to patrons at neighbouring tables;
3. sterilising return air before resupplying to patrons;
4. giving the best thermal comfort to patrons as the occupied zone becomes thermally stratified with the space above the tables being a warm air zone while the space below being a cool air zone. The cooled air moves upward by displacement current; and
5. improving the overall energy efficiency, thus saving energy, as the extraction is convection assisted.

Figure 3: The proposed UV sterilised indoor ventilation system

Since the 1950s, UFAD has been utilised in office buildings where raised floors are desirable for cable management, data centres, command centres and server rooms with large cooling loads for electronic equipment and requirements for routing power and data cables ³. There are many examples of such installations overseas. An example of an UFAD system in Hong Kong is the HSBC Data Centre at Shek Mun. Unfortunately, no restaurants in Hong Kong have ever utilised

this system.

This deficiency is understandable because, despite the many advantages, at least three obstacles must be overcome especially for existing installations, as follows:

1. Being non-conventional, the existing system must be substantially modified with cost and time implications.
2. Delivering supply air to patrons uniformly is difficult. Patrons in the middle section of the dining hall normally receive far less air supply from the sides.
3. Resolving design, construction, operational and maintenance issues ^2,5.

The concern in (1) above is real and genuine; however, the economic loss due to tight restriction on the dining hours and number of patrons in a table has caused the catering industry a loss of at less 4 billion dollars during the lunar new year period, according to Mr Wong Ka Wo, Chairman of the Hong Kong Federation of Restaurants and Related Trades. Restaurants could make use of a low business period to shut down for a relatively short period in order to modify the existing ventilation system with assurance that restrictions can be lifted or relaxed. The resulting economic recovery could more than offset the modification cost and the loss of business during the modification period, depending on the scale of the modification works.

In order to overcome the concern in (2) above, it is proposed that the floor should be raised to enable supply air ducts to go underneath the tables to supply air at strategic locations. Raising the floor with flat and rigid support would reduce the ceiling height by approximately 200 mm to 300 mm which should not normally be significant for large restaurants. Figure 4a shows schematically the proposed layout of the return air grilles and supply air grilles in a dining hall. Figure 4b shows a cross section of the layout with the supply air diffusers concealed under low tea tables to avoid obstructing the views of the patrons. Louvres are also tilted downward (details not shown) to improve the thermal comfort of nearby patrons. Floor diffusers, as installed in many installations such as data centres, are inappropriate for restaurants.

Figure 4a: The proposed indoor ventilation system for dining halls (plan view)

Figure 4b: The proposed indoor ventilation system for dining halls (section view)

Regarding the various issues identified in the design, construction, operation and maintenance of the system in (3) above, it is understood that many of the disadvantages have been resolved with the advancement of science and technology. In addition, with more experience gained in both the design and construction of the system overseas and in Hong Kong, performance should be improved ^2,5.

Conclusion

Conventional air ventilation systems in restaurants are designed to deliver cool air uniformly to patrons in the dining hall from the ceiling with far less design considerations for the return air. This overhead mode of ventilation systems causes horizontal spreading of exhaled air among tables. Acrylic sheeting between tables helps to minimise horizontal spreading to some extent but is insufficient as the sheets cannot achieve satisfactory enclosure of patrons seated at the tables. Furthermore, exhaled air from patrons can be extracted through the return air grilles and subsequently resupplied to other patrons over the whole dining hall, creating poor indoor air quality in the dining space and causing widespread contamination.

A conceptual design of a non-conventional UFAD ventilation system for restaurants is proposed to address this pitfall in the hope it may alleviate the problems with the conventional overhead ventilation systems in restaurants and similar indoor environments. It should be emphasised that this system requires substantial modification of the existing systems with time and cost implications. However, the said modification should be manageable with no insurmountable obstacles with more experience gained and lessons learnt. Finally, this system may also be applied to similar indoor environments where groups of people gather for various activities, such as department stores and fitness centres.

About the Authors: Ir Dr H. F. Chan is a Fellow of the HKIE in the Environmental and Energy Disciplines, the Past Chairman of the Environmental Division for Session 2000/2001 and the Past Discipline Representative of the Environmental Discipline for Sessions 2002/2003 to 2004/2005.

Ir Philip K. S. Chiu is a member of the HKIE in the Building Services, Control Automation and Instrumentation, Electrical, Energy and Fire Disciplines.

References

1. Yuguo Li, Hua Qian, Jian Hang, Xuguang Chen, Pan Cheng, Hong Ling, Shengqi Wang, Peng Liang, Jiansen Li, Shenglan Xiao, Jianjian Wei, Li Liu, Benjamin J. Cowling, Min Kang, (2021), Probable airborne transmission of SARS-CoV-2 in a poorly ventilated restaurant, Building and Environment , 196 (2021), 107788.
2. Underfloor Air Technology Home Page (2000). Centre for the Built Environment – Underfloor Air Technology. [online]. Available at: < https://cbe.berkeley.edu/underfloorair/>. [Accessed on 20 April 2022.]
3. Wikipedia (2022). Underfloor air distribution – Wikipedia. [online]. Available at: < https://en.wikipedia.org/wiki/Underfloor_air_distribution>. [Accessed on 20 April 2022].
4. Accessfloorstore (2019). Underfloor Air Distribution System Guide - Advantages, Applications, Work Principle & What is Underfloor Cooling System. [online]. Available at: <https://www.accessfloorstore.com/news/51--underfloor-air-distribution-system-guide-advantagesapplications-work-principle--what-is-underfloor-cooling-system>. [Accessed on 20 April 2022]
5. Larry Luskay and Dave Sellers (2002), Why the “Simple Underfloor Air Distribution System” Isn’t Quite as Simple as It Seems, Commercial Building: Technologies, Design, Performance Analysis, and Building Industry Trends , pp. 3.225