CovidSAFE HVAC

 

There's no HVAC system that's CovidSAFE, quite simply because they move air from one location to another. Other than cleanroom air handling systems of course.

Whilst we cannot turn HVAC systems in commercial office, retail, hospitality and healthcare building into "cleanrooms", it's possible to borrow some ideas from cleanrooms and adapt them to help retard the spread of contagious pathogens.

In recent times we've learnt much about how this virus spreads. Especially from the cruise ship sector and quarantine hotels.

Recapping quickly. Coronavirus is contracted from surfaces that contain active virus particles. It's also contracted from aerosolized particles that are suspended in the air. The greater the number of active particles that a person is exposed to, the greater the probability of that person being infected. Hence, the rate of virus transmission is greatest in poorly ventilated enclosed spaces, such a cruise ship cabins without balconies, small densely populated restaurants and low budget hotels.

The bulk of the legacy cruise ship fleet uses ducted air conditioning. This involves each cabin being connected via ducting to a centralized air heating, cooling and filtration units. The centralized air handling unit will also facilitate a some fresh outdoor air being introduces to replace the stale air circulating in the systems. Forced fresh air introduction also ensures acceptable carbon dioxide levels are maintained. The reason the bulk of the air is re-circulated is to reduce heating and cooling energy consumption. However, re-circulation also facilitates the rapid distribution of any airborne substances, especially when the provisions to filter out those substances are poor. More modern cruise ships do employ air filters that are of higher efficiency than the filters we have in our home a/c systems. In response to the lessons learnt, cruise ships are now being equipped with HEPA, high-efficiency particulate absorbing filters that are capable of removing coronavirus to an efficiency of at least 99.9%.

Ducted System Multi Split-cycle System

 

Whilst these days most buildings use split-cycle air conditioning systems, many older commercial buildings, including some hotels still employ ducted air conditioning similar to that described above for cruise ships.

 

The advantage of split-cycle systems is that they don't convey air between rooms. They instead convey a heating/cooling fluid to finned heat exchange coils located within the room. Air within that room is then circulated through the coils with aid of a fan.

The disadvantage of split-cycle systems is that they have no or extremely low efficiency air filters. Units that do have filters are unfortunately inadequate at removing viruses. A further disadvantage of some split-cycle systems is no or negligible provision of fresh intake. Many of these installations rely on unassisted delivery of fresh air from the room's window and or door openings. As such, they are prone to situations where the concentration of virus particles builds up exponentially.

Hotels where this exists must never be used for quarantine purposes. Hotel rooms with balconies is somewhat safer. Most modern hotels that use split cycle air conditioning systems and that have no opening windows or balcony now included a duct that draws a small amount of air from the corridor immediately outside the room.

 

This method of drawing in air from corridors into outside room is a possible virus transmission pathway. Especially in cruise ships and hotels.

Now that we understand the risks associated with various deployments of air conditioning systems, we're in a better position to deliver solutions that provide better protection and reduced transmission of the virus.

The same strategies that we're now developing for CovidSAFE HVAC systems in cruise ships, hotels and hospitals can also be deployed in other commercial buildings, such as apartments blocks, offices, conference centers, retail outlets, restaurants, shopping centers, airports, gymnasiums, exhibition centers, libraries, warehouses and factories.

Solutions

HVAC systems are part of the virus transmission problem. However, with relatively minor alterations, such as the inclusion of UV-C germicidal disinfection lighting, HEPA filters and or substantially increasing the amount of fresh outdoor air, HVAC system can become part of the solution.

ASHRAE

Last March in response the Covid-19 pandemic, The American Society of Heating, Refrigerating and Air-Conditioning Engineers issues supplimental recommendations, including;

• Building designers, owners, and operators should give high priority to enhancing well- designed, installed, commissioned, and maintained HVAC systems with supplemental filtration, UVGI, and, in some cases, to additional or more effective ventilation to the breathing zone. Filtration and UVGI can be applied in new buildings at moderate additional cost and can be applied quickly in existing building systems to decrease the severity of acute disease outbreaks. Indoor Air Quality Guide (ASHRAE 2009) contains information about the benefits of and techniques for accomplishing these enhancements.

• New healthcare facilities, including key points of entry such as emergency, admission, and waiting rooms; crowded shelters; and similar facilities should incorporate the infrastructure to quickly respond to a pandemic. Such infrastructure might include, for example, HVAC systems that separate high-risk areas; physical space and HVAC system capacity to upgrade filtration; the ability to increase ventilation even as high as 100% outdoor air; the ability to humidify air; and receptacles at the upper room and ceiling heights of at least 2.4 m (8 ft) to enable effective upper-room UVGI. Once the building is in operation, rapid availability of filter elements and upper-room UV fixtures should be arranged for rapid deployment in an emergency.

More outdoor air

In most commercial buildings the amount of outdoor air that is introduced results in the air within the building being changed once to twice each hour. By increasing the number of air changes per hour to more than fifteen serves to dilute the concentration of virus particles and in so doing reduces the probability of someone being infected. It is however the least effective method of protecting people and results in increased running costs.

HEPA Filters

Many existing HVAC systems may not be able to easily accommodate HEPA filters, either due to physical restrictions or sufficient fan capacity to overcome the added flow resistance. However this difficulty should not dissuade building owners from investigating this upgrade option.

UV-C GI

Retrofitting UV-C lighting into HVAC systems is less challenging and does not impose added flow resistance. In certain instances, UV-C light reduces flow resistance as it mitigates the growth of mold and bacteria in addition to killing coronavirus.

 

Studies show that AC coils irradiated with UV-C light, particularly in moderate to highly humid climates, produce a differential pressure reduction in the region of 15%. Heating and cooling transfer efficiencies also improve by about 10%. This produces reduced cooling/heating flows. The overall result being lower fan and pump energy consumption. In additional to energy cost savings, maintenance costs are also reduced.

Whilst these solutions can deliver direct financial benefits to building owners, they also deliver reputational benefits by way of demonstrating real care for the occupants of the building.

Most importantly however is the fact that these measures not only contribute to solving the Covid problem, but they will also address the sick building syndrome problem and thus contribute to improving the health of occupants and help raise productivity.

The timing has never been better the HVAC industry, architects, builders and commercial property owners to play a major role in not only impeding the spread of coronavirus, but also in solving the Sick Building Syndrome problem.

 


More

Ultraviolet Germicidal Irradiation Fundamentals and Indoor Environment Applications

 

Simulation of Covid-19 Aerosol Spread in Office Environment

 

Leap Australia & LCI Consultants explain - HVAC Systems & Pathogen Spread Using CFD

 


 

 

 

 

 

The above article includes images, videos and links from other organisations that at the time of publishing was openly available in the public domain. The author acknowledges, thanks those organisations and claims no rights.

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