Practices

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How HVAC systems help to reduce the airborne transmission of COVID-19

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by Adil Rasool (Virkraft Engineering)

The purpose of HVAC systems is to provide and maintain environmental conditions, including proper airflow, relevant humidity, heating, and cooling in a specific area to ensure thermal comfort and indoor air quality. In the context of recent threats, special attention should be paid to the built environment. People usually spend their 90% time indoor; it is essential to understand the potential transmission dynamic of COID-19 within the building environments. The building's environment plays an important role in infection transmission. Dry indoor air was associated with higher rates of infectious and inflammatory diseases in the building occupants.

 

Airborne transmission through small particles, which may stay airborne for hours and can be transported long distances. These are also generated by coughing and sneezing and talking. Small particles (droplet nuclei) form from droplets that evaporate and desiccate. The size of a coronavirus particle is 80-160 nanometre2, and it remains active for many hours or a couple of days. Such small particles stay airborne and can travel long distances carried by airflows in the rooms or in the extract air ducts of ventilation systems.

 

Figure WHO reported exposure mechanisms of COVID-19 SARS-CoV-2 droplets

 

Airborne transmission in the closed complex, shopping malls grocery store is a huge problem for all of us. Malfunctioning systems can greatly facilitate the spread of viruses and bacteria. The virus will be present in the ecosystem and the only effective method to limit its spread by proper environmental conditions.

 

The transmission through the air should be controlled. To reduce the airborne exposures, it's necessary to change the building HVAC operations. Proper ventilation can reduce the concentration of viruses in the air and, therefore, the risk of transmission reduced. HVAC filters, along with other strategies, like flow rate and relevant humidity may help to reduce virus transmission while removing other air contaminants that may have health effects.

 

If there is a lack of good ventilation, its range is up to 10 m because central exhaust and air conditioning systems are very often used are based on recirculation. There are solutions that significantly reduce the spread of germs guaranteeing good air quality and ensuring thermal comfort. 

 

  1. Operate air handling units with 100% outdoor air
  2. Increase outdoor air ventilation to maximum, it will increase the effective dilution ventilation per person.
  3. In buildings with mechanical ventilation systems extended operation times are recommended.
  4. Disable demand-controlled ventilation.
  5. Increase the central air filtration at least to the Minimum Efficiency Rating Value 13 (MERV-13), or High-Efficiency Particulate Air (HEPA)
  6.  Keep toilet ventilation 24/7 in operation
  7. Maintaining an indoor relative humidity between 40-60%
  • When RH is in this optimal range, infectious aerosols released from a sick person quickly settle out of the air and can be wiped away from surfaces.
  • Many viruses and bacteria carried in droplets are less infectious in this midrange RH zone. Conversely, when indoor air is dry, their infectivity is higher. This goes against many people’s intuition.
  • Relative humidity between 40-60% is the range that optimizes the ability of our immune system to fight viral and bacterial infections. When indoor relative humidity is lower, our respiratory immune system is less able to protect us from infectious microbes.

Below is the figure showing humidity impact according to different kinds of microorganisms, a decrease in bar width indicates a decrease in effect optimum zone. The optimum Humidity is 47%.

Ventilation systems should be properly evaluated in shopping malls, as the risks in those areas are much higher. Although not every shopping mall has the same design and content in terms of air conditioning and ventilation systems. We can summarize generally used systems as follows;

 

Usually, atrium or common areas in the closed shopping malls are predominantly resolved with air handling units or package air conditioners, has the use of fresh air. Because of the high human traffic in these common areas these devices usually to be selected in order to provide 100% fresh air when required.

 

But in the stores, indoor units usually placed predominantly heat pumps or fan coils. These devices work mainly with indoor air. However, each store also has a fresh air system fresh air that is fed around 10-20%. The only thing that can be done in the stores is to place the UVGI lights in the return duct of the indoor unit, so in this way, it will be possible to remove the virus up to 99.9% inside the stores. The selection of UVGI lights should be done individually on the basis of the volume flow rate, velocity, and duct size of the indoor unit.

 

Depending on the application (flow conditions, disinfection goal), the required fluence maybe 100s of µW/cm2.  UVGI lights should not be exposed to a human. It is necessary to use UVGI lights in all closed premises where no possibility to deliver %100 fresh air.