By Clive Samuels, P.E.

Supermarkets and food stores are considered essential and have remained open throughout the COVID-19 pandemic. In fact, the sale and distribution of food has remained extremely buoyant throughout this ordeal. Despite the huge growth in home delivery and curbside pickup, direct shopping visits to supermarkets and other food stores by the public have remained intense, despite the controls in place to limit the population density within these facilities and to maintain social distancing.

According to the WHO (World Health Organization), “The COVID-19 virus spreads primarily through droplets from the nose when an infected person sneezes.” However, current research reveals that breathing and talking release airborne micro-particles which carry the virus. With no external air movement, airborne particles can remain afloat for long periods of time, as they drift around propelled by thermal currents and any air-movement that may exist. Closed spaces with poor ventilation can enable these particles to remain afloat for periods sometimes measured in hours.

The Impact of Interior Humidity

Indoor humidity does play a role in distribution and transmission of these virus-laden particles.

“Transmission is greater in dry air, infectivity is higher in dry air, and the ability of a human being to fight infection is impaired,” said Dr. Stephanie Taylor, a graduate of and lecturer at Harvard Medical School. “Any one of those would be important, but all three of them are in play.”

Airborne particles carrying viruses can travel farther in air that is not sufficiently hydrated, and for reasons researchers are still probing, it also seems that viruses appear to be more infectious in lower humidity conditions, and this is exacerbated in colder climates. Buildings should be maintained between 40 and 60 percent relative humidity, particularly during the winter months.  This is an issue for most stores in colder climates where auxiliary heat is necessary to maintain comfortable interior conditions. In these stores, winter humidity levels can be extremely low often well below 20%, depending on the people count and the latent impact of food preparation.

Recommendations for Supermarket HVAC Operations

• Increase the fresh air intake to its maximum capacity within the limitations of the HVAC system design. This increases the effective dilution ventilation per person within the conditioned space.
• Disable demand-controlled ventilation (DCV), and other controls designed to limit the outdoor air quantity based on exhaust hood operation etc.
• If the HVAC system supply air fans incorporate variable frequency drives (VFD), and the fans do not operate at maximum speed continuously, purge the building at least once per hour for a minimum of 10 minutes by increasing the fan speed to its absolute maximum (beyond 60HZ if within the motor’s short term operating parameters). This will increase the air-movement within the store and create flow turbulence destroying the infected particles ability to float endlessly. In addition, this process will maximize the quantity of filtered and conditioned fresh air entering the building.
• Eliminate night setback algorithms which allow the supply air fan to shut down.
• If the HVAC design incorporates low air return, adjust the return air volume to maximize the air being returned beneath the refrigerated cases or from other low return locations. This may assist in creating airflow patterns within the store conducive to forcing the infected buoyant particles to flow downwards towards the low return grilles.
• Maintain the interior relative humidity no lower than 50%. This should have a relatively small effect on the refrigerated display cases performance and energy consumption, particularly with today’s large bias towards closed glass door cases. Investigate installing humidifiers designed to operate during the heating season in an effort to maintain relative humidity values of at least 50%. The application, installation and maintenance of commercial humidifiers should be evaluated carefully.
• Incorporate the most effective filtration possible within each of the store’s HVAC units. If compatible with the filter rack and the unit fan performance and capabilities, MERV-13 or better filters should be installed, and a carefully planned and scheduled filter replacement program enforced.
• Strong consideration should be given to the installation of UVGI (Ultraviolet Germicidal Irradiation) lighting within the primary HVAC air-handling units. This process is designed to reduce pathogens as the air passes around these UVC lights to reduce airborne disease transmission.
• The UVC lamp quantity, location and operation is important for maximum effectiveness as are safety maintenance and other considerations. However, the UVC spectrum has proven to be very effective against viruses, and serious consideration should be given to incorporating these fixtures within the conditioned air path. Further strategically located dedicated UVC fixtures may also help in reducing virus transmission within the store.

It must be noted that direct UVC light itself can be harmful to humans and can degrade organic materials. As such their locations must be evaluated carefully.

New Store Opportunities

Looking to the future, new store designs should consider the following:

• Incorporation of UVGI lighting technologies within the sales area.
• HVAC systems with increased airflow capabilities, greater fresh air capacity and the opportunity for more low air returns.
• UVGI designed and installed within the HVAC units.
• Incorporation of humidification into the sales area HVAC units.
• BMS systems with a function enabled during periods of high virus/bacteria concentrations or in response to pandemics or local outbreaks. The HVAC designs and equipment would be engineered to respond accordingly.
• Built in UVGI technology into refrigerated display cases and walk in cooler/freezers.

While the above are some of the new design parameters retailers should consider when moving to the new normal, each building is unique. We encourage retailers and grocers to contact a design professional to develop a specific plan that works for your building and store operation objectives. Please feel free to contact us to start discussing your solution.