Mechanical ventilation system for Post COVID Era

30 Jun 2021

COVID-19, the disease caused by the SARS-CoV-2 virus, a pandemic declared by the WHO, brought into focus, rather sharply, on not only how quickly but also how effectively we are creating & adapting the ‘new normal’.

HVAC systems can have a major effect on the transmission of infectious microscopic airborne particles and contaminated aerosol droplets from being the primary hosts to the secondary hosts. Ventilation, filtration, and air distribution systems and disinfection technologies have the potential to limit airborne pathogen transmission through the air and thus break the chain of infection. Transmission of SARS-CoV-2 through the air is sufficiently likely that airborne exposure to the virus should be controlled. Dilution and extraction ventilation, airflow distribution and optimization, mechanical filtration, ultraviolet germicidal irradiation (UVGI), and humidity control are effective strategies for reducing the risk of dissemination of infectious aerosols in buildings and transportation environments.

Most of the people have opined to restrict use of Air conditioning to increase cross ventilation.

Various industries / institutions like ASHRAE (American Society of Heating, Refrigerating and Air-Conditioning Engineers), ISHRAE (Indian Society of Heating, Refrigerating and Air-Conditioning Engineers), REHVA (Representatives of European Heating and Ventilation Associations), BlueStar, JLL etc. have opined to remodel the air-conditioning system in the new normal. As per McKinsey analysis, filtration, irradiation, thermal and ionisation are the various methodologies to improve indoor air quality (IAQ), in which air filtration is the most common and preferred technique followed by irradiation.

Improvisation in air filtration

The filters of the air handling units (AHUs), depending on their filtration media & efficacy, restrict or capture the microscopic airborne particles and contaminated aerosols.

As per the above chart from Azimi & Stephens, Building & Environment 70 (2013) 150-160, MERV 13 has nearly 14 per cent lower influenza infection risk as compared to the usual MERV 7 filters, while the change from MERV 13 to HEPA would barely reduce the infection risk by 4 per cent. Further, it needs high investments. Thus, MERV 13 (melt-blown, non-washable) is more suitable filter for AHUs and MERV 5 & MERV 8 filters before MERV 13 help to reduce the dust loading and increase the life of MERV 13 filter.

Sterilisation of AHU Coils with UVGI

In HVAC systems, UVGI (ultraviolet germicidal irradiation) is being increasingly used for surface sterilization of cooling coils and condensate drain pans and as a supplement to filters for air purification. Surface disinfection refers to either the disinfection of building & ventilation system internal surfaces. Contaminated surfaces are often a source of airborne microbes, and airborne microbes often produce surface contamination. The disinfection of cooling coil surfaces removes mold spores from the coils and prevents subsequent aerosolization, thereby helping keep the air clean. Ultraviolet light has been demonstrated to be capable of destroying viruses, bacteria and fungi.  UV of wavelength (254 nm) i.e. UVC is effective in penetrating the cell membrane, breaking the DNA structure of a microorganism. DNA sterilisation inhibits reproduction. There are a number of factors to be considered that affect the deployment of UVC viz. residence time, intensity, reflection, temperature etc.

Air Change Rates

Air change rate represents the number of times the entire volume of air in the room is replaced each hour. One of the methods is to infuse more fresh air into the occupied spaces. Fresh air is a natural disinfectant with rich natural oxidant properties that act as a barrier against the coronavirus and other pathogens. Higher number of air changes of ventilation per hour reduces the exposure to microbes.

Similarly, certain quantity of air is recirculated in the space through the air filtration system of the air handling unit. The number of times the air gets churned through the filtration system results in more of the particulate matter and the microbes getting filtered out of the air. This recirculation of air is also expressed in air changes per hour. System needs to be reconfigured to ensure 8-10 ACPH (recirculation) rates to improve filtration efficiency.

Enhanced Cleaning of Air Conditioning components

The methodologies and frequency of cleaning of filters, coils and distribution network needs to be relooked and improved. Robotic duct cleaning of air duct helps to eliminate the particulate contaminants that may act as potential carriers of the virus, to maintain healthy indoor environment and also enhance the life of fine filters (MERV 13). It is essential to also clean AHU coils to render them more effective.

Further, maintaining humidity between 40 per cent and 70 per cent and temperature from 24°C to 27°C helps to activate the virus killing properties in the air.

Results: We, at Godrej & Boyce, reengineered our central air-conditioning systemsin a similar context as aforementioned and appointed an accredited (ISO/IEC 17025) laboratory for pre & post measurement of outcomes. There is a drastic reduction of 84% and 90% in fine particles and microbes (at rest condition) respectively, which are responsible for transmission/spread of virus through air conditioning.

Conclusion: The project outcomes have been recognized by CII & showcased in National level publications viz. Air Conditioning & Refrigeration Journal by ISHRAE and Thermal Control Business update. Industries have started to see it as a benchmark and accepted the same. Provision of clean air acts as a bio-weapon in the fight against the virus.

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