Waste heat recovery to eliminate the novel coronavirus in HVAC systems

The World Health Organization (WHO) recommends COVID-19 patients to be isolated in effectively ventilated negative pressure rooms with a minimum rate of 12 air changes per hour. According to the US Centre for Disease Control and Prevention (CDC), isolation rooms should have a minimum air change rate  of 6 per hour or 12 per hour for new construction or renovation, along with the use of HEPA filtration of the incoming air, if re-circulated. However, there are no recommended guidelines for other aspects of HVAC systems such as temperature and relative humidity. ^[1] Yet, propagation of the SARS-CoV-2 aerosol depends on the characteristics of HVAC system settings and air circuits.

 

Several studies have concluded that the main criteria for coronavirus elimination are higher temperature and lower relative humidity conditions, which seem to reduce the number of live viruses. Some studies indicate that exposing SARS-CoV-2 to a temperature of 60°C for 5 minutes reduces the viral concentration by more than 99%. ^[2] Unfortunately, this process is not easy to implement in clinical practice. [1]

In a recent experimentation, researchers hypothesized that in cleanroom HVAC systems, SARS-CoV-2 could be eliminated completely by warming the exhaust air before filtration. To solve the problem of SARS-CoV-2 droplets exiting from HVAC systems and spreading outside of hospital buildings they propose to design a heat exchanger to warm the exiting air so that the viability of SARS-CoV-2 can be limited.^[2]

 

A common cleanroom HVAC system consists of three main components: an outdoor air intake and air exhaust ducts and controls, an air handling unit, and air distribution systems. An air handling unit by itself is composed of an HEPA filter, a humidifier, a cooling/heating coil, and ultraviolet light emitters.

 

The proposed cleanroom HVAC system under study is simple to install. The only difference between a common cleanroom HVAC system and the proposed system is applying a longitudinal air to air heat exchanger to the waste heat recovery of the chiller condenser. With the proposed cleanroom HVAC system continuously operating at its optimum efficiency, the longitudinal air to air heat exchanger warms the exhausted air. The longitudinal air to air heat exchanger has a long channel for heat transfer and also contains a copper plate to generate effective heat transfer. The heat sanitizing process occurs inside the longitudinal air to air heat exchanger.

 

The maximum air change rate (ACR) in the designed cleanroom was equal to 23 per hour (520 m3/h). Results from the experimentation show that the system produced exhaust air with a temperature range of 50 °C–80 °C and a relative humidity range of 40%–50%, conditions under which SAR-CoV-2 was observed to rapidly disappear.

Furthermore, it should be noted that cleanroom HVAC systems are also a potential source for heat recovery applications. The authors suggest that drier and warmer air can be converted into electrical energy or even used to dry clothes.

 

Sources:

^[1] Saran, S., Gurjar, M., Baronia, A. et al. Heating, ventilation and air conditioning (HVAC) in intensive care unit. Crit Care 24, 194 (2020). https://doi.org/10.1186/s13054-020-02907-5

^[2] Rezaei, Naser, et al. "A novel methodology and new concept of SARS-CoV-2 elimination in heating and ventilating air conditioning systems using waste heat recovery." AIP Advances 10, 085308 (2020); https://doi.org/10.1063/5.0021575