Technical
cleaner xxv . Potential calculations include those for contaminants to be controlled , space size and layout , noise , air distribution , ventilation , and the amount of clean air needed .
Figure 4 : In-duct UVGI
To determine the dose value requires analysis of the entire system .
Typically , single pass ducted systems are used . In-duct air disinfection systems should be designed to have the desired single-pass inactivation level under worst-case conditions of air temperature ( minimum and maximum temperatures in duct ) and velocity ( maximum allowable duct velocity ) in the irradiated zone .
The worst-case performance to reflect the combined effect of the ( a ) number / power of UV-C fixtures ( b ) air residence time ( which is inversely proportional to air velocity ) ( c ) lamp / ballast characteristics ( including the effect that the wind chill factor and lamp depreciation has on the lamp ). Lamps may be located anywhere in an air duct system provided their position allows for good maintenance access .
Figure 5 : Mobile air cleaning UVGI units ( approximately 3 to 5 ft high )
Advantages
• Suitable for all climates
• Enclosed system
• Treats re-circulated air
• Treats cooling coils to keep clean of mould
• Recommendations and guidelines available
Disadvantages
• Potential occupational exposure and lamp distance
• Expensive to install and maintain in resource-limited settings
• Degrading filter media
• Possible material degradation
MOBILE AIR CLEANING UVGI UNITS Mobile air cleaner units disinfect re-circulated air in occupied spaces and serve the greatest benefit in confined areas where UV-C cannot be used effectively . Larger air cleaners are more effective than small cleaners in small rooms , but both units are impractical and ineffective to high levels of disinfection compared to upper room UV-C in open spaces .
When HVAC equipment does not meet ASHRAE recommendations for ventilation and filtrations , calculations should be performed to select the appropriate mobile air
Figure 6 : Mobile air cleaning UVGI units
Advantages
• Suitable for all climates and ventilation conditions
• Enclosed unit
• Portable
Disadvantages
• Quality can vary
• Can require costly filter media ( if HEPA )
• Possible material degradation RACA
End notes continued on page 50
Using UV light as disinfectant – Part 2 will continue in the next issue of RACA Journal
www . hvacronline . co . za RACA Journal I November 2021 49