UVC deep dive

Get your engineering partner to size the UV-C and consider these 3 main factors.

1) Exposure time (efficacy) - Main factor is the air velocity in the air stream. Typically, you place the UV-C on the wet side of the coil and avoid exposure to casings, plastics and filters. The UV-C will destroy non-metallic components over time. Another good place to install UV-C will be in the return ductwork. The advantage here is that you can treat less air since the main air handler will be diluting the air supply with fresh air but the air maybe moving too fast. This could give you the ability to treat more air with less UV-C. Slow the speed of the air near the UV-C, by enlarging the duct at return air boots. Target under 400 feet per minute or less for the UV-C installation location. Faster the speed, the less UV-C exposure time you will have. Compensate by adjusting the intensity of the UV-C lights. The faster speeds will also cool the UV-C lights and make them less effective.

UV-C inactivates viruses by stopping the replication by disrupting the RNA/DNA molecules. This occurs over time and the more passes of air over the UV-C, the better the inactivation rate will be. Let's say the UV-C, inactivates 90% of the viruses that it comes in contact with, then the next pass we assume that 10% of the air still has active viruses and the UV-C then inactivates 90% of that remaining 10%. The point is that to understand a manufacturer's stated inactivation rates of 99.9% killed, understand how many passes across the UV-C it takes in an hour. With a sick person or a transmitter in a space, there is new generation of virus quanta occurring every minute. So, sizing the UV-C has to be balanced with multiple passes and one must account for the generation of new quanta in a space.

If the interior of the duct has high reflectivity, it will increase the effectiveness of the UV-C. With installations in systems that have sound lining or non-reflective surfaces, there will be lower effectiveness than reflective surfaces.

When you size the UV-C, you must also accommodate and plan for variable air velocities and ensure the UV-C is still effective at various air flows.

2) Light intensity

There are several types of lamps that will generate UV-C: low and medium pressure mercury lamps, pulsed xenon air lamps, LED's and krypton-chlorine excimer lamps. The most common lamps are the low pressure mercury lamps that work by passing an electric charge through a rare gas like argon or kryton-chlorine. The electric input power generates radiant energy in the from UV. Roughly 45% of that energy falls in the UV-C band of 253.7 and the next major emission is in the 184.9 nm range. This later range is not as effective in virus inactivation. Confirm the glass of the lamps absorbs the UV light in ranges above 280 nm. Most lamps do this as a matter of course. But we do not want the higher band UV entering the air stream as it can generate ozone.

The light intensity can vary based on your lamps, your distance from lamps to air, and air velocity. Based on IES guidance from studies on dosing, specify 17 mW of 254 nm lamp radiant power per cubic meter of air. You should create a detailed calculation and compare it to the IES guidance to see where you stand. Another benchmark unit you will encounter is microwatts/cm^2. Here be between 1200 and 4000 maximum, but work with the manufacturer.

https://www.ies.org/standards/committee-reports/ies-committee-report-cr-2-20-faqs/

3) UV-C bandwidth range

The best wavelength is the 253.7 nm UV-C. The beautiful research done in 1930, by Gates (Gates' original bactericidal action spectrum for Bacterium coli,a modern germicidal action spectra and the relative output of a low-pressure Hg germicidal lamp) still applies today.

For more detailed background and the credit, visit NCBI's decade old research paper that still applies.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2789813/