May 13, 2020 The Science Behind UV-C Energy: How UV Light Works
Modern UV lighting for HVAC systems resembles fluorescent lamps typically found in commercial office buildings. Both types of lamps have similar form and operate using identical electrochemical processes: an electric discharge through argon gas strikes mercury vapor to generate a photon with a wavelength of 253.7 nm (typically called UV-C), which is invisible.
Unlike fluorescent lamps, however, UV-C lamps’ glass envelope is a highly engineered, transparent glass, allowing the 253.7 nm wavelength to transmit unfiltered (fluorescent lamps use ordinary glass that is coated with phosphors on its interior surface).
A typical germicidal ultraviolet light for heating and cooling systems produces about 90 percent of its energy in the UV-C wavelength. About four percent of its energy is given up as heat, and the rest (~5 percent) is in the visible light range that is medium blue in color, which results from the argon gas in the envelope.
The similarities between UV-C lamps and fluorescent lamps provide many benefits. Both can be constructed on the same type of machine and in the same form factors, reducing manufacturing, packing, and shipping costs to offset much higher material costs. They can also be stored and recycled in the same manner. UV-C lamps are typically warranted to provide more than 80 percent of their initial output over a 9,000-hour period. Because UV-C lamps should be operated continuously, the corresponding 8,760 hours of a 24/7 schedule also fits conveniently into annual re-lamping schedules.
Attempting to run UV-C lamps longer than 9,000 hours produces individual lamp outages, so maintenance staff must monitor them routinely to know what to replace. Replacing lamps as they burn out requires a larger inventory of replacement lamps for when they begin to fail in larger numbers.
UV Lamp Types
- Single-ended lamps have all of the starting and ending terminals (pins) contained in the lamp base. They are used in several lamp systems, some of which allow the lamps to be inserted through a plenum or duct into the airstream, typically downstream of the cooling coil. Sometimes, this insertion into the air-flow stream enables 360-degree irradiance.
- Double-ended lamps have pins at both ends, come in many varieties, and are installed into specific length fixtures usually containing the ballast like a fluorescent fixture.
Both lamp types are available in high output (HO) and standard output (SO) varieties. The difference between them is their Watt rating and ballast size. HO lamps are usually recommended because they are less expensive on a per-lamp-Watt basis.
Another consideration is opting for encapsulated lamps, which have a transparent fluorinated ethylene propylene (FEP) coating over the glass envelope. Encapsulated lamps hermetically seal UV-C lamps in case of breakage. Should an accident occur, broken glass and mercury will remain within the lamp protective shell.
The next part of this series will discuss UV light in HVAC effectiveness and how the lamps can clean cooling coil surfaces, drain pans, air filters, and ducts to attain and maintain “as-built” airflow conditions and indoor air quality. It will also discuss some frequently asked questions about the UV light and UV air.