How UV-C energy works in HVAC applications: Part 2
Part 1 of this three-part series covered the nature of UV-C light and how UV-C is harnessed by lamps, which are used in HVAC systems. This installment discusses how lamps similar to fluorescent lamps generate UV-C.
UV-C lamps and lamp replacements
Modern UV-C lamps are very similar to fluorescent lamps typically found in ceiling fixtures. Both types of lamps are manufactured on fluorescent lamp machines in similar form factors (lengths and diameters), and they 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.
UV-C lamps differ slightly from their fluorescent counterparts in that the UV-C lamp’s glass envelope is a highly engineered, UV-C transparent glass. This allows the 253.7 nm wavelength to transmit through the lamp envelope unfiltered. Fluorescent lamps, however, use ordinary glass that is coated with phosphors on its interior surface. The UV-C energy is contained to excite the phosphors to glow (fluoresce) in the visible light range.
That being said, what gives UV-C lamps their characteristic blue hue, as shown in Figure 1?
A typical UV-C lamp produces about 90% of its energy in the UV-C wavelength. About 4% of its energy is given up as heat, and the rest (~5%) is in the visible light range that is medium blue in color. This blue color results from the argon gas in the enveolpe.
The similarities between UV-C lamps and fluorescent lamps providemany benefits. They 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% 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 the lamps begin to fail in larger numbers.
Like fluorescent lamps, UV-C lamps come in a variety of types and sizes, including single-ended and double-ended lamps. The 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.
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. Typically, all 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 polytetrafluoroethylene (PTFE) 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 encapsulate.
In the last installment of this three-part feature, learn how UV-C lamps are applied within HVAC systems to clean cooling coil surfaces, drain pans, air filters, and ducts to attain and maintain “as-built” airflow conditions and indoor air quality.
Forrest Fencl is president of UV Resources. He is the writer or co-writer of 15 patents, is an ASHRAE Fellow, and formerly an ASHRAE Distinguished Lecturer. He has authored numerous papers and articles and several ASHRAE Handbook chapters related to ultraviolet air and surface treatment.