How UV-C energy works in HVAC applications: Part 3
After determining how much light is needed, engineers need to select the types of lamps that will provide the necessary light energy. Among the considerations are single-ended (Figure 2) and double-ended lamps (Figure 3). Double-ended lamps are used in specific length configurations and may confine the design in certain air handling units (AHUs). Single-ended lamps provide a lot of flexibility relative to a given plenum’s width because they can be overlapped (Figure 4). Single-ended lamp fixtures can also be used in hard-to-access plenums and smaller rooftop units, as they are installed and serviced from outside the plenum (Figure 5).
Another consideration is whether to use PTFE encapsulation for safety. Encapsulated lamps trap the glass and mercury within a protective envelope should the lamp be broken. In most all applications there is a risk of lamp breakage. Encapsulation is recommended because the cleanup procedures for broken lamps can be extensive. Otherwise, guidelines for handling broken lamps can also be found in the 2011 ASHRAE Handbook – Applications, Chapter 60.
When using single-ended lamps, lamps of a single length can often be selected for the entire facility. This minimizes the number of spare lamps that must be kept on site, and it increases the purchasing power for buying in bulk when re-lamping on an annual schedule. As mentioned, this approach simply overlaps lamps and eliminates having to have combinations of sizes to get a perfect fit from one end of the coil bank to the other.
For a complete UV-C installation design, engineers may want to specify certain other aspects of their design. This could include the calculated distance of 12 in. from the coil, and a lamp holder that will assure that the lamps are properly held and can be easily replaced. The installation design should also specify the required electrical power. Ballasts today are typically offered in 120-277 Vac designs for flexibility.
UV-C systems have relatively simple controls, most of which pertain to safety. A typical control package includes a cutoff switch located just outside the UV light installation’s plenum door. Also included in that control circuit are the door interlock switches that turn off the lights when an access door is opened. Access doors can also be equipped with a view port to facilitate lamp inspections.
Another traditional control option is the radiometer, which can display lamp operating hours and a relative indication of UV-C output. However, radiometers can only monitor one lamp, and if that lamp stays on while others have failed, the measure may be meaningless. Also, lamps are much more reliable today and only lose as little as 15% of their initial output during 9,000 hours of operation, so the radiometer has lost favor.
Simple, self-powered current sensors that show whether a particular lamp/ballast combination is on or out are in greater demand today. Multiple lamp/ballast sensors can be fed into a replicator that allows one signal to the building management system (BMS) to represent up to eight lamp/ballast combinations (Figure 6). They also can be chained together to represent an infinite number of lamp/ballast combinations with one signal. Additional programming can be added to alert operators if a lamp or ballast is out, which eliminates the need to visit each AHU to check for failures, especially as the 9,000-hour useful life expectancy window approaches.
When controls are designed into the UV-C system, commissioning providers need to check that they are documented appropriately and functioning properly.
UV-C light is an incredibly effective and affordable technology for keeping critical components of commercial HVAC systems clean and operating to “as-built” specifications. Benefits of applying UV-C lamps in HVAC systems include greater energy efficiency, lower operating expenses, fewer occupant complaints, and better IAQ.
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.