May 14, 2020 The Science Behind UV-C Energy: UV In HVAC Systems
Three tiers of benefits
UV-C systems provide three levels of benefits when applied to HVAC systems.
Level 1—HVAC system efficiency: UV-C energy eliminates and/or prevents the buildup of organic material on the surfaces of cooling coils, drain pans, and interior duct surfaces. This improves airflow, returns and maintains the heat-transfer levels of cooling coils to “as-built” conditions and reduces maintenance.
Level 2—IAQ: UV-C improves airflow levels and eliminates organic material on surfaces, which helps improve indoor air quality (IAQ) by reducing pathogens and odors. This improves occupant productivity, boosts comfort levels and reduces sick time.
Level 3—Economic impact: The impact that UV-C has on mechanical systems and occupants translates into substantial economic benefits. These include reductions in energy consumption, energy cost, and carbon footprint; reductions in hot/cold complaints and maintenance actions associated with complaints; reductions in system downtime and staff time needed for chemical or mechanical cleaning; and increases in occupant satisfaction and productivity. On average, UV-C slashes 10 percent to 25 percent of HVAC energy use.
Sizing, selection, and specification
In order to achieve these benefits, engineers need to determine:
(1) How much UV-C energy is needed to “do the job”
(2) The lamp/ballast characteristics required to meet the individual application’s operating conditions
(3) The required quantity and configuration of UV lamps needed.
In its 2015 ASHRAE Handbook, Applications, Chapter 60.1, ASHRAE Technical Committee, TC2.9, established minimum irradiation levels of 50-100 µW/cm2 (microwatts per square centimeter) for cooling coil applications. This requirement must be met across the entire coil surface, including plenum ends and corners.
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 and double-ended lamps. 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. Single-ended UV lamp fixtures can also be used in hard-to-access plenums and smaller rooftop units, as they are installed and serviced from the plenum exterior.
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.
Controls
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 be equipped with a view port to facilitate lamp inspections.
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. 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.
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. UV light in HVAC effectiveness translates to greater energy efficiency, lower operating expenses, fewer occupant complaints and better IAQ.
