Forrest Fencl
Forrest Fencl

UV-C lamps are a simple, cost-effective, and highly reliable technology that keeps HVAC systems running at ‘as-built’ conditions. Application guidance is found in ASHRAE’s handbooks, “Air Quality Guide,” and other technical articles. This article serves as an introductory guide for sizing and selecting UV-C systems for HVAC applications.

UV-C disassociates elemental bonds in organic matter, degrading them similarly to how UV light from the sun degrades vinyl and rubber, etc. UV-C eliminates biofilms, mold and fungi, and other organic contaminants on coils, drain pains, and plenum interiors.

In new systems, such buildups won’t form because of the continuous UV-C irradiation. In retrofit applications, UV-C degrades the organic matter that has accumulated over time, and then prevents it from returning. It also penetrates all the way through coils to eliminate the build-up that mechanical and chemical washing often misses.

Although UV-C is simple, many contractors are mystified about how UV-C works and how to apply it cost-effectively. These aspects and other misunderstood issues are addressed here by using the guidelines mentioned above.

UV-C Basics

All of us are familiar with the harmful effects of sunlight’s UV-A and UV-B, such as sunburn, and yet the UV-C wavelength has more than twice the energy, and it is well absorbed (not reflected) by all organic substances, increasing its ability to destroy molecular bonds. We don’t protect ourselves from UV-C because it is filtered out by the ozone layer and atmosphere, and doesn’t reach earth.

A 2010 study commissioned by ASHRAE found that even the most sophisticated organic compounds suffer from exposure to UV-C. Because UV-C operates 24/7, a well-distributed dose is all that’s needed, using no more energy than commercial fluorescent lights.

UV-C Lamps and Replacements

Fluorescent and UV-C lamps are manufactured on similar machines; however, UV-C lamps differ in that their envelope consists of a highly engineered glass to allow the UV-C wavelength to escape the envelope unfiltered. UV-C lamps’ characteristic blue hue is from argon gas, as shown in Photo 1.

UV-C lamps provide 80 percent of their initial output over a 9,000-hour operating period, meaning the 8,760 hours of a 24/7 schedule fits nicely into annual re-lamping schedules.

UV-C lamps come in single- and double-ended types. Single-ended lamps offer more design flexibility and are favored. Double-ended lamps require specific length fixtures and are harder to fit in. Both are available in standard output (SO) and high output (HO), with the difference being their watt ratings. HO lamps are less expensive on a per lamp-watt basis.

Another consideration is encapsulated lamps, which have a Teflon coating. This seals and contains the broken glass and mercury in case of breakage.

UV-C Benefits

UV-C systems provide HVACR system efficiency, occupant comfort and IAQ, and environmental, and economic impacts.

System efficiency: UV-C eliminates and/or prevents the buildup of organic material on coils, drain pans, and interior air-handler surfaces to improve airflow and to return/maintain heat-transfer levels to “as-built” capacity. This means less energy is needed to provide the necessary amount of cooling and airflow to maintain/return system energy efficiency. UV-C installed in older systems reduces energy use by 10-25 percent on average.

Comfort and IAQ: Clean systems don’t contribute to foul odors, allergens, or pathogens in airstreams. They help sustain design temperatures and airflow. This translates to performance communicated by codes, standards, and the owner’s project requirements to deliver comfort, IAQ, occupant productivity, lower incidences of sick days, reduced hot/cold calls, and other service requests.

Environmental impacts: UV-C is a green technology that can eliminate chemical and mechanical (water) cleaning to reduce waste disposal issues. Efficient air-handler units not only save energy — they reduce carbon footprints.

Economic impact: Reducing energy costs, sick calls, service calls, and system downtime for maintenance translates into significant cost savings. Buildings with highly functioning HVAC systems also increase the value of building tenant leases by lowering building occupant turnover.

UV-C Life Cycle

To realize these benefits, a UV-C system is engineered, installed, operated, and maintained — just like other systems. The 2011 ASHRAE Handbook, Chapter 60.8, recommends 50-100 µW/cm2 (microwatts per square centimeter) for coil applications. This amount is also the threshold across the entire coil surface, including ends and corners.

Microwatts are unfamiliar to most of us. In lighting, sizing generally resolves to lamp watts. Converting microwatts to lamp watts is done using a form-factor translation of a 1-square-meter surface with a 1-meter-long lamp located midway up the surface on a horizontal plane. Average lamp watts and output from lamp manufacturers’ published data shows that a 1-meter, HO lamp is rated at 80 lamp watts with an output of 245 µW/cm2, at 1 meter distance from a lamp to a surface. UV-C lamps are usually installed 12 inches from the coil, so the irradiance needs to be interpolated for that distance. Using the industry-accepted cylindrical view factor model, the resulting irradiance is 1,375 µW/cm2 at 12 inches.

This number is more than the 100 µW/cm2 recommended by ASHRAE, but other operating conditions must be taken into account. Air temperature and velocity de-rates lamp performance and reflection positively affects it. In a 500 fpm, 55°F airstream, lamps are de-rated by about 50 percent. So, the 1375 µW/cm2 would now be 688 µW/cm2 — at 12 inches from the coil surface.

Next is the distance to the plenum corners. The View Factor model shows this to be 25 percent of the highest mean value. From the earlier example, 688 µW/cm2 is then multiplied by 0.25, which results in 172 µW/cm2 at the farthest corners of the plenum.

Dosage increases by the amount of energy bouncing off of the top, bottom, and sides of a plenum toward the coil. Reflectivity sends UV-C everywhere, assuring that all surfaces are clean and disinfected. Different materials have different reflectance multipliers. Using galvanized steel as an example, the multiplier is 1.50 (a 50 percent increase); therefore 172 µW/cm2 x 1.50 = 258 µW/cm2.

Even without reflectivity, recommended UV-C dosages are achieved at the farthest distances, so, should lower dosages be used? Because more energy provides a greater airborne kill ratio of infectious microbes (colds, flu, etc.), and because no significant cost savings exist for using fewer microwatts, it is not recommended.

The example results can be used for future UV-C lamp sizing as follows. Using a 1-meter-long 80-watt HO lamp on a 1-square-meter (10.76-square-foot) surface allows the lamp wattage to be divided by the square footage, or (80/10.76) = 7.43 W/square foot. This simpler < 7.5 W per square foot exceeds ASHRAE’s recommendations and can be used as a guideline on most any size coil.

After determining watts, one can determine the type of lamp. There are single- and double-ended lamps. Double-ended fixtures confine the design in most air-handler units. Single-ended lamps, even of a single length, provide more flexibility for a given plenum’s width because they can be overlapped. They also can be used in hard-to-access plenums such as rooftop units, as they are installed and serviced from outside the unit.

Single-ended lamps of a single length minimize the number of spare lamps and increase the purchasing power for re-lamping. This approach also eliminates the requirement of size combinations to obtain a perfect fit.

Installation and Controls

Once the 7.5 watt per square foot calculation is achieved and a single-ended lamp of a fixed length is chosen, using a lamp holder that assures lamps are properly held and easily serviced is needed. Also, consider the electrical power as ballasts today are offered in 120-277 vac.

UV-C systems use simple controls such as a cutoff switch just outside the UV plenum’s door and door interlock switches that turn off the lights when any access is opened. Doors can also be equipped with a view port for lamp inspections.

Simple, self-powered current sensors can show whether a particular lamp/ballast combination is on or out, and multiple lamp/ballast sensors can be fed into a replicator that allows one signal to the building management system (BMS) to represent any number of lamp/ballast combinations. This can alert operators if a lamp or ballast is out, which eliminates the need to visit each air-handler unit to check for failures, especially as the 9,000-hour life window approaches.

Facility staff should be trained on UV-C systems, and commissioning providers need to check that systems are documented appropriately and functioning properly and safely.

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 include greater energy efficiency, lower operating expenses, fewer occupant complaints, and better IAQ. UV-C is relatively easy to apply — it’s basically installing a bank of UV-C lamps in an air handler, or in a rooftop system, and then replacing the lamps once per year.

Publication date: 7/14/2014

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