How UV-C energy works in HVAC applications: Part 3
The last installment of this three-part series describes how UV-C lamps are applied in HVAC systems to clean cooling coil surfaces, drain pans, air filters, and ducts to attain and maintain “as-built” capacity and indoor air quality.
The first two parts of this series covered the nature of UV-C light and how lamps similar to fluorescent lamps harness UV-C light.
This final installment explores how UV-C lamps are applied in HVAC systems to clean cooling coil surfaces, drain pans, air filters, and ducts to attain and maintain “as-built” capacity and indoor air quality.
Three tiers of benefits
UV-C systems provide three levels of benefits when applied to HVAC systems.
Level 1—HVAC system efficiency: UV-C 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” capacity, 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, including 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% to 25% of HVAC energy use.
To receive these benefits, engineers need to apply simple methods of sizing, selection, and specifying a UV-C system during installation design. Contractors must correctly install the UV-C system, and facility staff must change the lamps annually and possibly perform other routine service. These activities can be grouped into the lifecycle phases of system design, installation, activation/commissioning, and operations and maintenance.
This article covers the lifecycle phase of system design, which includes sizing/selection of lamps, specifying the installation configuration and equipment, and selecting and specifying the controls.
Sizing, selection, and specification
For a complete design solution, engineers need to determine:
- How much UV-C energy is needed to “do the job”
- The lamp/ballast characteristics required to meet the individual application’s operating conditions
- The required quantity and configuration of lamps needed.
In its 2011 ASHRAE Handbook, Applications, Chapter 60.8, 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 as a “minimum” across the entire coil surface, including plenum ends and corners.
These engineering units, however, are unfamiliar to most practitioners. In lighting applications, sizing will generally resolve to lamp Watts. One accurate way to convert microwatts to lamp Watts is to use a form-factor translation consisting of a 1 sq meter surface with a 1-meter-long lamp located midway up the surface on a horizontal plane. The average lamp Watts and output of lamp manufacturers’ published data shows that a 1 meter, high-output (HO) lamp is rated at 80 lamp Watts with an output of 245 µW/cm2, at 1 meter distance (i.e., lamp surface to coil surface). UV-C lamps are usually installed at 12 in. from the coil surface, so the irradiance needs to be interpolated for that distance. Using the industry-accepted “cylindrical view factor model,” the resulting irradiance is 1375 µW/cm2.
While this number seems to be more than enough to meet the 100 µW/cm2 recommended by ASHRAE, all operating conditions must first be taken into account. Some conditions effectively lessen or “de-rate” the performance of the lamps, such as air temperature and velocity. In fact, changes in these variables can positively affect design performance. In typical conditions of 500 fpm velocity and 55 F air temperature, lamps are de-rated by about 50%. Hence, the 1375 µW/cm2 generated from a conventional high-output 80 lamp Watt bulb would now yield a dose irradiance of closer to 688 µW/cm2—at 12 in. from the coil surface (Figure 1).
The next consideration factor is distance of the UV-C lamp to the plenum corners. The Kowalski view factor on the 1-meter example (Figure 1) shows this to be 25% of the highest mean value. Following through our earlier example, 688 µW/cm2 is multiplied by 0.25, which results in 172 µW/cm2 at the farthest points, or corners of the plenum.
The good news? UV-C dosage is increased based on reflectivity from the plenum’s surface, or the amount of UV-C energy bouncing off of the top, bottom, and sides of a plenum toward the coil and elsewhere. Reflectivity sends UV energy everywhere to assure “all” surfaces are clean and disinfected. Different materials have different reflectance multipliers, as shown in Table 1. Using a galvanized steel plenum as an example, the multiplier is 1.50 (a 50% increase in UV-C energy); hence 172 µW/cm2 x 1.50 = 258 µW/cm2.
Even without considering reflectivity, the ASHRAE minimum UV-C dosage levels would be achieved at the farthest distance from the lamp to the coil. So, should less light be used? Because more light positively affects airborne microbial kill levels and because there is no significant cost savings for trying to use fewer or less-intense UV-C lamps, the 80-Watt HO lamps are highly recommended.
By working through the 1-meter example, the results can be used for future UV-C lamp installations as follows. The lamp was a 1-meter-long, 80-Watt HO lamp, irradiating a 1-sq-meter surface, or 10.76 sq ft. If the lamp wattage is divided by the square footage of the surface, it becomes (80/10.76) = 7.43 Watts/sq ft of coil surface area. This simpler method exceeds ASHRAE’s recommendations of 100 µW/cm2 at the farthest point, under typical operating conditions, when the lamp is located 12 in. from the coil surface!