ASHRAE Journal - August 2008 - (Page 34)

A B Open UVGI for High (>9 ft) Ceilings (>2.7 m) Louvered UVGI for Low (8 ft to 9 ft) Ceilings (2.4 m to 2.7 m) Figure 5: Typical upper-air UVGI installations. Figure 5a (left): Wall-mounted fixture in a health clinic. Figure 5b (right): Ceilingmounted fixture in a homeless shelter. levels in occupied spaces (Figure 6). These fixtures inactivate airborne microorganisms by irradiating them as air currents move them into the path of the UV energy. Some louvered fixtures use small fans to enhance air mixing (Figure 5b), which is a critical component of overall effectiveness. Where an in-duct UVGI system may not be feasible, or where additional UVGI is desired to further reduce airborne infectious disease transmission, upper-air systems can provide an effective solution. Application and placement criteria for upper-air UV fixtures are provided in various publications, and manufacturerspecific advice on placement and operations should always be followed. A rule of thumb for upper-air installations has been one 30 W (nominal input) fixture for every 200 ft2 (18.6 m2) of floor space to be irradiated.16 Many effective systems have been designed to this criterion, yet it is important to note that not all 30 W lamps provide the same output of UVC energy. UVC output is dependent on the type of lamp, the lamp manufacturer, and various other factors. Recent studies have suggested installing fixtures to maintain a uniform UV distribution of around 30 μW/cm2 to 50 μW/cm2 in the upper portion of the room.17 While essentially “normalizing” the recommended output over all lamps, this level of irradiance should be effective at inactivating most airborne droplet nuclei containing Mycobacterium, and would presumably be effective for inactivation of most viruses as well. The overall effectiveness of upp:er-air UVGI systems improves significantly when the air within the space is well mixed. Although convection air currents created by occupants and equipment can provide adequate air circulation in some settings, mechanical ventilation systems that maximize air mixing are preferable. If air mixing with mechanical ventilation is not possible, fans can be placed in the room to ensure adequate mixing. General UVGI System Design Parameters Figure 6: Typical upper-air UVGI installation showing wall mounted fixtures. Depending on the ceiling height, louvered fixtures or open fixtures are used to irradiate the air in the upper portion of the room while maintaining UV levels in the lower, occupied space at safe levels. Convection air currents and/or mechanical ventilation moves air through the irradiated zone. Ceiling-mounted fixtures can also be used. Relative Humidity Relative humidity (RH) has no significant impact on the performance of UV lamps, and its effect on the susceptibility of microorganisms (k-value) is not well understood. Attempts to correlate susceptibility of microorganisms to RH have yielded inconsistent results but it appears to be organism specific.18,19 The relationship between RH and k-values seems complex, but most research reported effects only as RH values increased above 70%. It is recommended that UVGI systems be operated below 60% RH, which is consistent with recommendations from ASHRAE and other organizations for providing comfort, acceptable indoor air quality, and minimizing indoor microbial contamination. Most upper-air UVGI systems are operated where the relative humidity is maintained below 60%. Conversely, in-duct systems are frequently operated at higher humidity levels. Depending on the disinfection goals for an induct system, potential effects of high RH levels on inactivation efficiency may need to be explored in more detail. Air Temperature and Velocity In addition to those criteria mentioned previously, many other parameters should be considered when designing or selecting a UVGI system. The most important are discussed below. 34 ASHRAE Journal Air temperature and velocity generally do not affect microorganism susceptibility to UVGI. However, their combined effect on lamp temperature can cause significant variation in lamp output, and ultimately UV dose. Depending on the lamp used, the UV output for in-duct systems can vary by more than 60% across a range of temperature and velocity conditions typical of HVAC system operation, particularly in VAV systems where both can change simultaneously.20 Modern UVC lamps are designed to reduce the output variation experienced by lamps designed to operate at room temperatures and still-air conditions when they are used for in-duct applications. The impact of air temperature and velocity should be considered in the design of in-duct systems to ensure that desired performance is maintained across ashrae.org August 2008 http://ashrae.org

Table of Contents Feed for the Digital Edition of ASHRAE Journal - August 2008

ASHRAE Journal - August 2008 Contents Commentary Industry News Letters Meetings and Shows Maintain to Sustain—Delivering ASHRAE’s Sustainability Promise Ultraviolet Germicidal Irradiation: Current Best Practices Improving Humidity Control With Energy Recovery Ventilation Single- or Two-Stage Compression Data Center Cooling: Using Wet-Bulb Economizers Building Sciences InfoCenter Practical Pointers Products Emerging Technologies Washington Report People Special Products Classified Advertising Advertising Index

ASHRAE Journal - August 2008

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