ASHRAE Journal - August 2008 - (Page 33)

the length of the duct or AHU to achieve longer exposure times as air moves through the irradiated zone. In-duct air disinfection systems, frequently installed in airhandling units, are typically designed for an air velocity of around 500 ft/min (2.5 m/s), although systems can be installed in air ducts where the velocity is much higher. At 500 ft/min (2.5 m/s), an irradiance zone of 8 ft (2.4 m) in length achieves a one second exposure. The UV dose required to inactivate a microorganism is the same whether it is on a surface or is in a moving airstream. To compensate for shorter exposure times, air disinfection systems require higher irradiance levels than surface disinfection systems. This translates to higher output lamps, more lamps within the duct, sufficient reflectivity, a method that allows air to be exposed to UVC over a longer air path, or any combination of these. At 500 ft/min (2.5 m/s), air-disinfection systems should be placed in an area where they can achieve a 2 ft (0.6 m) minimum irradiation zone down the length of the duct. This will provide roughly 0.25 seconds of exposure time and the UVGI system should be sized to deliver a sufficient dose to inactivate the airborne microorganisms of interest within that time period. UVGI fixtures are most often located downstream of the heating/cooling coils. However, in some cases mounting fixtures upstream of the coil may result in lower air velocity and/or increased UVC exposure time for “on-the-fly” inactivation. The trade-off is forgoing the disinfection of the drain pan that lamps mounted downstream of the coil also provide. In-duct air disinfection systems used to reduce or eliminate the spread of airborne infectious diseases (e.g., tuberculosis or influenza) in buildings with continuous occupancy and/or with immunocompromised populations (e.g., hospitals, prisons, or homeless shelters) should be operated on a continuous basis. However, properly designed systems installed in more traditional commercial buildings (e.g., offices or retail) can be operated intermittently, for example, powered on during hours of normal building occupancy and powered off when the facility is empty. This may result in energy savings and require less frequent lamp replacement while providing acceptable indoor air quality during periods of occupancy. Intermittent operation must be factored into the initial system design, as cycling UV lamps on and off is one of the many variables that affect lamp output and life. Upper-Air UVGI Upper-air UVGI involves lamp fixtures suspended from the ceiling and/or installed on walls with the fixture bottom at least 7 ft (2.1 m) above the floor (Figure 5). Lamps are shielded to direct radiation upward and outward to create an intense zone of UV in the upper portion of the room while minimizing UV August 2008 www.info.hotims.com/16017-34 ASHRAE Journal 33 http://buildwithpropane.com http://buildwithpropane.com http://www.info.hotims.com/16017-34

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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