Airstream or Coil: Which UV Application Is Right?
September 28, 2009
Using ultraviolet (UV) energy to destroy biological contaminants may be rooted in science, but the question we must ask is, how do we deliver the UV intensity needed to destroy the biological contaminants that may be traveling through the building’s ventilation system, or the mold growing on the evaporator coil? It is partly a question of placement.
UV light is being promoted by some as a “magic bullet” that will solve issues ranging from poor IAQ, to improving energy efficiencies and reducing maintenance on equipment. The truth is that there are many misconceptions on what UV can do, and how the technology should be applied so that the end user can enjoy the benefits of a well-engineered UV system.
When asked by a commercial end user to size UV systems for a commercial building, the customer should be asked, “What is it you want to treat, the air or the coil?” It is important for the installer and end user to understand that there are two very different ways of delivering UV energy for two very different UV applications: the air and the coil.
UV PRINCIPLESTo understand how UV energy destroys contaminants, we must first understand the principles involved. Various invisible UV wavelengths occur just before the visible light spectrum. The UV wavelength band most effective at microbial sterilization is ultraviolet-C (UVC), which can be found at the 254-nanometer wavelength.
Intensity: UVC germicidal ultraviolet light is effective in penetrating the cell membrane, breaking the DNA structure of the microorganism. The intensity of UV light is measured in microwatts; virtually every biocontaminant requires a specific dose of UV intensity for destruction within a given period of time (per second).
For example, S. marcescens bacteria requires 3,400 microwatts-sec/sq cm for complete destruction; the B. atrophaeus bacterial spore requires 45,000 microwatts-sec/sq cm for complete destruction. A contaminant’s kill rate is a factor of intensity multiplied by time (I x T = kill rate), so it is crucial that the biocontaminant receive the required dose of intensity for destruction.
Dwell time: Microwatt intensity, although vital, is not the most important variable in achieving a high kill rate. The amount of time the contaminant is in contact with the UV energy will determine how much UV energy the contaminant actually receives. The longer the dwell time, the more microwatt intensity can be delivered to the contaminant, resulting in a higher dosage and kill rate.
AIRSTREAM, AIR HANDLERSThe 2008 ASHRAE Handbook of HVAC Systems and Equipment included a chapter on “Ultraviolet Lamp Systems” for the first time. Chapter 16 discusses the various fundamentals and practices that make up a UV lamp system. One of the most important sections is the explanation of the various methods of UV treatment: in-duct airstream disinfection and air handler component surface disinfection.
In-duct airstream disinfection: According to the handbook, in-duct airstream disinfection is defined as the design of a UV system to “disinfect an airstream in a building or room ventilation system. These systems are designed to treat the airflow and use available space within the duct. In-duct systems are generally engineered to achieve a required level of air disinfection and are often unique to each installation.”
Air handler component surface sterilization: This category of UV treatment is typically designed to prevent and destroy microbial growth from growing on the coil and surrounding areas. UVGI (ultraviolet germicidal irradiation) can be readily applied to HVAC systems to help maintain system cleanliness.
“It is used to complement system maintenance by keeping coils, drain pans, and other surfaces clean and free of microbial contamination. Stationary surfaces receive UVC doses many orders of magnitude higher than microbes in moving air do (airstream), making it relatively easy, using lower levels of UV, to maintain heat exchange efficiency, design airflow, and to improve indoor air quality by reducing growth of bacteria and mold on system components.”
WHAT DO YOU WANT TO TREAT?Now we can revisit that question, “air or coil?” If the end-user wants to prevent and destroy mold growth on the coil, then the UV systems required for such an application would consist of lamp and parabolic reflector assemblies mounted 18 inches or so away from the coil projecting the UV energy onto the coil face. Because the coil is stationary, the exposure time is not limited; as a result, over time (days, weeks, months, or years), the UV energy will destroy and prevent microbial growth on the coil.
Unlike coil treatment where the coil is stationary, airstream purification requires treating the contaminants in the airstream, which may be moving at velocities of hundreds or thousands of feet per minute. As such, there may only be a fraction of a second to deliver the required microwatt dosage to destroy the desired contaminants. UV air purification systems must be sized so as to deliver the predetermined dosage of UV energy within a very short amount of time.
Coil-treatment UV systems that are used to keep a stationary coil clean are not designed to maximize exposure to the moving air and deliver the necessary microwatt energy to the contaminants in the airstream. In order for the UV fixture to be most effective, the UV lamp assembly must be configured in an orientation to the airstream that will maximize exposure, providing sufficient dwell time between the UV energy and the specific biocontaminant to be treated.
Typically, a parallel orientation to the airstream will guarantee this increased dwell time and offers an optimum cost-to-kill ratio. A plurality of perpendicular orientation of lamps may also be used to increase dwell time, although this is not as cost effective. A properly sized UV air-purification system can destroy up to 99.9 percent of contaminants on a single pass (EPA/National Homeland Security Sanuvox Testing).
Although both UV applications are very successful when applied correctly, if an end-user is expecting IAQ benefits from UV systems mounted on a coil, they may be disappointed to find out that the results may be nominal. As explained in the 2008 ASHRAE Handbook, the benefits to UV lights mounted on a coil are limited to maintaining system cleanliness, improving energy efficiencies, and improving IAQ resulting from a dirty coil - but improving a building’s IAQ from other sources besides a dirty coil may not be one of them.
VARIABLESDeciding what type of UV system to use (air or coil), contractors must take into account the nature of the application. Is the air in the facility in need of improvement, and/or is there a need to prevent and destroy microbial growth on the evaporator coil? Does the facility want to improve energy savings by benefiting from a more efficient system with better heat transfer, and is destroying airborne viruses such as influenza an issue?
The answers to these and other questions will determine the appropriate UV system.
Air temperature, humidity, evaporator coil dimensions, air velocities, installation on the upstream or downstream side of the coil, duct size, percentage of fresh air, which biological contaminants to destroy, desired kill rate, UV lamp aging, lamp fouling, and lamp cooling are just some of the many important variables that need to be taken into consideration when sizing a UV application. Sizing a commercial UV job is much more than just installing a UV lamp into a duct, or mounting one onto an evaporator coil.
Most residential IAQ applications are simple, straightforward installations where each UV system is rated for specific square footage. It’s just a question of selecting the appropriate UV system for the right size application. The IAQ issues facing commercial customers may be great and varying, requiring sound sizing calculations to guarantee that the customer’s needs are met.
When selecting a UV product, the manufacturer should provide, upon request, calculations including sizing and placement diagrams, real-time kill rates per air change on the specified contaminant, and real-time microwatt intensities and end-of-lamp life intensities.
According to Normand Brais, Ph.D., president of Sanuvox Technologies, “Manufacturers and installers must take sizing UV installations very seriously. Only when you take into account all the variables that influence the effective dose can you go forward and size a particular installation. It is essential to make sure the customer’s needs are met; only when calculations are run can such a sizing proposal be made.
“A reputable manufacturer should go so far as to size all applications at the end of their respective lamp life,” he said, “which means the manufacturer’s disinfection rates and UV intensities are calculated at the end of one or two years of operation (based on manufacturer’s lamp changeout).”
Sanuvox, for example, has developed sizing software that takes variables that influence a destruction rate into consideration. By inputting these variables into the software, the contractor or designer can learn how the UV system will actually perform in real time after installation. This is a valuable tool, as the end-user will have at his disposal the destruction rate for any particular contaminants, as well as how and where to place the UV system(s).
UV air and object purification technologies can dramatically improve our air quality and reduce maintenance costs while improving energy savings, but different UV systems are required to do each job properly. There is no such thing as a magic bullet. The right UV system has to be matched to the right job, taking into account the many variables that influence the desired results that will lead to successful installation and a satisfied end-user.
Publication date: 09/28/2009