Ultraviolet Energy: The Dedicated Aid For Commercial HVAC
Ultraviolet germicidal irradiation or light in the UV-C wavelength (254nm) has been used extensively in commercial and institutional HVAC systems since the mid-1990s, initially to improve indoor air quality (IAQ), and later, to improve airflow, boost heat exchange efficiency, and reduce necessary maintenance.
This veritable HVAC hat trick is accomplished for an average of <$0.15 per cfm – a mere fraction of the 10-25 percent potential energy and maintenance savings yielded by the efficiency-enhancing technology.
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. In fact, UV-C is about as close to a dedicated helper as one might get. Benefits range from improved energy efficiency, lower operating expenses and fewer occupant complaints, to better and healthier indoor air. All of these system-enhancing efficiencies can be found in ASHRAE’s 2011 Handbook of HVAC Applications, Chapter 60.8.
There are two distinctly different methods of applying ultraviolet-C energy (UV-C) in HVAC/R equipment. The first is the popular surface irradiation of drain pans, cooling coils, and other interior surfaces of plenums to maintain heat transfer and other performance efficiencies. The other is airstream disinfection, which kills airborne infectious microorganisms in HVAC air. Discussion of airborne treatment is best left for a future article; however, it is worth noting that a surface irradiation application can provide a fair amount of airstream disinfection while maintaining heat transfer and other performance efficiencies. A single-pass airborne kill ratio of up to 25% can be achieved in typical applications, meaning that each time the air passes the UV-C lamps, 25% of the airborne microbes are destroyed. Let's investigate one of the most common uses for UV-C technology: HVAC surface irradiation, which represents well over 95% of North American installations.
UV-C for Surface Irradiation
As air conditioner units and equipment get older, their ability to maintain adequate space temperatures and humidity levels declines. Most often, the culprit is reduced coil heat-transfer efficiency, or the ability of air-handling-unit coil to remove heat from the air. Evidence shows this drop in performance can occur within five years of startup due to the accumulation of contaminants on coil surfaces, leading to coil fouling.
Of course, there are other cost penalties with coil fouling, such as the lowering of chilled water temperatures and the pumping of more water, consuming more energy to compensate for lost coil capacity. There are also a higher number of hot/cold calls and associated maintenance actions.
These performance losses have led many building operators to successfully retrofit their air-conditioning systems with UV-C systems. According to Chapter 60.8 in the ASHRAE 2011 Handbook, UV-C technology can reduce mold and biofilm, coil pressure drop, and coil-cleaning procedures. Further, it states that the use of UV-C can increase airflow and heat-transfer coefficient and reduce both fan and refrigeration-system energy use, and thousands of installations show this to be true.
Costs and Payback
Most users report that their cost for a high output lamp system was less than $0.15 per cfm. For a 10,000 cfm system, that’s <$1,500 and a 24 x7 operating cost of <$188/year at $0.10/kW. That is less than 1% of the average 18% power saved to operate the air conditioning system. Also, field reports indicate that the first-cost of a UV-C system is less than a properly performed coil-cleaning procedure, especially when considering the cost of system shutdowns, off-hours work, overtime wages, and/or contractor labor costs.
The benefits of UV-C surface irradiation systems include maintaining indoor air quality and comfort levels commensurate with the owner’s performance requirements of the HVAC system, and doing so with minimal wasted energy. For new construction (an OEM feature), UV-C systems maintain as-built conditions from start-up. For retrofit applications, UV-C systems remove organic growth on all coil surfaces, clean drain pans and similarly, all other interior surfaces. A few months following the UV-C application, the surfaces stay clean and remain clean thereafter, requiring less maintenance and overhead expense.
Benefits include greater energy efficiency, minimized occupant complaints, reduced operating expenses, and better IAQ. UV-C is easy to install and begin using in your facility – it’s simply installing lamps in an air handler, or rooftop system, and then replacing them once per year.