Microorganism growth can also increase airflow resistance and reduce heat transfer, lowering the capacity and energy efficiency of the cooling system. Manually cleaning the coils is a laborious process that only temporarily removes the contaminants.
Using ultraviolet germicidal irradiation, produced by lamps designed specifically for this purpose, can provide continuous, cost-effective coil cleaning. These lamps are designed to emit radiation in the wavelength of 253.7 nanometers (nm) to provide the greatest disinfection ability. (The range spectrum of 200 to 280 nm is the "C" range of ultraviolet radiation, hence the term UVC.) This radiation is absorbed by the DNA molecule of the microorganism, producing mutation, and eventually, deactivation. Thinner-walled viruses are most readily deactivated, followed by bacteria and then fungi.
WHAT ARE THE BENEFITS?
- IAQ may be improved since the coils that are continuously cleaned by UVC are no longer an incubation site for microorganisms. Air flowing through the coils is not contaminated, resulting in cleaner air being delivered to the classroom.
- Maintenance benefits may accrue from use of UVC lights to keep coils continuously clean, avoiding the laborious coil cleaning actions that will otherwise be required to return coils to a clean condition.
- Energy benefits include reduced pressure drop, improved heat transfer, and increased system capacity, potentially resulting in overall cooling energy savings.
HOW IMPORTANT IS IAQ?Evidence strongly suggests that poor environments in schools, primarily due to the effects of indoor pollutants, adversely influence the health, performance, and attendance of students and teachers. This evidence links high concentrations of several air pollutants to reduced school attendance.1 There is also persuasive evidence that microbiological pollutants are associated with increases in asthma and respiratory infections, both of which are related to reduced school performance and attendance.UVC lights offer an effective means of both reducing energy use and delivering fresh air to improve classroom air quality.
The lamps are designed to clean both the coil and drain pan surfaces in a few hours or a few days2 and to progressively penetrate between the coil rows and fins with time.
The sidebar discusses some of the issues related to coil fouling and the pros and cons of traditional methods and UVC for cleaning fouled coils.
WHAT ARE THE MAINTENANCE ISSUES?An effective, traditional coil-cleaning program cleans the coils three to four times per year. Use of UVC lamps can eliminate the need for these costly, labor intensive cleaning treatments that create system downtime and use chemicals, biocides, or pressure washing. Mechanical or chemical washing may also damage coils.
UVC lamps should be inspected to see if they are dirty and should be cleaned on a regular basis, as needed. Some installations have a view port to permit visual observation of the lamps without entering the AHU. The frequency of cleaning of the UVC lamps depends on the level of filtration and whether the lamps are upstream or downstream of the filter. Some practitioners suggest that if lamps are installed downstream of an effective filter, the lamps will not need to be cleaned at all before they need to be replaced. To clean the lamps, wipe with a soft lint-free cloth (when the lamps are off) moistened with isopropyl alcohol or glass cleaner, to ensure that the lamps are operating at optimal efficiency. Lamps lose their efficacy with age and are generally replaced annually or whenever the output falls below 70% of the initial output.
Some practitioners of UVC systems recommend manual cleaning of the coils prior to installation and operation of the UVC lamps. This allows the UVC lamps to keep the coil in a continuously clean condition without fear of dispersing deactivated mold and other microorganisms that might otherwise be present if the UVC lamps were used to deactivate microorganisms on a dirty coil and drain pan.
Another option that may work for school buildings is to initially operate the UVC system when the building will be unoccupied for a sufficient period, such as the summer vacation break, to deactivate the organisms and "flush" them from the building prior to occupancy.
HOW DOES IT SAVE ENERGY?Cooling system energy can be saved by removing microorganisms from the coil, drain pan, and plenum area; reducing airside pressure drop; increasing airside heat transfer; and increasing system capacity.
Lamps are generally operated continuously to achieve the most effective cooling system cleaning and IAQ improvement. The resulting lamp energy use must be less than the cooling system energy savings for overall savings to accrue. In a typical installation, the installed lamp power could be as low as less than 1% of HVAC system power for large systems and as high as 5% or greater for smaller systems. The savings produced by the lamps need to exceed these levels to achieve net energy savings for the installation.
UVC AND SUSTAINABLE BUILDING PRACTICESWhile UVC is not specifically addressed under the LEED® rating system, it can be directly responsible for additional points for superior energy performance in the "Innovation by Design" area. Cooling energy savings potential can result in additional points for exceeding energy code requirements.
WHAT KINDS OF UVC SYSTEMS CAN I USE?There are three main types of UVC systems generally used in buildings: in-duct, upper room, and air-handling systems. In-duct systems provide a high level of UV radiation sufficient to kill microorganisms in the air flowing past the lamps. Upper room units are installed in occupied rooms above the heads of the occupants, shielded from their view, relying upon personnel movement and heat sources to create currents that cause airflow through the units. They are most often used in rooms with low air turnover. Air-handling systems are placed near the cooling coil and drain pan in the delivery plenum and are designed to provide UV radiation that deactivates microorganisms that would otherwise foul the surfaces of the AHU. This application - irradiation of stationary surfaces - has long UVC exposure times and therefore lower intensity re-quirements than the other types of UVC systems that are trying to disinfect a moving airstream.
Low-pressure UVC systems use lamps that are designed to provide radiation at the 253.7 nm wavelength that is most effective in deactivating microorganisms. The lamps use low-pressure mercury vapor, operating on the same principles as a fluorescent lamp, but differing in that it does not contain phosphors that convert UV to visible light. Another difference is that UVC lamps are made of quartz or soda barium glass that transmits UVC, rather than common glass, which does not.
This article deals primarily with issues related to placement of UVC systems in AHUs in the proximity of the cooling coil and drain pan. In all cases, it is recommended that filtration be used in conjunction with the UVC system.
CAN UVC BE USED IN MOST HVAC SYSTEMS?Manufacturers of UVC systems have designed equipment configurations suitable for all major types of A/C systems used in schools, ranging from unit ventilators to unitary single package and split systems, and built-up systems used in site-built classrooms to the wall-mounted single package systems used in relocatable classrooms.
Equipment configurations suitable for retrofit are also available for these types of A/C systems used in existing schools. Care must be taken to ensure that the lamps are placed in the best position to maximize the view factor between the lamp and the coil/fin and drain pan surfaces.
HOW SHOULD THE LAMPS BE SIZED, LOCATED, AND OPERATED?Lamps operate most effectively in still air at 25 C. Temperatures both above and below 25 C result in reduced lamp performance. Lamps are most effective when they are new and clean, and they lose their efficacy with age and lack of cleanliness. Humidity has little effect on lamp output, but germicidal efficacy appears to decrease with increasing relative humidity3.
Since lamps lose their efficacy with age and operating conditions are often less than optimal, lamps need to be oversized so they can provide effective performance for a reasonable duration in a real world environment of dust, humidity, and cooling airflow. Manufacturers take this into account and manufacture lamps and reflectors that provide the appropriate lamp intensity for the installation of interest.
For coil surface cleaning, lamps should be placed to provide good coverage of the coil face. The travel path of the UV rays should be directly through the gaps between the coil fins. The placement and sizing of the lamps depends on the types of microorganism in the system, the dimensions of the installation, and the desired level of disinfection. Many design approaches are available for sizing UVC systems, including using catalogs, tables, analytical methods, and rules of thumb. In general, the manufacturer will take the responsibility of sizing the product to meet the conditions required by the application. One manufacturer suggests that 24 in. of high-intensity UVC tube length be used for every 4 sq ft of coil face area, and that the ideal distance between the fixture and the coil is half the distance between rows or half the height of a one row coil if it is less than 24 in.
Lamps should be operated continuously to prevent growth of microorganisms.
WHAT ARE THE SAFETY ISSUES?Excessive exposure to UVC causes temporary redness and inflammation of the conjunctiva of the eye. Both should resolve within 24 to 48 hours. The cornea is very sensitive to UVC but UVC does not penetrate the cornea, and therefore, adverse lens or retinal effects are not experienced, except for people who have had cataract surgery to remove their lens or cornea.4 View ports designed to see if the UVC lamps are operating properly or need to be cleaned, should be constructed of glass or Lexan since UV does not penetrate either of these materials.
The Illuminating Engineering Society of North America (IESNA) cited the exposure limits set by the American Medical Association in Table 1.
The American Conference of Governmental Industrial Hygienists (ACGIH) recommends threshold limit values (TLV) for UVC exposure in an eight-hr period of 6.0 mJ/cm2, which is equivalent to an irradiance of 0.2 µW/cm2 for an eight-hr period, and 0.4 µW/cm2 for a four-hr period. Above this level, erythema (skin redness) and photokeratitus (external eye inflammation) occur. UV exposure and leakage needs to be minimized. (A telltale blue glow provides a clue to UV leakage.)
UVC lamps should be designed to avoid emitting radiation below the 200-nm wavelength that produces ozone.
Plastic-coated wiring can be become brittle when exposed to UV and can create a fire hazard. Glues that hold filter pleats together or hold the filter to the frame, can be degraded by UV. UV exposure to these materials must be avoided.
While these hazards are real and care should be taken to avoid unsafe practices, experienced manufacturers and installers are well aware of the safety issues accompanying the use of UVC in occupied buildings and have designed fixtures, safety interlocks, and installation, servicing, and operating procedures to avoid any potentially adverse affects that could occur.
HAS IT BEEN DONE BEFORE?UVC has been used effectively in many commercial buildings including a number of K-12 schools. Some examples of the benefits of UVC installation in schools include:
- A UVC classroom installation5 in the Capistrano School District in California, claimed reduction in indoor air contaminants (skin cell fragments of 66% and pollen of 50%) and "every 15 to 20 minutes the air in that classroom will be purified resulting in a major improvement over previous conditions."
- The LaPorte Independent School District in Texas installed UVC lamps6 in a building that had been infected with fungal growth that had been treated with costly cleaning, inspections, and chemical sprays. The UVC installation eliminated the need for these costly, time consuming treatments and provided the ancillary benefit of a nearly 10% reduction in energy use compared to a similar facility that had fewer hours of operation.
- The Stepping Stones Center educational and therapeutic facility7 in Cincinnati used UVC lights to effectively remove mold from an otherwise unusable building.
- UVC lamp systems were installed in 36 packaged A/C units in three school districts across California8. Their performance was compared to 18 control units in those school districts over a six-week period starting in August 2005. Microbial samples were taken from the surfaces of the cooling coils for each of the units prior to the installation and operation of the UVC lamp systems, and also at the end of the test period. Each sample was subjected to fungal and bacterial testing.
The results in the California shools showed that the UVC lamps notably reduced the levels of microbial counts in the evaporator coils in the A/C units. (Total fungal and gram-positive bacteria reductions from 65% to 100% of colony forming units were found.) Airflow and efficiency measurements were also made on the units and showed a positive trend (1% to 2% improvement in airflow) in reducing pressure drop and improving airflow, but this trend was not statistically significant for the sample size and conditions evaluated. The study is continuing with analysis of student attendance records to determine if the UVC lamp systems had an effect on absentee rates. Recommendations for further study with larger sample sizes and conditions more conducive to coil performance degradation are being provided.
Examples of use of UVC in other types of commercial buildings include the following:
- Florida Hospital in Orlando installed UVC lamps in a 27-year-old AHU9, and within weeks of the installation, air velocity over the coil more than doubled and pressure drop was reduced by over 60%, saving at least 15% of HVAC system energy costs.
- Application of UVC in the coil/drain pan area of the HVAC system in an office building in Montreal10 found a 99% reduction in AHU surface microorganisms, a 25% to 30% reduction in airborne bacteria, a 20% drop in worker absenteeism, and a 40% drop in respiratory problems.
- American Electric Power, formerly the Central & South West Corporation11,12 of Dallas, installed 170 UVC lamps in the AHUs in their nearly 500,000-sq-ft (Continued on page 95) building in 1998, providing an approximately 28% reduction in A/C system energy use and coils that are free of mold and organic buildup without any use of chemical cleaning or biocidal treatment.
In GSA-2003, Facilities Standards for Public Buildings13, the U.S. General Services Administration requires that UVC be used in buildings in their jurisdiction. "Ultraviolet light (C band) emitters/lamps shall be incorporated downstream of all cooling coils and above all drain pans to control airborne and surface microbial growth and transfer. Applied fixtures/lamps must be specifically manufactured for this purpose. Safety interlocks/features shall be provided to limit hazard to operating staff."
COST ISSUESThe initial cost of the lamps and related control equipment and the annual/periodic replacement costs of the lamps are additional costs accrued with the UVC systems. This should be compared to the maintenance costs that will otherwise result from regular chemical, biocidal, or pressure cleaning.
Incremental energy use of the lamps must also be considered. Practitioners of these systems have asserted that the additional cost of UVC systems is more than offset by the elimination of costly air-handing system cleaning, and incremental coil energy use reduction, and that short paybacks are generally achieved (Sidebar).
Furthermore the quantification of the value of reduced absenteeism, and greater learning performance can greatly multiply these benefits. In the end, it may often be the promise provided by using UVC to improve indoor environments and to consequently enhance student and teacher health and productivity that turns the decision in favor of this technology.
ACKNOWLEDGEMENTSThe information presented in this article is based on work performed for the California Energy Commission under its Public Interest Energy Research program in conjunction with the efforts described in Reference 8. The work is part of an Indoor Environmental Quality effort that is described on the California Energy Commission's PIER IEQ Program website on UVC for K-12 schools (www.archenergy.com/ieq-k12).
CITED WORKS1. Mendell, Mark and Garvin Heath, "A Summary of Scientific Findings on Adverse Effects of Indoor Environment on Student's Health, Academic Performance and Attendance," LBNL for the U.S. Department of Education, Doc. # 2004-06, April 2004.
2. Kowalski, Wladyslaw, "UV Cooling Coil Disinfection," The Pennsylvania State Indoor Environment Center for American Air and Water, Inc., September 1, 2005.
3. Peccia, Jordan; Holly Werth, Shelly Miller, and Mark Hernandez, "Effects of Relative Humidity on the Ultraviolet Induced Inactivation of Airborne Bacteria," Aerosol Science and Technology 35, University of Colorado, (2001), pp 728-740.
4. Brickner, Philip; Richard Vincent, Melvin First, Edward Nardell, Megan Murray, and Will Kaufman, "The Application of Ultraviolet Germicidal Irradiation to Control Transmission of Airborne Disease: Bioterrorism Countermeasure," Public Health Reports, Volume 118, March-April 2003, pp 99-114.
5. "Breathe Easy: Capistrano Unified Reduces Indoor Air Contaminants by 66%," www.SchoolFacilities.com.
6. Freeman, James, "UVC Sheds New Light on School Mold Problems," La Porte Independent School District, La Porte, TX, Heat-ing/Piping/Air Conditioning, May 2001.
7. "UVC Lights Put an End to Mold Problem," Air Conditioning/Heating/Refrigeration News, August 9, 2004.
8. "Improving Indoor Environmental Quality and Energy Performance of California K-12 Schools, Project 3, Effectiveness of UVC Light for Improving School Performance," RLW Analytics for the California Energy Commission under Contract 59903-300, Draft Report, January 6, 2006.
9. Keikavousi, Firouz, "Florida Hospital Puts HVAC Maintenance Under a New Light," Florida Hospital, Engineered Systems, March 2004.
10. Menzies, Dick, Julia Popa, James A. Hanley, Thomas Rand, and Donald K. Milton, "Effect of ultraviolet germicidal lights installed in office ventilation systems on workers' health and wellbeing: double blind multiple crossover trial," Montreal Chest Institute, McGill University, The Lancet, Volume 362, November 29, 2003, www.thelancet.com.
11. "UVC Technology Sheds Light on IAQ Problems," Air Conditioning/Heating/Refrigeration News, July 10, 2000.
12. "How do Utilities Save Energy? With UVC for HVAC!," Contracting Business, December 2001.
13. GSA-2003, Facilities Standards for Public Buildings, U.S.GSA, Office of the Chief Architect, Section 5.9 HVAC Systems and Com-ponents.
14. Siegel, Jeffrey; Iain S. Walker, and Max H. Sherman; "Dirty Air Conditioners: Energy Implications of Coil Fouling," 2002 ACEEE Summer Study on Energy Efficiency in Buildings, 1.287-1.299.
15. Peterson, George; Rob deKieffer, John Proctor, Tom Downey, "Persistence #3A, An Assessment of Technical Degradation Factors: Commercial Air Conditioners and Energy Management Systems," Proctor Engineering Group for the CADMAC Persistence Committee, February 25, 1999.
sidebar: Fair Or Foul?Coil fouling is defined as an increase in pressure drop above 100% compared to a new coil. Reduced airflows from coil fouling can cause typical efficiency degradation of less than 5%14, but can be much greater for marginal or extreme conditions. An analysis of A/C coils15 showed that they were relatively insensitive to low and moderate amounts of airflow reduction due to fouling. When airflow was reduced by 35%, the coil had just a 6% drop in EER with the majority (4.6% of the 6%) occurring in last two years of the coil's 20-yr life projection.
Both of these studies indicate that substantial fouling is needed to produce modest (~5%) degradation in efficiency. The level of fouling needed to provide the opportunity to save significant amounts of energy as cited in the Texas and Florida studies just described (References 7, 9, and 10), is likely to be indicative of humid, warm conditions that have produced considerable microbial growth that may have gone untreated for some time.
PROS AND CONS OF COIL CLEANING TECHNOLOGIES
The following compares the perceived advantages and disadvantages of traditional coil cleaning methods that use chemicals, biocides, or pressure washing, to the attributes of UVC lights for coil cleaning. Both types of technologies lack well-documented quantitative studies of coil degradation and the subsequent benefit of cleaning methods and systems.
Pros - Surface cleaning is quick and effective. Continuous cleanliness is maintained, sustaining cleanliness benefits. Maintenance (lamp cleaning and replacement) is quick and simple.
Cons - Unclear how UVC light penetrates well below the surface envelope of the coil to disinfect and clean deep within the coil. UVC only addresses biofouling and does not affect other contaminants. Cleaning could take weeks or months to reach maximum effectiveness. The initial cleaning period may need to be coordinated with breaks/school shutdown periods to avoid transmittal of dead/deactivated organisms into the occupied space.
Traditional Coil Cleaning Technologies
Pros - Coil is cleaned to the full extent that is manually possible immediately after treatment. HVAC technicians are familiar with these technologies, and infrastructure exists for their deployment.
Cons - Pressure washing could drive contaminants deeper into the coils. Chemicals and biocides need to be carefully removed to avoid subsequent air contamination. Cleaning can require facility shutdown, and disassembly of equipment. The coil cleanliness degrades steadily immediately after initial treatment.