Written by: American Air & Water
The discovery that bacteria grow on cooling coils and cause respiratory infections and allergies prompted engineers to look for an effective disinfection solution. It was determined that UV could be used to control microbial growth and the first UV systems were installed to disinfect the air handling equipment.
The effects of ultraviolet (UV) light on microorganisms were discovered in the 1800s and several scientific studies on UV were published over a century ago. The first studies that attempted to quantify the effects of UV irradiation on microorganisms were published in the 1920s. In these studies the disinfection rates for bacteria were determined in terms of UV irradiance and UV dose. The first usable data on UV irradiation of different types of mold was published in 1949.
Ultraviolet light with wavelength shorter than 300 nanometers is extremely effective in killing microorganisms. The most effective sterilizing range of UV is within the C bandwidth or UVC. This ultraviolet range is called germicidal UV bandwidth or UVC. UVC penetrates the membrane of the microorganisms and breaks down their DNA.
UV system designed specifically to disinfect the surfaces of air handling equipment, including humidifiers and air filters, was detailed by Grun and Pitz in 1974. In 1977 Luciano published a book detailing many applications of UV, including health care applications in which the UV lamps are specifically placed upstream of the cooling coils and downstream of the air filters.
In 1985 Phillips published a design guide which presented the first definitive description of UV lamps applications for control of microbial growth on cooling coils. This UV design guide, "Germicidal Lamps and Applications" provides details of how to locate UV lamps at specific distances from cooling coils and referrers to existing UV installations.
In typical cooling coil disinfection systems, a UV lamp, or array of UV lamps, is positioned so as to irradiate the coil surface. The UV lamps could be installed to irradiate both the upstream and downstream sides of a cooling coil. Often, it is not possible to position UV lamps on both sides of a coil and only one side is irradiated. Germicidal UV lamps are often positioned in a crossflow arrangement in which the axis of the lamp runs perpendicular to the fins of the coil. The orientation of the UV lamp is not necessarily critical and lamps may be positioned horizontally, vertically, or at any angle relative to the coil surface. UV lamp position will impact the irradiance levels at the coil surface but adjusting the total wattage, number of UV lamps, reflectivity, and other factors can compensate for less than optimum positioning of the UV lamps.
Dr. Kowalski, PE, Ph.D., Penn State University, developed a strong report which proves that UV can be cost justified as a capital expenditure by the savings it provides. The UV installations provide savings in energy and maintenance costs as well as enormous benefits of a healthier indoor environment.
The report reviews the history and current literature on ultraviolet germicidal irradiation (UVGI) cooling coil disinfection and summarizes the available information on laboratory and field testing of UV installations. Information on the energy savings and payback period of cooling coil irradiation are provided along with examples of the computation of typical payback periods. Guidelines on UV cooling coil irradiation systems are summarized.
The report provides an independent study proving that irradiating cooling coils with UV will virtually prolong the life of the HVAC system. The study includes a literature search of journals and trade magazines, including results from existing laboratory or field tests on UV equipment used in HVAC systems. The study includes analysis of UV effectiveness for cooling coils disinfection. It provides formulas for calculating return on investment (ROI) and the savings in energy and dollars from improving or maintaining the efficiency and extending the life of existing coils. It can reasonably be expected that typical UV installations will produce payback periods of 2-4 years. The report shows an example where the payback period is achieved in only 8 months. That is, the cleaning action of UV on fouled coils is so effective that the reduced maintenance costs and the savings in energy are so significant that the retrofit of a UV cooling coil cleaning system typically pays for itself in about 2-4 years. The energy savings results from two effects - the first being the reduced pressure loss through the coils once the fouling is removed, and the second being the increased rate of heat transfer in the coils when the fouling film is gone. Both of these can be significant, as can the reduction or elimination of maintenance for the cooling coils. Since the coils will be maintained in a clean condition, there is likely to be no requirement for periodic cleaning of the coils.
The economic savings that can result from the installation of a UV cooling coil disinfection system can be estimated by comparing the operating costs after installation with the operating costs before installation, minus the cost of installing and operating the UV system. Ideally, operating data would be drawn from field test results. This necessitates installing of a UV system. Little published data is available for installed systems but there are testimonials suggesting that UV disinfection systems are fully capable of restoring a fouled cooling coil to approximately the original design operating conditions. The cost savings will then depend on how much coil fouling has occurred and how far the system capacity has been diminished in comparison with the original design conditions.
The elimination of microbiological fouling of the cooling coils can be demonstrated through surface sampling before and after installation of the UV system. An alternative indicator of the effectiveness of UV germicidal irradiation could be coil performance, since the elimination of surface contamination should theoretically restore cooling coil performance to original design values.
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