The integration of ultraviolet energy systems into HVAC/R equipment is rapidly growing. UV has been used for many years to disinfect air and in recent years has seen increasing growth in applications to irradiate air-conditioning coils and drain pans.  Initially, most of the installations of UV equipment were done in the aftermarket as retrofits. Recently, however, HVAC/R OEMs have begun to offer UV equipment as a standard or factory-installed option.

The wavelength of ultraviolet light is at the low end of the light spectrum, and below the visible light range. UV light is further classified into three wavelength ranges:

• UVA  (300 to 380 nanometer).
• UVB  (279 to 320 nanometer).
• UVC (220 to 280 nanometer).

UVA is generally used for sun tanning, and “black lights.” UVB has some medical uses, but it is UVC that is generally used for disinfection, as its wavelength range possesses germicidal properties. All microorganisms are susceptible to UVC irradiation. A microorganism’s DNA is damaged by wavelengths between about 230 and 290 nanometer, with about 260 nanometer considered ideal for that purpose.

Certain types of mercury fluorescent lamps, designed specifically for UVC emission, produce most of their light energy at 254 nanometers so they are ideal for germicidal applications. While all microbes are sensitive to UVC irradiation, their sensitivity varies. Generally, airborne viruses and bacteria can be readily destroyed with UVC. Fungi (including molds) are much more difficult to disable and the amount of UVC needed is higher. Research in this area has been performed for decades, leading to some well-established figures for the approximate dosages required to disable different types of microbes. These figures are readily available in the scientific literature.

When UV is used to disinfect an airstream, the UV levels must be high enough to supply an adequate dosage when exposure times are short, typically a fraction of a second.  When trying to disable fungi in a moving air stream, it may be necessary to combine filtration and UV in order to effectively contain the microbes.

When UV is used to irradiate a surface, such as a coil, filter, drain pan or duct wall, the UV lamps are typically operated continuously, making exposure time extremely long, essentially infinite. In these cases, therefore, only low levels of UV intensity are required to destroy microbes.

Why UV?

Installation of UV systems for a large hospital in Florida. This installation is typical of the configured approach embraced by OEM customers.

The concept of using UV to destroy microbes (such as bacteria and mold or fungi) has been understood for more than a century. UV has been utilized to disinfect both water and air. Thousands of municipal water treatment plants and wastewater treatment plants throughout the world utilize UV as a primary disinfectant. In air, UV has been used to deactivate microbes for many decades. In addition, UV has been used to irradiate the upper air in rooms and also to disinfect moving air streams in duct systems. The ability of UV to serve these applications is well understood and has led to its wide deployment. More recently, however, UV has been integrated into HVAC/R systems, on air-conditioning coils and drains pans for economic and air quality reasons.

Coils and drain pans provide ideal environments for the capture and growth of bacteria and mold. Moisture from the condensate and nutrients supplied from the air stream nourish the growth of bacteria and mold and eventually the equipment can become significantly fouled with organic matter. This fouling has many detrimental effects on the proper operation of the HVAC system. As the space between fins becomes fouled, the pressure drop increases across the coil. In addition, heat transfer from the coils is decreased. Maintenance and cleaning costs increase, or if maintenance is not performed, system performance deteriorates. Indoor air quality is degraded as the fouled coils can become a source of airborne organic matter and odors.

Applying UV light can provide many benefits, including reducing or eliminating the need for coil cleaning, saving energy (through improved heat transfer and lower pressure drop), and, most importantly, improving indoor air quality.

Market drivers in the HVAC/R market seem to be helping the rapid growth of UV technology. Important drivers include reducing energy costs and improving life cycle operating costs. One driver encouraging OEMs to consider factory installations is the GSA specification now in effect for new building construction of government facilities. This specification requires the installation of UV to irradiate the coil and drain pan. In addition, architects and engineers are beginning to specify UV in new equipment installations. Even commercial refrigeration manufacturers and manufacturers of food preparation and display coolers now find themselves exploring the benefits of UV.

Design considerations

UV system installed downstream of the air conditioning coil in a residential HVAC system (horizontal system). Residential UV systems are designed for simple, safe and convenient installation.

There are many variables that affect the output of a UV lamp and its performance in a system. Working with an experienced UV component manufacturer can make the job of designing the system and providing a proper UV dose much simpler. Some of the key considerations are:

• UV output. The amount of UVC generated by a specific lamp.
• Air velocity and temperature. This affects the output of the lamp.
• Exposure time. In a moving air stream, this depends on the duct length and air velocity.  On surfaces, this may be infinite.
• Reflectivity of system materials. Microbes must be hit by the light via direct line of sight or indirectly by reflection. It should be noted that UVC is reflected differently than visible light. (See Table 1.)
• The geometry of the system and placement of the lamps.
• The age of the lamps. UV output depreciates with age, depending upon the lamp.

Many methods for designing a UV system are available. Lamp manufacturers provided some rough tables more than 50 years ago. Now, better application data is available, and UV energy requirements are better understood. Some manufacturers have formulas or proprietary modeling software to facilitate proper design of a system. Nonetheless, there are many different approaches by different manufacturers and standards have not yet been established. Currently, there is a great deal of work being performed on application and testing standards by organizations such as ASHRAE and IUVA.

For moving-air-stream disinfection systems, the applications generally have many lamps mounted in the return air duct. The lamps should irradiate the air for a distance of at least 3 ft. to 4 ft. in order to provide a reasonable dosage. An added section of plenum may need to be added by the OEM.

For irradiating surfaces, particularly air-conditioning coils and drain pans, modest levels of UV will be adequate. Typically the lamps are installed downstream of the coils at a distance of 12 in. to 14 in. from the coil. Most large systems have this type of room available. The lamp fixture, or multiple fixtures, are installed in rows. The lamps should be installed end-to-end in order to cover the full width of the coil. Modular and track mounted fixtures can make this easier. Multiple rows may be required depending upon the height of the coils and the output of the UV lamps.

The power of an individual lamp is not important. What matters more is the mounting of the lamps, the proper number of lamps, and the previously mentioned design factors, to achieve a good level and distribution of UV energy over the coils. Although standards have not been agreed upon and established to date, it appears that levels in the 50 microwatts to 150 microwatts of UVC per square centimeter are good for most applications. 

Installation hurdles

Enlarge this picture Table 1

When UV equipment is installed in HVAC/R systems, special considerations are necessary. To fully realize the benefits of the UV, certain precautions must be taken to  avoid damage to equipment and to prevent safety problems. The UV equipment must be installed in the proper location, generally downstream of the air conditioning coil. The UV lamp fixtures need a suitable supporting structure and space for installation and future maintenance. UV lamp fixtures are generally installed the full width of the coil with spacing that allows the fixtures to evenly irradiate the entire coil, including the irradiation of the drain pan.

Electrical service must be supplied to the UV fixtures. The amount of UV intensity applied is extremely important, as too little may not keep the coils clean. Too much wastes energy, increases safety concerns, and may damage some organic components in the system. Fortunately, the intensity levels of UV for surface applications are very low, as the fixtures are generally left operating continuously so that the required dosage is achieved even at low intensity.

There are also safety and equipment concerns which must be considered and are relatively simple to handle.  The UV used in these applications is of the UVC wavelength (254 nanometer), which is harmful to skin and eyes. Therefore, safety interlock switches must be installed on doors and access panels to protect operators on locations where they could accidentally access a system and be exposed to UVC.

UVC can significantly degrade organic components over time. Almost all organic materials can be degraded by UVC. Some materials, like synthetic media filters and insulation, are damaged quickly. Other materials, such as black motor drive belts and certain plastics, are quite resistant to UVC effects. UVC levels at various points in a system should be evaluated to determine if certain components need to be shielded from the UVC or if those components can be fabricated from a more UV-resistant material.

Manufacturing solutions

UV system installed downstream of the air conditioning coil in a residential HVAC system (vertical system). Residential UV systems are designed for simple, safe and convenient installation.

These hurdles can make it more practical to incorporate UV into the product at the OEM level, where they can be more easily  overcome. During the design and manufacture of the equipment, HVAC/R components can be re-arranged to allow the space for the UV equipment. Electrical junction boxes can be included in the equipment wiring. Safety interlock switches can be installed at all user accessible panels and doors and integrated into the control system. UV levels in the system can be more accurately determined, and the appropriate UV-resistant material can be specified for all components that will receive significant UV irradiation. These issues are much more easily addressed during design and manufacture of the original equipment than they can be on a single unique system in the field.

These reasons, coupled with the growing demand for UV, are why many makers of coils, air handlers, and full HVAC/R systems are considering or beginning to offer UV as a factory-installed option. The OEM can readily take the necessary measures to avoid the safety or material problems  and provide for a robust simple installation. Several manufacturing approaches have been used to simplify installation of UV equipment at the OEM level. Generally, the UV equipment utilized is of a modular design. It can be easily mounted on a track or horizontal support and interconnected electrically. Units come in a variety of sizes to allow for proper sizing to any coil size.

As the range of sizes and the variety of the OEM equipment is very large, several manufacturing approaches have been utilized.

One approach is for the UV equipment supplier to provide a kit tailored for the OEM’s particular piece of equipment. This kit includes all the necessary supports, UV fixtures, switches, and accessories. It can be shipped and tagged with the OEM’s shop order to allow for easy installation during assembly of the system.

In another approach, application software available from the UV system supplier can be integrated with the OEM’s manufacturing software. The shop order will then select and add the proper UV equipment to the manufacturing bill of materials and production plans.

A third approach, one taken by a large OEM, is to stock a modest inventory of modular UV components. Then, when an order is received, a UV software program is run to specify the UV equipment required. The UV parts are delivered to the appropriate point on the assembly line and the stock replaced as necessary. 

Residential success

Example of UV system installed in a residential HVAC system. UV is applied directly downstream of the air conditioning coil to maximize energy savings benefits.

To date, a good portion of the OEM systems incorporating UV have been for large commercial systems. However, UV equipment is quickly gaining popularity in residential systems. Several OEMs currently offer UV appliances for residential equipment. One particularly successful application is the Honeywell line of UV products for residential central HVAC systems. The UV appliance has been designed to install easily and to integrate with their control systems. Logic has been incorporated in the UV appliance to control the duty cycle of the UV appliance and thus optimize lamp life. In addition, the home control system warns the user when lamp replacement is recommended, just as it does with filter changes.

As this development suggests, the growing application of UV equipment into HVAC/R systems has led more OEMs to consider the wisdom of incorporating the UV equipment, by design, during assembly of the system. In the controlled factory environment, they can provide optimal installations that will maximize the benefits of UV, while reducing or eliminating safety and material concerns.

For more information, email: sales@uvdi.com

Sidebar: Photocatalytic Oxidation

Another unique application of UV energy for improving indoor air quality, photocatalytic oxidation (PCO), has been under development for many years. It is not as widely used or commercially accepted as UVGI (Ultraviolet Germicidal Irradiation), but PCO use is growing. PCO has many uses and benefits, including the improvement of indoor air quality by removal of pollutants and odors from an air stream. In indoor air applications, PCO may be part of a standalone room air cleaner, or installed in larger ducted HVAC systems.

PCO is a room-temperature, catalytic chemical reaction. In its most popular form, it uses short-wave UV light to supply energy to a catalyst covered substrate. The most common catalyst used is titanium dioxide, and the substrate may be a structure such as glass filter media, aluminum mesh, screen or honeycomb. The structure allows the air to pass over or through the substrate, and pollutants from the air are adsorbed onto the surface of the catalyst. The UV light, typically UV light between 200 and 380 nanometer, is applied to the surface. This creates electron-hole pairs and, with the moisture and oxygen in the air, generates free hydroxyl radicals that break down organic pollutants. Carried to completion, the reactions result in the conversion of the pollutants into carbon dioxide and water.

Carbon is currently the accepted method of removing pollutants and odors. These are absorbed on the carbon and the carbon is changed when it reaches capacity. Unlike carbon, PCO is a catalyst so the pollutants, are converted rather than captured, so theoretically the catalyst can last indefinitely.

Challenges facing the OEM employing PCO include:

• Presenting the catalyst on a substrate that allows sufficient surface area for contact with the air stream.
• Avoiding excessive pressure drop through or over the  substrate.
• Applying the optimum level of UV energy to the catalyst while matching it with the quality of the catalyst substrate.

Implemented properly, PCO can prove effective in air purifiers and HVAC systems.