How To Size UV Light For HVAC?

UV-C lighting systems are not a novel, cutting-edge invention. Since the middle of the 1990s, we have been using UV-C light energy for our benefit. The purpose of UV-C light is to considerably increase the HVAC/R airflow. Secondly, we use this light to increase heat-exchange efficiency, which can lower HVAC/R energy use by up to 25%. Moreover, we know the UV-C light energy by other names such as ultraviolet germicidal energy or light energy. Even while UV-C doesn’t save energy on its own, it increases the possibility of energy savings by restoring cooling and airflow.

Scientists discovered that, under reference conditions, exposing a fouled cooling coil to UV-C led to a 10% reduction in pressure drop and a 14.55% increase in heat transfer coefficient levels. 

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In new or OEM equipment, UV-C prevents organic accumulation on the surfaces of cooling coils, drain pans, air filters, and ducts. With the objective of preserving IAQ, airflow conditions, and cooling capacity, we use UV-C. UV-C eliminates organic matter that has gathered and grown over time in retrofit applications and stops it from returning.

Although UV-C is a pretty straightforward technology. We simply install the lamps onto surfaces. Moreover, technicians have also added simple on/off controls to aid maintenance. Many professionals are baffled by how UV-C functions and how to use it efficiently. The uses of UV-C technology that appear to be the most perplexing are in this article.

Lamps And Lamp Replacements For UV-C

Modern UV-C lamps resemble fluorescent lights, frequently used in ceiling installations. Both kinds of bulbs run on the same electrochemical principles. These procedures could involve an electric discharge through a mixture of noble gasses impacting mercury vapor to produce a 253.7 nm-wavelength photon (typically called UV-C). It is undetectable. About 90% of the energy produced by a typical UV-C lamp is in the UV-C wavelength.

About 4% of its energy is released as heat, with the other 5% occurring in the visible light spectrum, which has a medium blue color. The argon gas inside the lamp envelope is what gives the color blue.

UV-C Effects

When used with HVAC/R systems, UV-C systems offer various advantages, including efficiency, occupant comfort, indoor air quality, and environmental and economic effects. 

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You must develop, install, operate, and maintain a UV-C system to reap the benefits of system efficiency. Moreover, you can also use UV-C light to improve the indoor air quality provided by this technology. However, as seen below, these life cycle steps are relatively easy to complete.

Specification, Sizing, And Selection

In order to provide a comprehensive design solution, practitioners must decide:

1. The energy required by UV-C to “perform the job.”
2. The lamp/ballast specifications needed to satisfy the operating conditions
3. The quantity and type of bulbs that are necessary.

ASHRAE Technical Committee set the 50–100 W/cm2 as the minimal irradiation level by the (microwatts/square centimeter) for applications involving cooling coils. Including plenum ends and corners, the entire coil surface must meet this standard as a “minimum” barrier. However, the majority of practitioners are not aware of these engineering units. 

How To Size UV light For HVAC

We use units of lamp watts to indicate the Sizing in lighting applications. A 1-square-meter surface with a 1-meter-long light positioned in the middle of the surface on a horizontal plane is one accurate technique to translate microwatts into lamp watts.

A 1-meter, 36-inch high-output (HO) lamp is rated at 80 lamp watts with an output of 245 W/cm2, according to lamp manufacturers’ published data on average lamp watts and output. The irradiance must be extrapolated to account for the fact that UV-C lamps are typically located 12 inches from the coil surface. The corresponding irradiance, calculated using the “cylindrical view factor model,” is 1375 W/ cm2 at 12 inches.

How To Prevent De-Rating Of The Lamps

Although this appears to be more than sufficient to meet the 100 W/cm2 ASHRAE recommendation, all operational circumstances must be taken into account first. The performance of the lamps can be significantly reduced or “de-rated” by some factors, such as air temperature and velocity. Still, other adjustments may help to improve design yield. Lamps are derated by roughly 50% under typical conditions of 500 fpm velocity and 55 F air temps. So, at 12 inches from the coil surface, the 1375 W/cm2 produced by a typical 80 W high output lamp would now provide more of a 688 W/cm2 irradiance. You also need to take the distance between the UV-C lamp and the plenum corner into account next. Typically, we increase the UV-C dosage based on the amount of UV-C energy that bounces off the top, bottom, and sides of a plenum toward the coil and other areas.

Reflectivity And UV-C Lights

Reflectivity disperses UV-C energy so that “all” surfaces are cleaned and sterilized. Reflectance multipliers vary among various materials. The multiplier is 1.50 (a 50% increase in UV-C radiation), hence, using a galvanized steel plenum as an example, 172 W/cm2 x 1.50 = 258 W/cm2.

The ASHRAE minimum UV-C dosage levels would be attained at the furthest distance from the lamp to the coil, even without taking reflectivity into account. So, should less light be used? Because more light positively affects airborne microbial kill levels and there are no significant cost savings for using fewer or less-intense UV-C lamps, the 80-watt HO lamp or HO lamps are highly recommended by ASHRAE. All operating conditions must first be considered. Some conditions, such as air temperature and velocity, effectively decrease or “de-rate” the lamps’ performance. Yet other changes can positively increase design yield. In typical conditions of 500 fpm velocity and 55 F air temperatures, lamps are derated by about 50 percent.

UV-C light is incredibly effective and affordable for keeping critical components of commercial HVAC systems clean and operating to “as-built” specifications. The advantages of UV-C lamps in HVAC systems are more significant energy efficiency, lower operating costs, fewer tenant complaints, and better IAQ. UV-C is relatively simple to use for all these advantages; all that is required is installing a bank of UV-C lamps in an air handler or rooftop packaged system, followed by an annual bulb replacement.

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