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How to Size UV Light for HVAC

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Integrating UV-C light into HVAC systems effectively maintains cleanliness, boosts airflow, and enhances overall system performance. Ultraviolet germicidal irradiation (UVGI), particularly at 253.7 nanometers, has been incorporated into HVAC systems since the mid-1990s.

This technology helps prevent the accumulation of organic materials like mold, bacteria, and other microorganisms on components such as cooling coils and drain pans. To fully leverage the advantages of UV-C technology in HVAC systems, it’s crucial to accurately size the UV-C light for the specific application. This guide will delve into the process of correctly sizing UV-C light for HVAC systems to ensure optimal performance and energy efficiency

Why Use UV-C in HVAC Systems?

Understanding the advantages of UV-C light in HVAC systems is crucial before addressing the details of sizing. UV-C light plays a key role in enhancing both the operational efficiency and indoor air quality (IAQ) of these systems by keeping vital components clean. By halting the buildup of organic matter, UV-C light ensures that the system maintains its intended cooling power, airflow, and heat-exchange effectiveness, potentially leading to energy savings of up to 25%. Moreover, it boosts IAQ by reducing harmful microorganisms that could otherwise spread through the system’s air circulation.
 
Additionally, older systems can regain lost efficiency through UV-C light as it cleans dirty components, thereby enhancing the system’s heat exchange capability and reducing pressure drop across the coil. This cleaning process is especially advantageous in retrofit applications, where it can revitalize HVAC systems that have diminished efficiency over time due to biological growth.

Key Factors for Sizing UV-C Light in HVAC Systems

While UV-C technology is straightforward in application, correctly sizing it is crucial for optimal results. Key factors influence how much UV-C energy is necessary and the lamp configuration. Let’s delve into the main considerations:

1. Essential UV-C Energy

First, determine the UV-C energy needed for effective cleaning. Industry standards, like those from ASHRAE, provide clear guidelines for UV-C use in HVAC systems, especially on cooling coils. The 2015 ASHRAE Handbook – HVAC Applications suggests a UV-C dosage of 50 to 100 µW/cm² for cooling coil surfaces. This level must be achieved across the entire coil, even in hard-to-reach spots like plenum corners.

Calculating UV-C dosage involves more than just lamp selection. Factors such as lamp-to-surface distance, airflow rates, and coil shape can affect actual irradiance levels.

2. Lamp Output and Placement

In HVAC systems, UV-C lamps resemble fluorescent lamps but emit primarily in the UV-C spectrum. The most used UV-C lamp is a high-output (HO) 80-watt lamp. At 1 meter, it can generate 245 µW/cm² of UV-C energy. UV-C lamps are generally set 12 inches (0.3 meters) from the coil, producing about 1375 µW/cm² in irradiance. Despite this exceeding ASHRAE’s minimum, operational conditions like lower air temperatures (around 55°F) and higher airflow velocities (roughly 500 feet per minute) can reduce UV-C efficacy by up to 50%, dropping irradiance to 688 µW/cm².
 
Even with these reductions, UV-C output still surpasses ASHRAE guidelines. Ensuring complete coverage requires considering the distance to the plenum’s farthest corners. UV-C dosage tends to decrease the further from the lamp, reducing to around 172 µW/cm² in the corners, which is still above the minimum for effective microbial control.

3. Boosting UV-C Performance with Reflectivity

Reflectivity of surfaces around the coil is a factor often overlooked in UV-C lamp sizing. Surfaces reflect UV-C light to different extents, enhancing overall system effectiveness. For instance, galvanized steel plenums have a reflectance multiplier of 1.50, boosting UV-C energy by 50%. This reflectance can increase UV-C energy in difficult-to-reach areas, ensuring thorough cleaning and disinfection across the system.
 
Incorporating reflectivity into the design maximizes UV-C energy distribution throughout the HVAC system, improving performance

4. Simplified Sizing Formula: Watts per Square Foot

For a straightforward UV-C lamp sizing process, HVAC experts can use the rule of thumb: 7.5 watts per square foot of coil surface area. This calculation exceeds ASHRAE’s minimum and offers an easy way to determine the number of lamps needed for a system.
An 80-watt high-output UV-C lamp covers around 10.76 square feet of coil area. By dividing the lamp’s wattage by the surface area, an 80-watt lamp provides 7.43 watts per square foot of coil area—adequate for most HVAC uses

5. Choosing the Right UV-C Lamp Type

Two main UV-C lamp types are used in HVAC systems: single-ended and double-ended.

1.Single-ended lamps: Offer installation flexibility and can overlap for continuous coverage. Ideal for smaller systems or tight spaces like rooftop units.
2.Double-ended lamps: Designed for specific lengths, making them suitable for precise applications.
Selecting the right lamp type depends on the HVAC system’s needs and available installation space

Installation Considerations

When installing UV-C lights in an HVAC system, it’s crucial to position the lamps correctly for peak effectiveness. Typically, placing the lamps twelve inches from the coil is ideal for most systems, although this can vary with different configurations. Alongside proper positioning, it’s important that the UV-C system features controls for safe operation and easy maintenance. Safety measures like door interlock switches can automatically turn off the UV-C lamps when access doors are opened. Additionally, incorporating lamp failure indicators and self-powered current sensors can alert facility managers to any lamp outages, minimizing the need for manual checks

Maintenance and Lamp Replacement

To maintain consistent performance, UV-C lamps should be swapped out annually. Over time, their effectiveness declines, even if they still appear to be working. Regular maintenance and timely replacement are crucial to keeping the system’s cleaning capabilities intact and ensuring the HVAC system runs optimally.

Conclusion

Selecting the right size for UV-C lighting in HVAC systems is vital for maximizing the cleaning and maintenance of key components like cooling coils. By adhering to ASHRAE guidelines for UV-C dosage and employing the straightforward 7.5 watts per square foot sizing approach, HVAC experts can design and install UV-C setups that boost energy efficiency, enhance indoor air quality, and lower operating costs. With UV-C technology, HVAC systems can function at their designed capacity, offering cleaner air and improved efficiency for the long term.

UV Light Size Calculator for HVAC

Note: Adjust airflow and temperature values for accurate results.

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