For over 70 years, 253.7nm ultraviolet light has been used to save electrical energy and reduce maintenance frequency by installing lamps along the cooling coil. This practice can save up to US$150 per ton of cooling per year, while also improving air quality. Fungi and mold growing on the coil form a biofilm that acts as an insulator, and heat transfer is not optimal, also airflow can be reduced by up to 25%, producing higher energy costs.
Keeping the coil clean is purely a geometric issue; it must be ensured that the radiation over the coil is uniform and that the light reaches all points. Enough lamps of sufficient length must be placed to cover the entire coil, as UV light only works where it shines. The dose (mJ/cm2) depends on the product of time (s) and power density (uW/cm2), and since the exposure time on the coil surface is arbitrarily long, the lamp intensity can be low. ASHRAE indicates that values like 100 uW/cm2 are sufficient although at least 1000 uW/cm2 is recommended for faster action.

It is quite different if we wish to disinfect the airflow. Although there are specific ASHRAE standards to validate UVC disinfection in handlers and ducts (185.1.2020 /185.2.2020) and for infection control (ASHRAE 241-2023), the recommended values in the ASHRAE Handbook are more than 10,000 uW/cm2 to disinfect the airflow. This is because the exposure time for a high flow is very small, usually between 0.3-0.8 seconds; with such a small time, the power density must be very high to compensate and maintain a sufficient dose for inactivation. The distribution of lamps within a handler or duct must be such that at all points at least 10,000 uW/cm2 can be guaranteed, so that when the air passes through, during the time established by the flow, it receives enough radiation for a correct dose. If higher power densities can be guaranteed, we can have greater disinfection efficiency, which is referred to as the log of disinfection:
log 1 = 90% disinfection
log 2 = 99% disinfection
log 3 = 99.9% disinfection
log 4 = 99.99% disinfection … etc
For most bacteria, viruses, fungi, and mold, knowing one of the log values is enough; the rest is determined by the formula:
log 2 = 2log 1
log 3 = 3log 1
log 4 = 4*log 1 … etc
The best way to verify an installation is by measuring the power density with a radiometer calibrated at 254nm or using a dosimeter that measures the dose directly.
Therefore, disinfection is not just a matter of choosing the right lamp but is more critically about the installation; the lamp must be placed at an appropriate distance and for an appropriate exposure time.
Some important doses (mJ/cm2)