Clarifying Myths About UV and IAQ
Can Coil Cleaning Design be Applied for Airstream Disinfection?
The COVID-19 pandemic has spawned a broad proliferation of unproven, unvalidated UV systems which are often either undersized or oversized for the application without due consideration of the design factors discussed here. Some of the common misapplications which have been reported are systems which claim air disinfection, but have been sized for coil applications (i.e., sizing is based on intensity rather than dose), or can be simply upgraded by adding just a few more lamps. This is not true as installation specifications vary widely, based on the required dose and multiple parameters relative to the site characteristics. The following example illustrates this in practice:
According to ASHRAE guidelines required for airstream disinfection in a HVAC system, a minimum dose of 1500 microwatts /cm2-sec is recommended for a minimum irradiance zone of 2 feet. At the ASHRAE recommended intensity of the coil of 100 microwatts/cm2, this results in an applied dose of only 24 microwatts/cm2-s compared to a minimum dose required of 1500 mw/cm2-s. Even if we increase the intensity over ten-fold to, say 1250 microwatts/cm2 , that is still not sufficient to achieve the required dose for airstream disinfection because of the short exposure time. The effect is accentuated even further at higher air flow speeds.
While there is infinite time for coil disinfection, which requires relatively low UV intensity levels, there is only a fraction of second available to disinfect the virus particles moving in an air stream and therefore require much higher lamp intensities.