Why Porex Virtek PTFE lined UVC disinfection chambers can provide many times the average irradiance versus aluminum and other reflective materials
July 8, 2021
by Gerry DiBattista, VP of Marketing – PTFE
With the onset of COVID-19, the world will forever be changed. With heightened awareness of potential airborne viruses, ensuring the cleanliness of indoor air – especially in public spaces – has become a high priority and a marketing opportunity for proactive venues.
Despite the focus on indoor air quality, there remain many questions on the methodology and efficacy of air cleaning methods. High efficiency HEPA-type filtration can be effective, but many air handling systems may not be properly designed to handle the increased pressure drops needed or be properly maintained to be effective. UV-C light has come to the forefront as an effective way to reduce pathogens and molds – however the proper time needed to dose UV in a high airflow system to inactivate viruses is often insufficient. Simply adding a UV source to the filter system or condensing coils will do little to deactivate viruses (it is mainly used to control mold).
Inactivating a pathogen
To inactivate a pathogen in a moving air stream using UV energy, there are several things to keep in mind. First, understanding the UV intensity and how much total energy must be absorbed by the pathogen to render it inactive is critical – think about how long it takes you to get a sunburn on a summer day – a brief exposure just isn’t enough.
The IUVA has an excellent database of the required exposure levels to reduce activity of pathogens (known as log reductions) called Pathogen UV Sensitivity which can help as a guideline – however current studies have shown that as little as 2-10 mJ/cm2 of exposure in the germicidal range (typically 240-280nm) can reduce viral loads by over 2 logs (or 99%).
Using Sumpner’s Principle
So, in an actively moving airstream, how do you ensure that all pathogens are being properly exposed to render them harmless? The link above discusses fluence or dose needed – but how is that determined? To properly dose an airstream, you will need some sort of reaction chamber where the UV source can do its job and the air has some residence time to dwell for it to become properly exposed. The great thing about an air disinfection reactor is that the chamber can be void of objects and reflective media can be used to recycle UV energy to increase this fluence (or irradiance) level well above the UV source alone. This concept can be described using Sumpner’s principle, which interrelates mean surface illuminance and its direct and indirect components in ideal conditions. The equation is fairly basic, stating that the total average irradiance (E) is the sum of the direct (ED) and indirect (ER) irradiance:
Etotal = ED + ER
Direct irradiance (ED) is easy – that is just the energy output of the UV source to the surface. So, for simplicity, let’s just assume that it is 1 unit of energy (1 mW/cm2). Now for the more complicated indirect (or reflected) irradiance (ER). In an ideal lined reactor where there are no losses outside of the reflective surface losses (which is impossible as the UV source itself will absorb UV energy!) – the additional reflective irradiance is equal to the direct irradiance times reflectivity divided by 1 minus the reflectivity of the reflective surface or:
ER = ED x (R/(1-R))
R is the percent reflectivity of the reflective surface stated as a fraction (i.e., 70% reflectivity = 0.7). Therefore, for an ideal aluminum-lined reactor that has a reflectivity of 70%, and a UV source energy output of 1, the total average irradiance would then be:
Etotal = 1 + (1 x (0.7/(1-0.70)) = 1 + 2.33 = 3.33
You can see how important the reflective surface is as you get 3.33 times more energy having a reflective surface versus the source itself (in ideal situations….) that is an average of 3.33 total versus just 1 from the UV source.
Reflectivity of Steel
If the UV reflectivity is only 30%, which is typical for stainless or galvanized steel, then the total average irradiance in the same setup would be:
Etotal = 1 + (1 x 0.3/(1-0.3)) = 1 + 0.43 = 1.43 or about 43% of the total average irradiance versus a 70% reflectivity material. That means a material with about ½ the reflectivity (30% vs. 70%) yields a significant loss in total average irradiance (57%). At these levels, the equation seems fairly linear where percent reflectivity and average irradiance move at similar rates. If the reflectivity is zero, then the average is simply the UV source output.
Highly Reflective Materials
What is truly amazing is if you go further and increase the reflectivity of the surface to a high-reflectivity lining such as Porex Virtek® PTFE with reflectivity values of up to 97%. In this case, the average total irradiance with a starting point of 1 unit of energy becomes:
Etotal = 1 + (1 x (0.97/(1-0.97)) = 1 + 32.3 = 33.3
Yes – 33.3 versus 3.33 or 1.43! This is 10 times that of aluminum and 23 times better than stainless steel. So even though in raw percentage numbers, it seems that going from 30% to 70% to 97% should yield similar differences, in reality the differences are dramatic and are in fact exponential once you get above 90% (see chart below.) Think of it like a well-insulated thermos versus a simple cup. A thermos will keep liquids hot or cold for extended periods because the insulation resists heat flow, whereas a cup will get to room temperature fairly quickly. Using Porex Virtek® PTFE lining in your reaction chamber acts in the same way, as very little energy is lost when it strikes the surface and thus energy is reflected numerous times, creating a dramatic increase in overall irradiance versus a material that is only 70% reflective.
Now please keep in mind these are ideal conditions with no other points of loss, which is impossible. You will have some absorption from the UV source itself, edges and openings will be difficult to fully cover, dirt and oils can reduce performance, and any openings for the air to circulate will cause reflected energy to be lost. But in a well-designed UVC air disinfection system you could expect four or more times higher average irradiance using Porex Virtek PTFE versus aluminum. For stainless steel, the improvement can be over 10 times.
Total Irradiance in a Closed System
ETotal = ED + ER
Diffuse vs specular reflectivity
Another major benefit of Porex Virtek PTFE is that it provides a highly Lambertian reflection, which means that it has highly diffuse reflectivity (light reflects in all directions) that creates a very uniform energy distribution. Basic metal reflectors tend to be more specular (light bounces mainly in the same direction), which creates areas of higher and lower energy distribution. Poor energy distribution will lead to areas of bypass (underexposed) within a reaction chamber, which creates additional disinfection uncertainties.
Using Porex Virtek PTFE in your lining
So let’s go back to the introduction, where we discussed needing a specific level of energy absorption to deactivate pathogens. As you can imagine, if you have a high airflow and a relatively low residence time in your system, you will need some very high-intensity UV lamps or you will need to line a very long reaction chamber with UV lights to get enough UV energy distributed to be effective in disinfection. This can lead to very high upfront costs or simply not enough space to design an effective system. Utilizing Porex Virtek PTFE reflective media in your design and gaining four to ten times the average irradiance can offer numerous benefits including:
- Much lower operating costs
- Much lower initial costs
- Higher efficacy rates
- Higher levels of system confidence
There are numerous additional advantages, but the benefits of using Porex Virtek in UVC reflectivity for air disinfection applications are very clear.
Imagine how much more cost-effective a UV disinfection system can be if you can gain 4 times the average irradiance in the reaction (disinfection) chamber. Utilizing Porex Virtek PTFE as your reflective media provides much higher levels of design freedom, a high disinfection safety factor, and much lower overall upfront and operating costs. Porex Virtek PTFE is also highly resistant to UV energy above 200 nm and does not oxidize like metals can under UV energy and humidity, which can cause major losses in reflectivity. A well-maintained system utilizing Porex Virtek PTFE as the reflective media can have a very long and reliable operating life and provide a safe environment for your workers, customers, friends and family.
Check out our demo video and other UV resources
Demonstration Video: UVC Disinfection Chamber
See all Porex UV Resources in: Porex UV reflectivity Resource page