# Thread: UV Lights and Air Cleaning

1. Regular Guest
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May 2008
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## UV Lights and Air Cleaning

About ten years ago I heard an EPA rep say that UV lights were good for keeping mold off of coils that it shines on directly, but that to just have it in the return air stream, the air is moving to fast to have any effect on it.

Has anyone seen definitive proof either way?

2. Wrote this on a rebuttle on a hospital project that was trying to spec just UVGI lamps on a project
Lamps alone are not effective in a fast-moving airstream. Here are our calculations for Airhandler 7:
Return side dimensions: 20’ x 16’
400 FPM = 6.67 FPS
Supply duct = 100” x 80”
2,016 FPM thru supply duct = 33.6 FPS
This works out to be 112000 CFM thru the air handler.
Consider Aspergillus niger (black mold). The k-value to destroy this mold with 90% confidence is 0.00000128 cm2/mW*s (Kowalski 2006). Consider lamps with an intensity of 10,000mW/cm2 12” away (very high intensity lamps!).
Time to Destroy = 1/(k * I)
Time = 1/(0.00000128 * 10,000)
Time = 78.125 seconds
This is fine for surface cleaning. If your mold is growing on a surface, it will take only 78 seconds to kill. However, in an airstream, this proves difficult.
How far does a particle travel in an airstream in 78 seconds?
In the airhandler:
Distance traveled = Time * Speed
= 78.125S * 6.67 FPS
= 520’ in direction of airflow
The airhandler is not that long, let’s try the ductwork.
Distance traveled = Time *Speed
= 78.125S * 33.6 FPS
= 2,625’ in direction of airflow
Not many jobs have ½ mile of ductwork.
Now, we have to light the ductwork to keep 10,000mW/cm2 on the particle.
First, assume we can find a 10,000mW/cm2 lamp, 48” long. This will allow 12” spacing between lamps.
4 lamps across the section, 6 lamps high = 24 lamps in one array. Install this array for every foot of ductwork.
24 lamps per foot * 2625 feet = 63,000 48” lamps. These run at 1 amp each, so count on 63,000A @ 120v all day long, so A * V = 7,560,000 Watts.
Cost per Day to run @ 6.26 cents per KWh - \$11,360 for 1 single AHU. There is no payback time on this.

3. Good read genesis! Really shows how futile it is to install UV lights on the return.

Put the UV lights in near the coil so they kill the mold and bacteria that want to grow there.

Then put in a good air cleaner such as a MERV 16 on the return side of the equpment.

I'm assuming you are talking about residential equipment?

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