Covid 19/ Ventilation

[nicolai]

With the current pandemic going on
what is the anticipated upgrade expected of guidelines
especially on recirculating air conditioning systems
what would be the best technology to fit on the return air side of FCU's etc
HEPA filtration will result in huge pressure losses

whast your take?

 

[itsmoked]

Likely UV somewhere in the plenum.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[miningman]

Its already a real thing , up and running in my house for three months

https://www.eng-tips.com/viewthread.cfm?qid=471665

 

[IRstuff]

I think you meant this one: thread1088-466759.

Nevertheless, it's unclear whether such a system is effective and whether it's worth the money. Moreover, it looks like far-UVC (<222 nm) is possibly a better choice

https://www.nature.com/articles/s41598-020-67211-2

> lower power density to kill pathogens
> less harmful to humans

The lower kill density means that for the same power level, it'll kill coronaviruses faster, which means it might actually work in an HVAC system. Furthermore, since it's not harmful to humans one could actually use in continuously in the living space itself, thereby sanitizing surfaces as well as the air. And since the air isn't moving as fast in the living space, there's a much better chance of kill the airborne virus

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

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[AdrienneGC]

I met with a vendor last week who did a tremendous amount of research regarding the coronavirus. His presentation was very academic and my notes are not up to par, but as best as I can, here is a summary:
[ul]
[li]Since the virus is so small, < 0.1 microns, it is dictated by Brownian motion rather than standard physics, e.g., gravity[/li]
[li]A technique to capture the virus is to alter the shape of the virus by something like a "power generator air purification system", or to conglomerate the particles to increase their size[/li]
[li]UV lights work well to kill anything in the AH. You could also incorporate portable UV lights to be used when the space is unoccupied or fixed UV lights in occupied spaces (taking proper safety precautions)[/li]
[li]He recommended designing DOAS with 150% of the OA required[/li]
[li]He recommended a non-ozone ionizer[/li]
[/ul]

Edit: He also recommended reading ASHRAE's documentation on the reopening of schools and universities, and their documentation on medical facilities.

 

[IRstuff]

Since the virus is so small, < 0.1 microns, it is dictated by Brownian motion rather than standard physics, e.g., gravity

This is not strictly relevant; the virus is emitted from the body with droplets or aerosols, with the only aerosols being in the sub to 1 um diameter regime. Even after over 6 months, there's still a lot of uncertainty about what the actual sizes are and which are the ones most prevalent for infection.

The issue with UV lights, either UVC or far-UVC, is the exposure time required to kill the virus, which is in the multiple minute range. But, assuming a 900 cfm in a 4 ft^2 duct results in 3.75 ft/s air velocity. Let's say the UVC source covers 2 linear ft of the duct at its widest point (this is actually unrealistic); that results in 0.53-s dwell time of the air being irradiated by the UVC, which is orders of magnitude less that required to directly kill COVID19 virus. However, when running the AC, the evaporator acts as a dehumidifier, which results in desiccating any droplets or aerosols in the air stream. This alone will inactivate the virus in about a minute.

I'm not convinced that a non-ozone ionizer is the ticket, since ozone has sterilizing capabilities of its own; if the ionizer is buried in the AH, that might be similarly effective in killing the virus, particularly if it's embedded in aerosol or droplet.

TTFN (ta ta for now)
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[itsmoked]

IR; I'm not on board with your multi minute requirement. It makes no physics sense at all. It takes very few photons of UV energetic light to utterly disrupt a virus.

A 60W light spews 2 x 10²⁰ photons per second! It's estimated there are 100 times fewer grains of sand on planet Earth. (Per second!)

A single sub-second corner-of-the-eye flash from a big arc welder can do enough damage to stick you in a dark room for several days and that's an enormous eye ball. Think about a 120nm virus.

I totally disagree with minutes for air-suspended UV sterilization. A puddle of snot on a surface? Sure I'll agree with minutes.

There's no reason any particular virus in a forced air environment wouldn't pass thru the air handler dozens of times.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[miningman]

I believe the last two posts here are the best summary of this UV discussion. With the most open minded attitude all of us here are diseminating less than accurate facts / theories.
I certainly have my own beliefs and have put my money on the line. Other than wasting cash . I cant see any downside in my approach while I wait for consensus to develop.

 

[IRstuff]

You can believe what you want, but data says otherwise:

https://www.cuimc.columbia.edu/news/far-uvc-light-safely-kills-airborne-coronaviruses

https://www.medrxiv.org/content/10.1101/2020.05.31.20118588v3

The latter article only listed the required dosage for UVC as 1.5 J/cm^2. A normal UVC bulb, puts out around 25W, but that's going to be spread out over, say, the 4 sq ft I used earlier. So that drops the dose to 6.7 mW/cm^2. To achieve 1.5 J/cm^2 total dose, we therefore need to irradiate for 223 seconds, or nearly 4 minutes.

As with sunburn, medicines, and poison, there is some threshold of dosage above which damage or death occurs. Moreover, the total population of the virus needs to be reduced not just any one particle. I've stated my position that A/C is a low-likelihood infection pathway anyway, since it's pretty obvious that direct infection from inhaling virial particles from someone, even 6-ft away is much more likely than waiting for the virial particles to travel 20 or 30 ft through the A/C filter, ducting, and evaporator and some number of feet from the register to you.

TTFN (ta ta for now)
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[miningman]

Unfortunately theres no numbers here as to wavelength or exposure time, but theres some semi serious $$$ being tried out here

https://bc.ctvnews.ca/video?playlistId=1.5043888&cache=yesclipId104062?clipId=104070

 

[miningman]

https://www.mansionglobal.com/artic...uality-ahead-of-a-summer-spent-indoors-215103