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Airflows and HVAC
#2
+1 Great info.
All of the points you listed from HEPA to Ozone are purification methods are mostly in the ducts, right? The local purification methods are the room sized purifiers (which included up-to-HEPA filters, ozone (not in CA), and ionization). I hadn't seen hydrogen peroxide mentioned before

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The final bullet point is opposite what I would have thought. I've definitely seen it said that planes could have been safer if they had exit vents above people and put the blowers at the windows.

If an infected person enters my house (one central exit vent in the ceiling with numerous floor registers), they will exhale air that contains droplets and aerosols that contain the virus. For all air-conditioned space and for most of the year in un-airconditioned spaces, the exhaled air will be 20-30 degrees F hotter than the room air and will rise toward the ceiling. (Based on remembered observation of exhaled smoke, I'd guess it rises about 0.5 ft/sec.)

Large droplets (100u and larger) will eventually fall to the floor (< 100u will evaporate to become aerosol before hitting the floor). However, plumes from people's body heat will lift those droplets up, and small eddies will mix them up.

Droplets that fall to the floor won't be removed by air filtration, so we can ignore those. If there were no airflow, the exhaled air would cool and tend to fall toward the floor (denser due to higher humidity). If there is a decent amount of airflow, it would seem to be better to pull the air upward and out of the room at the ceiling. Once that air & aerosols are in the open space above peoples heads, they should have a mostly clear path to ceiling exhaust vents without solid objects (bodies, furniture) that disrupt the path of the air. It would seem incorrect to cause the airflow to work against the natural pluming effect of exhaled air and blow it down, where eddies might make it swirl up into the area of other people's noses & eyes.

Is something wrong with my lay understanding of the airflow of small droplets and aerosols?

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The room-sized air purifier I have blows up toward the ceiling (actually at a slant), taking air from near the floor. I believe most work that way. I find I enjoy walking into that room. Probably that is due to the almost imperceptible air flow over my face causing a perception of fresh air. Possibly I am sensing a more evenly distributed temperature vertically.

The problem for COVID or colds/flu is that the air flow is not one-way. What goes up must come down. Where it comes down is where the exhaled air near the ceiling is in danger of being inhaled. There is also lateral flow - near the ceiling, away from the purifier; near the floor, toward the purifier.

Trying to figure out how to set up the purifier (or, more complicated, two) so that the downward flow doesn't happen where people are is challenging, because people want to spread out throughout the room. If one end of a room is set up with equipment (TV, printers, etc.) that is a candidate area but the heat from those devices will counter the downward flow.
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Messages In This Thread
Airflows and HVAC - by cardcrimson - 01-12-2021, 12:45 PM
RE: Airflows and HVAC - by M_T - 01-12-2021, 03:10 PM
RE: Airflows and HVAC - by cardcrimson - 01-12-2021, 05:52 PM
RE: Airflows and HVAC - by cardcrimson - 01-13-2021, 08:35 AM
RE: Airflows and HVAC - by ChrisGreene - 01-15-2021, 10:24 AM
RE: Airflows and HVAC - by M_T - 01-15-2021, 05:21 PM

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