Airflows and HVAC

[cardcrimson]

Not sure this is the right place for this, but an answer to M_T's question in the car airflow.

There has been a ton written about HVAC and Covid, though at this point, there are no clear solutions.

Background: 

• Sick building syndrome has been around for decades, and costs $60 billion in lost productivity annually per the EPA.
  
• Causes: VOCs, viruses, mold, bacteria, pollen, outdoor pollution brought indoors, etc.; brought indoors through three mechanisms: bringing in outdoor air; people and what they bring in (food, pets, viruses, etc., poor hygiene); the building itself (outgassing of materials, leaks creating mold, rodent infestation, etc)
  
• Energy savings tend to tighten buildings and exacerbate the issue.
  
• HVAC systems are designed primarily for comfort, energy efficiency, and installed cost; minimize outside air, install low pressure drop filtration (MERV 8)
  

Enter Covid:
CDC and ASHRAE (look it up) recommend bringing in more outside air and tighter filtration (HEPA); unfortunately, that won't really work.

• Systems aren't designed to bring in increased levels of outside air, requiring significant more energy and equipment capacity to cool in the summer and heat in the winter
  
• Systems aren't designed for the added contaminants either (smoke from fires, pollutants from cities, pollen)
  
• Systems aren't designed for high grade filtration; inadequate airflow across the filter will ultimately damage the compressors; adding increased fan capacity is an expensive retrofit
  

The alternatives:
HEPA Filtration

• Will trap contaminants, including the virus
  
• Confusingly, filters are measured on their effectiveness in trapping .3micron particles, which for some reason are the most difficult to trap. Smaller particle, such as Covid (around .1 micron) are easier to trap due to Brownian motion. Disappointingly, many new entrants to the market are stretching the claims of how good their filters are, because of the Brownian motion. "Our filters remove 99.9999% of all particles". While that may be true, the standard metric for comparison is how they perform with the .3 micron particles and they are misleading the uninformed masses. Shame. . . .
  
• As mentioned, systems aren't necessarily designed for the pressure drop over a HEPA filter (MERV 8 has a drop of less the .4 inches of water, HEPA over 1 inch of water)
  
• Importantly, filters in the duct work don't help at all with person to person transmission within the space, and the same can be said for many of the centralized purification strategies. As mentioned in the other post, a major hospital, working in concert with HVAC contractors, found that there is zero evidence of transmission of Covid through an HVAC system.
  

UVC

• Dangerous to humans, okay in ductwork. 
  
• Dwell time is key (the time the airflow is exposed to the UVC). Multiple bulbs that are 5-6 feet long are required to actually kill the virus as it's moving through the system. Systems are on the market and probably will kill the virus.
  
• UVC LEDs may be fine to kill the virus, if it is stuck on the evaporator coils, but they are certainly not strong enough to kill in the air streams. Not even close yet.
  
• Far UVC LEDs. Safe for humans (allegedly), but not strong enough at this point, either.
  

Bi-polar Ionization

• Positive and Negative ions attach themselves to impurities in the air, and drop them to the floor or attach them to walls.
  
• Half life of an ion is 5 to 90 seconds. Probably not very effective in large spaces where the virus may exist. Imagine an in duct ionizer with long duct runs. The ions will capture stuff in the duct, but won't survive long enough to protect those in the space themselves.
  
• Localized bi-polar ionization may indeed provide some protection, especially in smaller spaces
  

Ionized hydrogen peroxide

• Cold plasma created hydroxyl radicals and H2O2 ions appear to effective on the virus and other pathogens
  
• Half life is a question mark, as ions don't tend to survive for long
  
• I'll know more in a couple of days, as a client is developing some new tech here.
  

Ozone

• Very effective at killing mold, bacteria, viruses
  
• Considered a pollutant, and dangerous to humans at higher levels, okay at lower levels
  
• Much longer half life than ions
  
• California has strict regulations regarding purifiers that use ozone
  
• Ozone purification is common in Asia and Europe
  

So what to do now?

• I’m a big believer in localized purification; though there is a lot of snake oil out there (see Molekule)
  
• I think a combination of the above can be helpful, especially when combined with a device that pushes ions/ozone that can go into the space locally and attack the contaminants
  
• Forgot the most important suggestion, focus on CFM and air changes. The more the better, and at least 4X per hour, preferably 8X for a conference room, and higher for small spaces.
  
• The airflows of the purifiers should positioned to pull in from the inhabitants, and blow air down, away from people. Pushing air up or out is not good.
  

Long term

• There is a $15trillion installed base of HVAC equipment in the US. Changing that will not be easy.
  
• Eventually, new designs will be similar to that in planes. Air will flow from the HVAC system through diffusers in the ceiling and exit the space through returns on the floors or lower walls. That way, air will not flow readily from person to person as it does today. (Currently both the diffusers and returns are typically in the ceiling, and many homes have the registers on the floor and the returns in the ceiling. No bueno)
  

[M_T]

+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

----------

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?

------

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.

[cardcrimson]

+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

----------

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?

------

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.

Actually, all of the alternatives can be either in duct or stand alone. Ozone is allowed in CA, as long as it's under 50ppb, and there is some disagreement what level is required to inactivate Covid. I have some test results, but those pesky NDAs. Ozone is certainly allowed in CA for "professional" services, such as mold and odor remediation, but only when the space is unoccupied if you're going above 50 ppb (OSHA is 70ppb).

The issue with returns on the ceiling is the mixing of the air. It isn't perfect, or even close. The aerosols will tend to circulate, with the particles remaining aloft and dispersing throughout the room. 98.6 vs 72, in the scheme of things, is not a huge difference.

The best way to configure the purifiers is through computational fluid dynamics, but that's not available to everyone. Also important is the existing HVAC system and how that flows. Best to try and have them complement each other using common sense. Most work the way you mentioned, and there in lies an opportunity to exploit. Stay tuned. . . .

[cardcrimson]

M_T,

Understand your point about the feeling of fresh air in your face when you enter the room. Two things, the purifiers are indeed pulling crap out of the air, and that's a good thing. Your question about the air flows was still bouncing around in my head this morning, when it dawned on me that all of these purifiers were designed before the pandemic, so blowing fresh air into a face was a good thing. Now, if someone who is shedding the virus is between you and the machine, an air flow to the face is a bad thing. Similarly, an air flow to the face of an infected person is going to spread the aerosolized virus throughout the room even more.

Getting the exhaled "air" away from a person and cleaned quickly is now the key. . . .

[ChrisGreene]

M_T,

I believe humid air is less dense. It "feels" heavy, but it's not. Think of it this way, all air molecules in the room have the same average distance. There's N2, O2, CO2 and H2O. The relative weights of the molecules are 28, 30, 42, and 17.

[M_T]

You're right about the humid air. I screwed up there.