Failure to recognise when an aerosol isn't a droplet 26

[LittleInch]

FacEngrPE dropped this in an obscure post and it reads very well.

Basically the whole epidemiological world though various dieses and viruses were spread by "droplets" which landed on surfaces and then infected people or were sneezed at you.

And a lot of times they are probably right.

But there was a magic 5 micron cut off between droplets and aerosols. Why? Read on

https://www.wired.com/story/the-tee...t-helped-covid-kill/?utm_source=pocket-newtab

So is this a disaster - Well you tell me.
But it goes to show that just because a lot of people write something, it doesn't mean that they originally got the wrong end of the stick and then the error repeats itself until it becomes fact.



Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[RedSnake]

I am not sure what kind of ventilation systems you have over there.
Here we never recycle the same air, new air comes in and get heated by the old air going out.

We also was hit hard in the retirement / nursing homes in the beginning.
The reason was a lot of circulation on short term employed staff, shortage of mask and visirs.
The ones that managed the best was the facilities that early on set up to different physical restricted areas for infected and none infected persons and never mixed staff or things in-between.

Best Regards A


“Logic will get you from A to Z; imagination will get you everywhere.“
Albert Einstein

 

[electricpete]

As you must know, that is
ΔT = Q x Mass x Cp

yes absolutely.
Therefore we can also say ΔT = Q x Volume*Density x Cp

It is about mass.
Volume is not in the equation

In my thought process water is relatively constant density. The ratio of volume to mass is constant (that ratio does not change between large and small particles)
You have not postulated any change in density, so you have the same thought process.
We're all saying the same thing.

... that is, unless we put beer into the equation, then I'm not sure what happens to the density when the bubbles come to the surface!


=====================================
(2B)+(2B)' ?

 

[moon161]

Ar/V = 3 I'd say 3 is constant, but I don't care.

This is no longer a ratio of volume to area. It's a dimensionless group. The units of volume/area would be L or L³/L²

You're asked for a paint estimate to protect a 100,000 gallons of oil storage, as

1 x 100,000 gallon tank
10 x 10,000 gallons tanks
100 x 1,000 gallon tanks

Discuss. Assume similar geometry for all tanks.

 

[1503-44]

Water's density can change guite a lot, so its better to work without density in general and C_p values, because we have constant pressure, so it is more useful to us than C_v, at constant volume. Plus for evaporation, we need to add another 50% to the heat required to cross the phase change. But all we need to know here is simply the smaller drop has far less mass. Gravity would also affect the trajectory of each size equally, but larger particles probably would acquire lots more drag in air and fall shorter than smaller sizes. As to if the smaller ones survive to travel longer distances before evaporating, i think probably not. Maybe there is an optimal size that maximizes trajectory, but I'll let somebody else work on that one.

Statements above are the result of works performed solely by my AI providers.
I take no responsibility for any damages or injuries of any kind that may result.

 

[FacEngrPE]

Redsnake answer your question about how the HVAC systems handle fresh air, Most systems - except for specific occupancy recirculate most of the air used for heating or cooling. A minimum amount of outside air is required, Some larger systems can use up to 100% outside air to get free cooling in winter.

It is unusual for a typical office or store to be designed for significantly larger than required amounts of outside air, and sometimes these occupancy attempt to go greener by controlling outside air on space CO2 levels.

Heat exchanges on the outside air exchange are unusual, the economics generally do not work.

 

[EdStainless]

I suspect that another underlying issue with the 5um fallacy is based on some unstated assumptions about how may virus particles are critical for infection. I am sure that in many cases the fraction of the total virus load in the very fine particles is a small fraction of the total and ignoring them is not a big deal. But in other cases this is not true.
Stacking assumptions can get unlessant fast.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[RedSnake]

Some larger systems can use up to 100% outside air to get free cooling in winter.

Hmmm... maybe that is the difference. ;-)
We defiantly don't need free cooling in the winter , -20 C , brrrrr

This building norm here.
100% outside air comes in and is heated up by the outgoing air.
The incoming air then goes to all living areas, bedrooms, livingroom.
The outgoing air is taken from all rooms that produce most, bathroom, toilettes, kitchen.

Best Regards A



“Logic will get you from A to Z; imagination will get you everywhere.“
Albert Einstein

 

[moon161]

^ Yes, I used to work in this factory and it seemed like the whole building vented through the no-flush urinal.

 

[moon161]

I think another reason that the size dependant ratio of surface area to volume matters with droplets is more readily observable than heat/mass transfer and that's settling velocity. The smaller the droplet or particle is, the longer it is likely to stay airborne. This is why we see air handling as a hazard and a mitigator, depending.

If the proportion of body force to surface force on a falling droplet was the same for a droplet of any size, I don't think we'd see this.

 

[dik]

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[dik]

link...

https://uvresources.com/solutions/infection-control/

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[Compositepro]

"You just said, "volume to surface area of a sphere is r/3". Does that change?"

Yes it does. 'r' is is not only a variable, it is the variable that this whole discussion is about.

The ratio of surface area to volume changes linearly with 'r'. And yet you say that the density of water changes "a lot". For the purpose of this discussion, it does not. Volume = mass.

 

[1503-44]

If you wish.

Statements above are the result of works performed solely by my AI providers.
I take no responsibility for any damages or injuries of any kind that may result.

 

[RedSnake]

Well one thing I know is that one 1 liter water aprox. 1 dm3 aprox. 1kg is much more in dm3 if it is froze, ;-)
especially at -30 C.
The mass is the same but the volym is much larger.
I am not sure a snowflakes counts as droplet though.

/A

“Logic will get you from A to Z; imagination will get you everywhere.“
Albert Einstein

 

[RedSnake]

And thanks dik for the translation.
I was a bit lazy, but I see "google tranlate" works from Swedish to English as well.

/A

“Logic will get you from A to Z; imagination will get you everywhere.“
Albert Einstein

 

[1503-44]

If you freeze it, vol is almost +10%. One of the relatively few substances that expands on cooling.

Statements above are the result of works performed solely by my AI providers.
I take no responsibility for any damages or injuries of any kind that may result.

 

[davidbeach]

If it didn’t expand on freezing there might not be life on a planet with most of the water frozen.

I’ll see your silver lining and raise you two black clouds. - Protection Operations

 

[dik]

I just wanted to make sure the titles were what I thought they might be... two of them, I didn't have a clue... Swedish... I thought it was German...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[dik]

Water is most dense, I understand, at +4C...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[dik]

think tartigrades... tough little mothers...

https://phys.org/news/2021-05-tardi... effort, the,also survive high-speed impacts.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik