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A/C for Room Virus Removal

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10 minutes ago, Airbrush said:

Any volunteers for math modeling?  All I can say is if you have many vent fans sucking air out the ceiling, rather than one big fan that creates a wind tunnel, you distribute the motion and increase the area of the air moving up and out the ceiling.  Therefore no wind tunnel.  This will be expensive but better than nothing in a world where deadly pandemics can pounce on us at any time.

How is one big fan vs lots of small fans different? 

If you can’t model the behavior, how can you predict what happens?

 

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Posted (edited)
9 minutes ago, swansont said:

How is one big fan vs lots of small fans different?

Air moves faster through an air duct when you constrict the duct.  Water flows faster out a hose when you constrict the nozzle.  Having many smaller ceiling vent fans EXPAND the size of the exit.  When you increase the size of the exit door, more people can flow through the door walking, not running.

Edited by Airbrush

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Just now, Airbrush said:

Air moves faster through an air duct when you constrict the duct.  Water flows faster out a hose when you constrict the nozzle.  Having many smaller fans do the opposite of constriction.  When you increase the size of the exit, more people can flow through the door.

You said yourself a wind tunnel has a big fan. Where is the constriction? I ask again: how is one big fan vs lots of small fans different? (same area)

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Posted (edited)
5 minutes ago, swansont said:

You said yourself a wind tunnel has a big fan. Where is the constriction? I ask again: how is one big fan vs lots of small fans different? (same area)

Not the same area.  By having many fans you can increase the total area of the openings that the air is passing thru.  That slows down the air flow even though the same amount of air is expelled.  You could also have one giant fan the size of the ceiling turning slowly, pushing air out a giant hole 50 feet in diameter, but that is not practical for construction.

Edited by Airbrush

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Just now, Airbrush said:

Not the same area.  By having many fans you can increase the total area of the openings that the air is passing thru.  That slows down the air flow even though the same amount of air is expelled.  You could also have one giant fan the size of the ceiling turning slowly, pushing air out a giant hole, but that is not practical for construction.

Changing the question doesn’t make the answer correct.

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If people sneeze  at 100 miles per hour then, to make sure that the particles go up not across, you need an upward velocity near 100 MPH
You need that all across the room- because you can't be sure where someone will be standing.
Such rooms exist...
 

 

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Posted (edited)
8 hours ago, John Cuthber said:

If people sneeze  at 100 miles per hour then, to make sure that the particles go up not across, you need an upward velocity near 100 MPH
You need that all across the room- because you can't be sure where someone will be standing.

False.  100 mph only near the sneezing nose or coughing mouth.  After a few feet the water droplets slow down RAPIDLY due to air resistance.  Heavy droplets fall to the floor in one second.  The lighter droplets are deflected over the heads of people after the sneeze droplets travel about 10 - 15 feet.  I have seen high-speed video analysis of droplet projection from coughing, sneezing, or shouting.  The droplets do NOT travel across the room, only about 15 feet.  Even if they did travel across the room, it would not matter since the bad air would be over people's heads in seconds.  After only a FEW seconds of AAR (Accelerated Air Replacement) the strong suction fans in the ceiling will suck the air upward at about 2 mph.  If you have enough fans, of adequate diameter, sucking air out the ceiling, turning at the proper rate, you can adjust the vertical air flow so all the air in the room leaves in a few minutes without a wind tunnel.  You can move the air in a vertical direction over the heads of people in a few seconds.

Also, you can catch Covid-19 by droplets reaching your eyes!  So you need eye protection as well as an N95 mask.

Edited by Airbrush

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How does one large fan compare to many smaller? Other things being equal the single larger fan will be more energy efficient. But other things are never equal and the centralisation of the duct sytsem will come with more resistant.

One thing being missed is static v volume (in the fan selection). So those :"smaller" fans will still have the same static and aren't really smaller at all in the one metric that dictates sizze of motor/power etc.

Another thing - ducts can't really be pushed at more than 10m/s, for many reasons. Most designers will work to around 6m/s if they can get away with it. A lot of what is proposed here is impracticable.

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41 minutes ago, druS said:

How does one large fan compare to many smaller? Other things being equal the single larger fan will be more energy efficient. But other things are never equal and the centralisation of the duct sytsem will come with more resistant.

One thing being missed is static v volume (in the fan selection). So those :"smaller" fans will still have the same static and aren't really smaller at all in the one metric that dictates sizze of motor/power etc.

Another thing - ducts can't really be pushed at more than 10m/s, for many reasons. Most designers will work to around 6m/s if they can get away with it. A lot of what is proposed here is impracticable.

Energy efficiency would seem to be orthogonal to the discussion. Airbrush seems to be implying that a large area of small fans somehow inherently gives rise to a different flow rate than a large fan

"All I can say is if you have many vent fans sucking air out the ceiling, rather than one big fan that creates a wind tunnel, you distribute the motion and increase the area of the air moving up and out the ceiling."

Changing the are will matter, but in the above quote it's not at all obvious that the example is changing the area (from the term "big fan" and that wind tunnels generally have a fan as big as the tunnel) It's the speed of the air that matters. The speed of the air being greater near a nozzle is only true near the nozzle, and it's not a given that there is any nozzle in this problem.

Mainly what I want is Airbrush to present some science or engineering to back up a claim, rather than a WAG. This being a science discussion site and all. The scenario is proposing that air is only around for a few seconds, which sounds like you are getting 10 or so room changes per minute, instead of per hour. To do this, the air has to move faster, regardless of how you do it. My point is having the air move 60x faster is not an imperceptible change, as was claimed. 

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10 hours ago, Airbrush said:

he lighter droplets are deflected over the heads of people after the sneeze droplets travel about 10 - 15 feet. 

By what?

Also, many or most rooms are only about 10- 15 feet across.

10 hours ago, Airbrush said:

The droplets do NOT travel across the room, only about 15 feet.

Again, that's a matter of wealth, not physics.

10 hours ago, Airbrush said:

Also, you can catch Covid-19 by droplets reaching your eyes!  So you need eye protection as well as an N95 mask.

Yep.

You will see people wearing it in clinical situations.

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Posted (edited)
4 hours ago, swansont said:

Energy efficiency would seem to be orthogonal to the discussion. Airbrush seems to be implying that a large area of small fans somehow inherently gives rise to a different flow rate than a large fan

"All I can say is if you have many vent fans sucking air out the ceiling, rather than one big fan that creates a wind tunnel, you distribute the motion and increase the area of the air moving up and out the ceiling."

Changing the are will matter, but in the above quote it's not at all obvious that the example is changing the area (from the term "big fan" and that wind tunnels generally have a fan as big as the tunnel) It's the speed of the air that matters. The speed of the air being greater near a nozzle is only true near the nozzle, and it's not a given that there is any nozzle in this problem.

Mainly what I want is Airbrush to present some science or engineering to back up a claim, rather than a WAG. This being a science discussion site and all. The scenario is proposing that air is only around for a few seconds, which sounds like you are getting 10 or so room changes per minute, instead of per hour. To do this, the air has to move faster, regardless of how you do it. My point is having the air move 60x faster is not an imperceptible change, as was claimed. 

A large area of small fans do not inherently give rise to different flow rates than a large fan.  You can have one giant fan moving the SAME amount of air as many smaller fans.  I only thought it would be impracticable to build such a large fan.  Cheaper to use a number of smaller ceiling-suction-vent-fans.

The speed of air will be greater near a nozzle constricting the area that the air is passing thru.  For example, if you have a room of 100 square feet (10' x 10'), you can have about 9 ceiling vent fans with a diameter of 18 inches or 1.5 feet.  Area of each circular vent is PI X diameter (3.142 x 1.5' = 4.7 square feet per vent).   Times 9 ceiling fan vents (9 x 4.7 = 42) square feet of vent area.  That means almost half the square footage of the ceiling will be venting area circles sucking out the used air.  With that much venting area you can move a lot of air slowly.  You can have air entering the room faster around the perimeter near the floor.

What is a "WAG"?  You are misunderstanding my proposal.  Air is not "only around for a few seconds."  In a few seconds the droplets projected from a sick person will float above the heads of people.  It would take several minutes for all the air in the room to be removed.

I have never heard of AAR (Accelerated Air Replacement) have you?  Did I just invent the term?  In the future engineers will be considering AAR in new construction I suspect.

Edited by Airbrush

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9 minutes ago, Airbrush said:

A large area of small fans do not inherently give rise to different flow rates than a large fan.  You can have one giant fan moving the SAME amount of air as many smaller fans.  I only thought it would be impracticable to build such a large fan.  Cheaper to use a number of smaller ceiling-suction-vent-fans.

Wind tunnels literally have fans like this, but irrelevant if you are getting the same airflow from multiple fans. The air speed will be the same if you are moving the same amount of air through the same area.

 

9 minutes ago, Airbrush said:

The speed of air will be greater near a nozzle constricting the area that the air is passing thru.  For example, if you have a room of 100 square feet (10' x 10'), you can have about 9 ceiling vent fans with a diameter of 18 inches or 1.5 feet.  Area of each circular vent is PI X diameter (3.142 x 1.5' = 4.7 square feet per vent).   Times 9 ceiling fan vents (9 x 4.7 = 42) square feet of vent area.  That means almost half the square footage of the ceiling will be venting area circles sucking out the used air.  With that much venting area you can move a lot of air slowly.  You can have air entering the room faster around the perimeter near the floor.

But if the air is moving slowly, it will not get sucked out of the top quickly. Here's where analysis comes into it, where you can avoid making potentially contradictory statements like this. Continuity matters in situations like this. You can't be moving more air in than you are moving out. Moving the air slowly is not in keeping with your desire to move air away from people quickly.

 

9 minutes ago, Airbrush said:

What is a "WAG"?  You are misunderstanding my proposal.  Air is not "only around for a few seconds."  In a few seconds the droplets projected from a sick person will float above the heads of people.  It would take several minutes for all the air in the room to be removed.

Wild-Ass-Guess i.e. a proposal with no basis in modeling or analysis

I suppose I was thrown by your statement in the OP where you said "You want the air to circulate so fast that if someone sneezes the water droplets will remain inside the room only a few seconds before it is sucked out the ceiling vent. "

I've been basing my responses on what you said. Is this the scenario, or not?

 

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Posted (edited)
46 minutes ago, swansont said:

Wind tunnels literally have fans like this, but irrelevant if you are getting the same airflow from multiple fans. The air speed will be the same if you are moving the same amount of air through the same area.

But if the air is moving slowly, it will not get sucked out of the top quickly. Here's where analysis comes into it, where you can avoid making potentially contradictory statements like this. Continuity matters in situations like this. You can't be moving more air in than you are moving out. Moving the air slowly is not in keeping with your desire to move air away from people quickly.

Wild-Ass-Guess i.e. a proposal with no basis in modeling or analysis

I suppose I was thrown by your statement in the OP where you said "You want the air to circulate so fast that if someone sneezes the water droplets will remain inside the room only a few seconds before it is sucked out the ceiling vent. "

I've been basing my responses on what you said. Is this the scenario, or not?

Sorry for the miscommunication.  My intention was to say that when someone sneezes, with AAR the water droplets will be lifted over people's heads in a few seconds, removing most threat of infection.  It will actually take minutes for ALL that air to travel out of the ceiling suction vents.  But remember in the 10' x 10' room you have almost half the ceiling area (42 square feet) is venting used air.  Lots of air can move out at a slow speed.

You can have fewer intake vents blasting air into the room along the room's perimeter near the floor.  These can match the same air flow as the exhaust vents.  People may experience wind blowing near their feet, but it won't blow skirts up.

Also when we sneeze at 100 mph the droplets will not bounce off the walls.  The droplets meet air resistance that causes the mist to mushroom into a larger area so air resistance increases even more with distance from the sneeze, decreasing droplet speed dramatically, slowing the mist to well under 100 mph.

4 hours ago, John Cuthber said:

By what?

Also, many or most rooms are only about 10- 15 feet across.

Again, that's a matter of wealth, not physics.

"The lighter droplets are deflected [upward] over the heads of people after the sneeze droplets travel about 10 - 15 feet" by the upward motion of air moving 2 mph.

As to the size of the rooms, I'm thinking of AAR for larger, commercial spaces like hospitals, warehouses, offices, factories, businesses, or the homes of the wealthy.

Edited by Airbrush

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1 hour ago, Airbrush said:

A large area of small fans do not inherently give rise to different flow rates than a large fan.  You can have one giant fan moving the SAME amount of air as many smaller fans.  I only thought it would be impracticable to build such a large fan.  Cheaper to use a number of smaller ceiling-suction-vent-fans.

The speed of air will be greater near a nozzle constricting the area that the air is passing thru.  For example, if you have a room of 100 square feet (10' x 10'), you can have about 9 ceiling vent fans with a diameter of 18 inches or 1.5 feet.  Area of each circular vent is PI X diameter (3.142 x 1.5' = 4.7 square feet per vent).   Times 9 ceiling fan vents (9 x 4.7 = 42) square feet of vent area.  That means almost half the square footage of the ceiling will be venting area circles sucking out the used air.  With that much venting area you can move a lot of air slowly.  You can have air entering the room faster around the perimeter near the floor.

What is a "WAG"?  You are misunderstanding my proposal.  Air is not "only around for a few seconds."  In a few seconds the droplets projected from a sick person will float above the heads of people.  It would take several minutes for all the air in the room to be removed.

I have never heard of AAR (Accelerated Air Replacement) have you?  Did I just invent the term?  In the future engineers will be considering AAR in new construction I suspect.

try pi X diameter squared /4

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2 minutes ago, J.C.MacSwell said:

try pi X diameter squared /4

Please explain what you mean.  I thought a circular venting area would be pi X diameter.  What's with the "squared /4"?

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1 hour ago, Airbrush said:

Please explain what you mean.  I thought a circular venting area would be pi X diameter.  What's with the "squared /4"?

pi * diameter does not get you an area. Wrong units.

A = pi * r^2 = pi * d^2/4

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1 hour ago, Airbrush said:

Please explain what you mean.  I thought a circular venting area would be pi X diameter.  What's with the "squared /4"?

That's the circumference. The area is pi X r squared, or (pi X diameter squared)/4.

cross posted with Swansont

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2 hours ago, Airbrush said:

You can have fewer intake vents blasting air into the room along the room's perimeter near the floor.

That would lead to a lot of Marilyn Monroe, skirt blown up around the waist, type incidents.
Is this thread becoming a little silly ?

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Posted (edited)
On 5/15/2020 at 10:55 AM, J.C.MacSwell said:

That's the circumference. The area is pi X r squared, or (pi X diameter squared)/4.

cross posted with Swansont

Thanks for the correction.  That means in a room 10' x 10' with 9 exhaust vent fans, 9" radius each, in the ceiling you will have 3.14 x 9^2 = 3.14 x 81 = 254 square inches/144 = 1.77 square feet of venting area per vent, times 9 vents in the ceiling = about 16 square feet of venting area per 100 square feet of indoor space.  Don't you think the air is passing through such a broad exit it can move at 2 mph?

On 5/15/2020 at 11:11 AM, MigL said:

That would lead to a lot of Marilyn Monroe, skirt blown up around the waist, type incidents.
Is this thread becoming a little silly ?

Then you have whatever number of intake vents you need to equalize the volume of air being sucked out the ceiling through 16 square feet of venting area per 100 square feet.

I just thought it would be more comfortable working with a cool breeze on my ankles. :D The more floor vents you have the more air you can move without blowing skirts up.  Just adjust the air inflow fans speed, size, and number to achieve moving air upward at 2 mph.

Edited by Airbrush

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2 hours ago, Airbrush said:

Thanks for the correction.  That means in a room 10' x 10' with 9 exhaust vent fans, 9" radius each, in the ceiling you will have 3.14 x 9^2 = 3.14 x 81 = 254 square inches/144 = 1.77 square feet of venting area per vent, times 9 vents in the ceiling = about 16 square feet of venting area per 100 square feet of indoor space.  Don't you think the air is passing through such a broad exit it can move at 2 mph?

That’s for you to show. 

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Posted (edited)
On 5/21/2020 at 1:43 PM, swansont said:

That’s for you to show. 

Air moving upward at 2 mph is equal to about 3 feet per second.  Will air moving upward at 3 feet per second blow a skirt up?  I doubt it.  The idea is to not allow the virus float around for hours or even minutes indoors.  If you can move air up over people's heads in a few seconds I suppose that decreases the risk of getting infected.

Edited by Airbrush

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10 minutes ago, Airbrush said:

Air moving upward at 2 mph is equal to about 3 feet per second.  Will air moving upward at 3 feet per second blow a skirt up?  I doubt it.  The idea is to not allow the virus float around for hours or even minutes indoors.  If you can move air up over people's heads in a few seconds I suppose that decreases the risk of getting infected.

This isn't an analysis, it's a hand-wave. You're making an assertion that isn't backed up by any analysis, and yet, this is a science site. It's not unreasonable to expect that this be backed up by some science and/or engineering. 

It also assumes you have a room where you can put that amount of vent area in the ceiling.

You want a certain result, but have done little to show if such a result is feasible.

 

You haven

't even done something simple, like showing that a 10' x 10' room with air moving at 2 mph / 3' per second in the middle means the air must be moving up to 12 mph at the vent (since it has to be moving ~6x faster if there's no change in pressure) and that this is moving 18,000 cfm through a single room. What kind of system handles that? What kind of pressure do you need in the ducts to get that kind of airflow?

IOW, you can't just say "lets move the air in the room at 2 mph" and ignore all of the ramifications and system requirements, like you have a magic wand to make these other problems go away. 

 

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35 minutes ago, Airbrush said:

Air moving upward at 2 mph is equal to about 3 feet per second. 

That simplistic analysis is for the case where the WHOLE floor is the inlet, and the WHOLE ceiling is the exhaust.
In any real world situation where the inlet and exhaust vents are fractions of the area, Mr. Bernoulli dictates speed and pressure changes.

I don't know how this idea deserves three pages of deliberation, but Swansont is right; your 'hand-waving' arguments aren't convincing anyone of its merits.

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Posted (edited)
1 hour ago, MigL said:

That simplistic analysis is for the case where the WHOLE floor is the inlet, and the WHOLE ceiling is the exhaust.
In any real world situation where the inlet and exhaust vents are fractions of the area, Mr. Bernoulli dictates speed and pressure changes.

No because the air can move much faster thru the vents than it moves thru the room.  The air flow can be adjusted by fan speed.  You may want to have a wind tunnel after the people are gone, you just turn up the fan speed and empty the indoor space of bad air in a short time.  Then during work hours the air is moving upward at a modest speed.  If 2 mph blows a skirt up, (it won't unless the skirt is made of tissue paper) then turn it down to 1 mph.  The adjustable  inlet and exhaust vents push as much air as you need for a modest upward flow of air.  With Mr. Bernoulli's speed and pressure, air moves faster thru the vents than thru the room.

To compare bad air that lingers in an indoor space for hours because of a few tiny vents which is the current norm, compared to rapid air replacement thru more powerful, adjustable air-flow vents in a few minutes it not a "hand-waving" argument.

Edited by Airbrush

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Posted (edited)

"In fluid dynamics, Bernoulli's principle states that an increase in the speed of a fluid occurs simultaneously with a decrease in static pressure or a decrease in the fluid's potential energy.[1](Ch.3)[2](§ 3.5) The principle is named after Daniel Bernoulli who published it in his book Hydrodynamica in 1738.[3] Although Bernoulli deduced that pressure decreases when the flow speed increases, it was Leonhard Euler who derived Bernoulli's equation in its usual form in 1752.[4][5] The principle is only applicable for isentropic flows: when the effects of irreversible processes (like turbulence) and non-adiabatic processes (e.g. heat radiation) are small and can be neglected."

https://en.wikipedia.org/wiki/Bernoulli's_principle

Can anyone translate this into common English?  How does this disprove my idea of accelerated air replacement (AAR) without a wind tunnel?

Edited by Airbrush

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