3 ms·

and they will cycle the air four to five times per hour in a room that size, Is that all the air or part of it? And does that matter? i.e. anyone knows whether

by stinos 9y ago

and they will cycle the air four to five times per hour in a room that size, Is that all the air or part of it? And does that matter? i.e. anyone knows whether smoke particles etc automatically dillute or rather stay in the same concentration given one volume of air? I mean, I can imagine if you put one of these things in the center of the room, it will take in air and spit it out, but unless there is some constant process also taking volumes of air away from the far away corners, behind tables/beds/couches and getting it to be sucked up by the machine, the machine is always cleaning the 'same' air hereby leaving volumes of air far away from it untouched and hence possibly still polluted? Or isn't that how it works because there always is some kind of air flow going on somehow leading to all air making it through the machine?

dmlorenzetti 9y ago
Most rooms do have dead spaces where the "age of air" is higher than the well-mixed parts of the room. So you can expect some of the room air to go through the filter more, and some less. There are a few reasons this effect probably doesn't matter so much. First, the dead spaces tend to be places where people don't breath -- in closets, under furniture, in corners, near ceilings, etc. Second, where people do go, they bring thermal plumes and activity-generated eddies, which tend to mix up room air. In fact, the act of breathing also provides mechanical power that will promote mixing. The filter itself also blows air, giving more mixing than if the filter wasn't present. The net effect is that the participating volumes of the room, where people tend to breath, probably get mixed at a rate similar to, or greater than, the rate at which the filter scrubs the air. Any non-participating volumes act as slow-filling and slow-releasing reservoirs, which delay the room's response to changes in outdoor concentrations. I would worry more about the filter only cycling the air 4-5 times per hour. That's probably faster than the effective release rate of the indoor sources with which Mr. Hefferman (the expert cited in the NYTimes article) seems to have have experimented. However, it's the same order of magnitude at which outside air gets into most houses (especially in the parts of California affected by these fires). My gut feeling is that implies the filter isn't going to keep up so well -- though I'd want to model it to say anything definitive.
- stinos 9y ago
  Thanks, that was a nice explanation.
  - dmlorenzetti 9y ago
    Except the part where I spelled "breathe" incorrectly :-)
- subwayclub 9y ago
  FWIW I believe it helps to run ordinary fans in the room in tandem with a filtration unit to combat the airflow issues.

by stinos 9y ago

dmlorenzetti 9y ago

Most rooms do have dead spaces where the "age of air" is higher than the well-mixed parts of the room. So you can expect some of the room air to go through the filter more, and some less. There are a few reasons this effect probably doesn't matter so much. First, the dead spaces tend to be places where people don't breath -- in closets, under furniture, in corners, near ceilings, etc. Second, where people do go, they bring thermal plumes and activity-generated eddies, which tend to mix up room air. In fact, the act of breathing also provides mechanical power that will promote mixing. The filter itself also blows air, giving more mixing than if the filter wasn't present. The net effect is that the participating volumes of the room, where people tend to breath, probably get mixed at a rate similar to, or greater than, the rate at which the filter scrubs the air. Any non-participating volumes act as slow-filling and slow-releasing reservoirs, which delay the room's response to changes in outdoor concentrations. I would worry more about the filter only cycling the air 4-5 times per hour. That's probably faster than the effective release rate of the indoor sources with which Mr. Hefferman (the expert cited in the NYTimes article) seems to have have experimented. However, it's the same order of magnitude at which outside air gets into most houses (especially in the parts of California affected by these fires). My gut feeling is that implies the filter isn't going to keep up so well -- though I'd want to model it to say anything definitive.

stinos 9y ago
Thanks, that was a nice explanation.
- dmlorenzetti 9y ago
  Except the part where I spelled "breathe" incorrectly :-)
subwayclub 9y ago
FWIW I believe it helps to run ordinary fans in the room in tandem with a filtration unit to combat the airflow issues.