Thinking About Switching to Coal

 
ddahlgren
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Post by ddahlgren » Tue. Nov. 22, 2016 1:24 am

I have always wondered why the fine HEPA filters seem to have the same area as a coarser one as they restrict the air flow more.

 
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Post by franco b » Tue. Nov. 22, 2016 12:12 pm

ddahlgren wrote:I have always wondered why the fine HEPA filters seem to have the same area as a coarser one as they restrict the air flow more.
I prefer the bags for that reason. I think they have the most area, and are cleaner to just throw away. One bag easily lasts for a season.

 
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Post by lobsterman » Tue. Nov. 22, 2016 8:17 pm

ddahlgren wrote:I have always wondered why the fine HEPA filters seem to have the same area as a coarser one as they restrict the air flow more.
For given area of filter, higher micron has bigger holes but lower micron has more smaller holes, resulting in same airflow, much like a screened window lets in essentially as much as air as an open window. Ha ha having a glass of wine and made this up
but believe it is true.


 
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Post by ddahlgren » Wed. Nov. 23, 2016 10:37 am

lobsterman wrote:
ddahlgren wrote:I have always wondered why the fine HEPA filters seem to have the same area as a coarser one as they restrict the air flow more.
For given area of filter, higher micron has bigger holes but lower micron has more smaller holes, resulting in same airflow, much like a screened window lets in essentially as much as air as an open window. Ha ha having a glass of wine and made this up
but believe it is true.
Sorry to say you are wrong as there is little flow at the edges of the holes. If the screen was zero restriction why do they blow out on very windy days?? :lol:

 
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Post by Lightning » Wed. Nov. 23, 2016 11:00 am

lobsterman wrote: For given area of filter, higher micron has bigger holes but lower micron has more smaller holes, resulting in same airflow, much like a screened window lets in essentially as much as air as an open window. Ha ha having a glass of wine and made this up
but believe it is true.
Yeah like Dave mentioned the more smaller holes have more cumulative perimeter friction for the air to work around. Larger holes in the screen make it easier for air to pass thru :)

 
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Post by lobsterman » Wed. Nov. 23, 2016 2:40 pm

Of course, it is lesser flow per smaller hole but what you are missing is the packing fraction with smaller holes may be bigger by design to keep filter size constant. So the screen analogy should be compared to a piece of plywood with only a few bigger holes cut in it. At some size hole and number of holes the screen will have equal flow. You can equalize air flow in a smaller micron filter by adding more holes.


 
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Post by ddahlgren » Wed. Nov. 23, 2016 3:18 pm

lobsterman wrote:Of course, it is lesser flow per smaller hole but what you are missing is the packing fraction with smaller holes may be bigger by design to keep filter size constant. So the screen analogy should be compared to a piece of plywood with only a few bigger holes cut in it. At some size hole and number of holes the screen will have equal flow. You can equalize air flow in a smaller micron filter by adding more holes.

You can as long as losses from the edges of the hole venia contracta (sp?) a fancy term saying the edges only flow a small fraction compared to the center so it needs to be

area of small holes - losses from edges of holes =area of larger holes.

 
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Post by lobsterman » Wed. Nov. 23, 2016 6:59 pm

I don't know if they actually do that but if they did that flow would be equalized. I think anyway it does not matter, we are talking about smallish effects. Anyway the ash vac is great (thanks for pointing out its usefulness) because of that marvelous filter.

 
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Post by ddahlgren » Thu. Nov. 24, 2016 4:23 am

I would think well did after giving it some more thought that the holes per se do not exist. To keep out sheet rock dust and fine ash I think you would have to be in the single digit micron level. That is the pore in the paper. The horse has officially been beaten to death LOL

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