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| HEPA filters and fans |
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| Kleinstein:
--- Quote from: engineheat on March 25, 2023, 04:42:44 pm --- --- Quote from: thm_w on March 24, 2023, 09:29:49 pm --- 500Pa is ~50mm H2O which should be fine, as long as the HEPA filter is not tiny (high restriction to flow). --- End quote --- Can you explain the relationship between static pressure and filter size? The way I understand it, air flow rate (volume/time) = velocity of air across the filter cross section area multiplied by the filter cross section area. Pressure needed to force air through the filter is a function of velocity (ie, higher the air velocity, higher the pressure needed). --- End quote --- For a given air pressure it is simple: the air flow is proportional to the filter area. For a fixed filter size one gets more air, the higher the pressure is. For not too high an air flow one could get an air flow about proportional to the pressure, though details may vary. The pressure needed is kind of an economic balance - it makes little sense to increase the filter area very much. The time a filter could be used in a relatively humid environment is limited (e.g. 1 or 2 years). A large filter area can save on the fan, but it makes it more expensive, especially if so large that one exchanges the filter because of age (posisbly fungy growth) and not because they are clogged / dirty. To keep the noise down it can help to have some filter also on the other side of the fan, this can be a relatively simple one, but should still catch much of the coarse dust. I don't think 2 fans in parallel make much sense, if at all 2 fans in series for more pressure. |
| lfldp:
--- Quote from: Kleinstein on March 26, 2023, 10:24:42 am --- --- Quote from: engineheat on March 25, 2023, 04:42:44 pm --- --- Quote from: thm_w on March 24, 2023, 09:29:49 pm --- 500Pa is ~50mm H2O which should be fine, as long as the HEPA filter is not tiny (high restriction to flow). --- End quote --- Can you explain the relationship between static pressure and filter size? The way I understand it, air flow rate (volume/time) = velocity of air across the filter cross section area multiplied by the filter cross section area. Pressure needed to force air through the filter is a function of velocity (ie, higher the air velocity, higher the pressure needed). --- End quote --- For a given air pressure it is simple: the air flow is proportional to the filter area. For a fixed filter size one gets more air, the higher the pressure is. For not too high an air flow one could get an air flow about proportional to the pressure, though details may vary. The pressure needed is kind of an economic balance - it makes little sense to increase the filter area very much. The time a filter could be used in a relatively humid environment is limited (e.g. 1 or 2 years). A large filter area can save on the fan, but it makes it more expensive, especially if so large that one exchanges the filter because of age (posisbly fungy growth) and not because they are clogged / dirty. To keep the noise down it can help to have some filter also on the other side of the fan, this can be a relatively simple one, but should still catch much of the coarse dust. I don't think 2 fans in parallel make much sense, if at all 2 fans in series for more pressure. --- End quote --- writing pararell i mean in series just wrongly translate this for example bofa oracle uses 2 blowers x2 180m3/h |
| engineheat:
--- Quote from: Kleinstein on March 26, 2023, 10:24:42 am --- --- Quote from: engineheat on March 25, 2023, 04:42:44 pm --- --- Quote from: thm_w on March 24, 2023, 09:29:49 pm --- 500Pa is ~50mm H2O which should be fine, as long as the HEPA filter is not tiny (high restriction to flow). --- End quote --- Can you explain the relationship between static pressure and filter size? The way I understand it, air flow rate (volume/time) = velocity of air across the filter cross section area multiplied by the filter cross section area. Pressure needed to force air through the filter is a function of velocity (ie, higher the air velocity, higher the pressure needed). --- End quote --- For a given air pressure it is simple: the air flow is proportional to the filter area. For a fixed filter size one gets more air, the higher the pressure is. For not too high an air flow one could get an air flow about proportional to the pressure, though details may vary. The pressure needed is kind of an economic balance - it makes little sense to increase the filter area very much. The time a filter could be used in a relatively humid environment is limited (e.g. 1 or 2 years). A large filter area can save on the fan, but it makes it more expensive, especially if so large that one exchanges the filter because of age (posisbly fungy growth) and not because they are clogged / dirty. To keep the noise down it can help to have some filter also on the other side of the fan, this can be a relatively simple one, but should still catch much of the coarse dust. I don't think 2 fans in parallel make much sense, if at all 2 fans in series for more pressure. --- End quote --- So for a given filter size, air flow is proportional to pressure drop then? In another word, pressure needed is proportional to air velocity thru the filter? |
| engineheat:
I'm looking at this fan for the PAPR application https://www.delta-fan.com/products/BFB0712HD.html But it's a 12V (rated) DC Brushless fan. I currently only have some 18V batteries. Is it problematic to run it at 18V or should I get a buck converter? Thanks |
| thm_w:
--- Quote from: engineheat on April 09, 2023, 10:16:53 pm ---I'm looking at this fan for the PAPR application https://www.delta-fan.com/products/BFB0712HD.html But it's a 12V (rated) DC Brushless fan. I currently only have some 18V batteries. Is it problematic to run it at 18V or should I get a buck converter? Thanks --- End quote --- It says 13.8V max in the datasheet, you want a buck regulator. You can get an adjustable buck module for <$2 online. Then that gives you the ability to throttle down the fan voltage as well. |
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