General > General Technical Chat
HEPA filters and fans
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james_s:
Sure, you can get gauges made just for that for HVAC applications. https://www.amazon.com/General-Media-Cleaner-Filter-Gauge/dp/B0195UXV7O
lfldp:

--- Quote from: engineheat on April 15, 2023, 09:40:58 pm ---
--- Quote from: lfldp on March 26, 2023, 08:58:39 am ---
--- Quote from: engineheat on March 19, 2023, 01:57:55 am ---Hi,
I want to build a DIY air purifying system that incorporates a HEPA filter.

I'm a bit confused on the different types of fans available for filtration purposes. I know there must be sufficient air pressure.

When I tried to learn more about fans, I come across terminologies like "centrifugal" and "axial". The way I understand, axial fans are the common type that we are all familiar with, it can move lot of air but cannot generate high static pressure.

But I'm a bit fuzzy on the terms "centrifugal", "squirrel cage", "blower". Are they all pretty much the same thing? Any of those would be suitable for a HEPA filter provided the static pressure is sufficient right? Do you know what pressure is sufficient for a HEPA filter (assuming clean filter, just need a rough ballpark idea)
Thanks

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hi
if you need to use hepa h13 alone then blower from air cleaner should be ok but if you want to add carbon filter then you will need at last blower with high pressure like ebm papst like this https://www.ebmpapst.com/de/en/products/blowers/gas-blowers.html only gas blowers and modiffied gas blowers have high static pressures to manage both hepa + carbon filters but they have less airflow up to 180m3/h , in soldering fume extractors like bofa or weller you have one from these gas blowers or too both connected in pararrel 180m3/h x2 = up to 360m3/h , in chinese fume extractors blowers used too but less quality

i can explain you exactly about how you count calculate valid static pressure do fan/blower pairing etc. ;)
you need both hepa h13 filter documentation paper and blower speciffication

you have speciffication of hepa filter in attachment (for example the one which you wanna buy) at first you must obtain how much airflow you need - lets say 150m3/h
1. in filter airflow chart check about how many Pa(static pressure) the filter suck trought 150m3/h speed H13 UltraMet78 with 305 305 size like you see it give around of 170 Pa at 150m3/h airflow but this is not the end of calculation !
2. the 170 Pa is static pressure generated on clean filter but you will need static pressure on dirty filter , when there will be more dirt then filter make more static pressure - how calculate this ? just count 170 Pa x 2,5 = 425 Pa (this will be final pressure drop on this hepa h13 filter) so you will need blower which can manage 425  Pa trought 150m/3h airflow ;)
3. now go to the blower speciffication and check on its chart about how much pressure drop it can manage at 150m3/h speed

and btw. from my own experiments i can tell you the blowers from cheap air cleaners should manage valid pressure drop for hepa but if you want to add some carbon filters you will need high end blower

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Thanks. That was helpful.
Is it possible to use pressure sensors (manometers) to measure the pressure drop while the air purifier is in operation, that way you'll know when the pressure drop is too high and the filter needs to be changed?

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hello just read this post now , you can use something like i dont remember now - yes it checking actuall pressure and can trigger alarm but tbh. this solution is used only in high-end fume extractors and you dont need it in cheap chinese soldering fume extractors i think they just implement a current measure it if filter is heavy-loadded (contaminated) then blower sukcs more power from host and this triggering the alarm :)
lfldp:

--- Quote from: engineheat on March 30, 2023, 03:02:23 am ---
--- 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).

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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).


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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.

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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?

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dont understand what you mean exactly - this hepa filter have very big size because is pleatted inside tape more size of filter area give you less pressure drop if this would be just tape with hepa h13 cotton not pleated you will never push trought it air it is same case like you compare carbon filter tape with 5cm of carbon loadded will burn and damage usuall inline fan but if you put same value of carbon in kg to filter like can-filters then you can use it without problem with inline fan because the wall of filter will be much more smaller than 5cm but still same kg because case of this filters are not packed in tape but in tube with thick wall
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