Author Topic: HEPA filters and fans  (Read 3545 times)

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Offline engineheatTopic starter

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HEPA filters and fans
« 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
 

Offline thm_w

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Re: HEPA filters and fans
« Reply #1 on: March 20, 2023, 09:06:02 pm »
We have many threads here if you search:
https://www.eevblog.com/forum/reviews/(my-diy)-smoke-absorver-filtering-for-soldering/
https://www.eevblog.com/forum/projects/diy-20lt-drum-solder-fume-extractor-(hepa-active-carbon)/
https://www.eevblog.com/forum/reviews/fume-extractor-advice/
https://www.eevblog.com/forum/beginners/pwm-fan-frequency-different-from-speed-controller-frequency/
https://www.eevblog.com/forum/projects/diy-high-performance-solder-fume-extractor/

Squirrel cage is centrifugal, although usually when people say that it tends to be the wide bladed style.

You probably want static pressure of 20mm H20 or more, 120mm or larger. Depending on what you are trying to do.
Ideally you can get a good powerful inline duct fan and have it somewhere far away where the noise is not an issue. But if you are designing a box sitting beside you, might be different choice of fan.
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Offline Neutrion

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Re: HEPA filters and fans
« Reply #2 on: March 21, 2023, 02:30:57 pm »
I think squirrel cage is only in the rare cases called centrifugal, where the air goes in into the middle and gets blown outward.
While the fan which is normally called squirrel cage you find in fancoils often just "spoons" the air and it does not go into the middle, and can not produce too high  pressure.

A really good fan for such a project is to get a used IBM blade server fan(K3G180-AC40-07), which is a top notch industrial fan from EBM-Papst. It is with a BLDC motor and the controller, and you can get it for a much lower price than what such a thing would normally cost new.
You can get enough pressure with it on lower rpm, and if noise is not at issue, on max. power it is really powerful.
I have one, and also wanted to build an air purifyer with it, and it is still on schedule, only issue is, that there is no proper documentation for the control of it.
But if money is not an issue, you may can  find the same fan alone with some standard interface.
(Have to add that the control can be atually easily done with a variable DC voltage source as well, so if that is OK for you, than you can have a go with them)

Still if someone may knows about some more detailed doc., please let me know!


 

Offline engineheatTopic starter

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Re: HEPA filters and fans
« Reply #3 on: March 24, 2023, 09:08:58 pm »
We have many threads here if you search:
https://www.eevblog.com/forum/reviews/(my-diy)-smoke-absorver-filtering-for-soldering/
https://www.eevblog.com/forum/projects/diy-20lt-drum-solder-fume-extractor-(hepa-active-carbon)/
https://www.eevblog.com/forum/reviews/fume-extractor-advice/
https://www.eevblog.com/forum/beginners/pwm-fan-frequency-different-from-speed-controller-frequency/
https://www.eevblog.com/forum/projects/diy-high-performance-solder-fume-extractor/

Squirrel cage is centrifugal, although usually when people say that it tends to be the wide bladed style.

You probably want static pressure of 20mm H20 or more, 120mm or larger. Depending on what you are trying to do.
Ideally you can get a good powerful inline duct fan and have it somewhere far away where the noise is not an issue. But if you are designing a box sitting beside you, might be different choice of fan.

Thanks, I did briefly look over the threads you posted. I will go back and reread them to digest later. A lot of the links in the first thread are broken, due to age, but I did notice he bought an "inline duct fan" on Ebay. You also mentioned "inline duct fan." Based on my research, "inline duct fans" are not necessarily centrifugal. Some of them are axial type. Is that right?

Someone else I know built a DIY air purifer using an "inline duct fan" from AC Infinity:
https://acinfinity.com/inline-fan-systems/
Looking at the T6 model, it seems pretty quiet (32 dba) but I'm not sure if the static pressure (503 Pa) is sufficient for a HEPA filter.

I did some research on fans, and it seems centrifugal fans generates higher static pressure than axial. I took apart a commercial HEPA air purifier and it uses a centrifugal fan (squirrel cage) fan too. I found a company called Delta Fans that makes a wide array of DC centrifugal fans with a wide range of flow rate and static pressure. Have you or anyone heard of them? I do want to buy quality components for my project.


So right now, I'm deciding between an AC Infinity "inline duct fan" vs a DC centrifugal fan from Delta, to build a home air purifier for a 800 sq ft room. But it seems the Delta fans are pretty small, their flow rates only go up to 129 CFM or so and the noise (70 db) seems pretty high.
https://www.delta-fan.com/pfb0812ee-e.html

But that's because I chose one with a pretty high static pressure (6 water in, or 1500 Pa). Not sure if that's way too much for what I need...

Thanks
 

Offline thm_w

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Re: HEPA filters and fans
« Reply #4 on: March 24, 2023, 09:29:49 pm »
Yes inline duct fans, the small ones, are usually not centrifugal. Some of the larger units are.
Either way they have decent static pressure, as they are expected to be used in long ducts. 500Pa is ~50mm H2O which should be fine, as long as the HEPA filter is not tiny (high restriction to flow).

The Delta one is a crazy high RPM high performance fan, sure performance is good, but the noise (screaming sound) isn't really worth it.
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Offline engineheatTopic starter

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Re: HEPA filters and fans
« Reply #5 on: March 25, 2023, 04:42:44 pm »
500Pa is ~50mm H2O which should be fine, as long as the HEPA filter is not tiny (high restriction to flow).

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).

If the filter A has half the cross section of filter B, if you keep the air speed across the two the same, pressure needed is the same. But you get more air flow with filter B. For a given air flow, the smaller the filter, the larger the pressure needed.

Is that correct?
Thanks
 

Offline deadlylover

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Re: HEPA filters and fans
« Reply #6 on: March 26, 2023, 08:20:56 am »
The Corsi-Rosenthal box was popping off during the pandemic. It's a bit how ya doin but it probably has the best price to performance ratio ahahah.

The InovaAir E7 purifier I have uses a centrifugal fan, the ebm-papst R2E190-RA26-51. Maybe you can use their specs as a guide.
 

Offline lfldp

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Re: HEPA filters and fans
« Reply #7 on: March 26, 2023, 08:58:39 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
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
« Last Edit: March 26, 2023, 09:03:58 am by lfldp »
 

Offline lfldp

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Re: HEPA filters and fans
« Reply #8 on: March 26, 2023, 09:02:36 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
also you can forget about using about using duct fans to manage hepa h13 filters even the one from harmann company with EC motor isnt enough to manage hepa itself ;)
 

Offline lfldp

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Re: HEPA filters and fans
« Reply #9 on: March 26, 2023, 09:41:41 am »
500Pa is ~50mm H2O which should be fine, as long as the HEPA filter is not tiny (high restriction to flow).

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).

If the filter A has half the cross section of filter B, if you keep the air speed across the two the same, pressure needed is the same. But you get more air flow with filter B. For a given air flow, the smaller the filter, the larger the pressure needed.

Is that correct?
Thanks
for carbon filters if you wanna add them to hepa h13 you will need much more powerfull blower or even 2 blowers connected in pararell
you can use carbon filters in tubes patron filters like can-lite they have much more less pressure drop than conditionall tape carbon filters becoz the wall of of these filters is "thick"
for example loading more carbon to tape filters will puts more pressure drop whether loading more carbon to tube filters could reduce pressure drop ;)
you may consider something like that > https://www.smogbox.pl/ there you have connected tube carbon filters in pararell so you have few thick walls wheres air going trought this setup give you very low static pressure drop

for example in weller fume extractor you have very small filters 300 x 300 (tape filters) with just maybe 2kg of activated carbon and alumina but they have very high pressure drop thats why they use very expensive gas blowers installed in these fume extractors as i remember it can manage up to 2,5k - 3,5k Pa ;) but this blower cost big money
 

Online Kleinstein

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Re: HEPA filters and fans
« Reply #10 on: March 26, 2023, 10:24:42 am »
500Pa is ~50mm H2O which should be fine, as long as the HEPA filter is not tiny (high restriction to flow).

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).

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.
 

Offline lfldp

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Re: HEPA filters and fans
« Reply #11 on: March 26, 2023, 04:37:23 pm »
500Pa is ~50mm H2O which should be fine, as long as the HEPA filter is not tiny (high restriction to flow).

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).

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.
writing pararell i mean in series just wrongly translate this for example bofa oracle uses 2 blowers x2 180m3/h
 

Offline engineheatTopic starter

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Re: HEPA filters and fans
« Reply #12 on: March 30, 2023, 03:02:23 am »
500Pa is ~50mm H2O which should be fine, as long as the HEPA filter is not tiny (high restriction to flow).

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).

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.

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?
 

Offline engineheatTopic starter

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Re: HEPA filters and fans
« Reply #13 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
 

Offline thm_w

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Re: HEPA filters and fans
« Reply #14 on: April 10, 2023, 09:38:19 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

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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Offline engineheatTopic starter

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Re: HEPA filters and fans
« Reply #15 on: April 12, 2023, 04:53:53 am »

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.

Thanks. I'm pretty new to using buck converters. Can you provide an example of something that'll satisfy my need? When choosing a buck converter, you gotta find one that match your input voltage range, output voltage range, and the wattage of the application right?

https://www.amazon.com/Converter-DROK-Transformer-Regulator-Stabilizer/dp/B00JUFJ1GA

I found one above on Amazon but it seems like a poor quality one that'll break easily. It says output current : 2A so that seems to be the limiting factor regardless of output voltage?
« Last Edit: April 12, 2023, 04:55:25 am by engineheat »
 

Offline james_s

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Re: HEPA filters and fans
« Reply #16 on: April 12, 2023, 06:28:07 am »
I've used these quite a few times, they're cheap and work fine.

https://www.ebay.com/itm/332711109230

The IC is not really what it says it is on the ones I have, it runs at the wrong frequency but it works fine. If you need more current you can easily get similar regulators that are rated for 5, 10 amps or more. I wouldn't run one of those Chinese modules past about 60-70% rated current though.
 
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Offline engineheatTopic starter

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Re: HEPA filters and fans
« Reply #17 on: April 15, 2023, 04:30:22 am »
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

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.

If I understand correctly, you can vary the speed of a DC fan such as the Delta fan above with PWM right? This one have 2 wires.

Because I read there's also "PWM" fans with a dedicated wire for PWM.


 

Offline engineheatTopic starter

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Re: HEPA filters and fans
« Reply #18 on: April 15, 2023, 09:39:37 pm »
I've used these quite a few times, they're cheap and work fine.

https://www.ebay.com/itm/332711109230

The IC is not really what it says it is on the ones I have, it runs at the wrong frequency but it works fine. If you need more current you can easily get similar regulators that are rated for 5, 10 amps or more. I wouldn't run one of those Chinese modules past about 60-70% rated current though.

Do you know how efficient those buck converters tend to be?
 

Offline engineheatTopic starter

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Re: HEPA filters and fans
« Reply #19 on: April 15, 2023, 09:40:58 pm »
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
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

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?
 

Offline james_s

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Re: HEPA filters and fans
« Reply #20 on: April 16, 2023, 03:50:38 am »
Sure, you can get gauges made just for that for HVAC applications. https://www.amazon.com/General-Media-Cleaner-Filter-Gauge/dp/B0195UXV7O
 

Offline lfldp

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Re: HEPA filters and fans
« Reply #21 on: October 21, 2023, 08:00:02 pm »
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
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

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?
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 :)
 

Offline lfldp

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Re: HEPA filters and fans
« Reply #22 on: October 21, 2023, 08:13:42 pm »
500Pa is ~50mm H2O which should be fine, as long as the HEPA filter is not tiny (high restriction to flow).

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).

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.

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?
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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