Fans that'll drive a HEPA filter and active carbon filter for air purification

[engineheat]

Hi,

I'm looking to build a DIY room air purifier and I need a fan (AC or DC) that's enough to power a HEPA filter (similar to those used in Filtrete room air purifiers) and an active carbon filter that I got (for hydroponics). The room is about 700 sq ft.
I want to buy a quality, name brand fan that has high enough pressure and flow rate for my purpose. Ideally one that I can control the rpm of using a microcontroller (via PWM perhaps). But I'm a newbie when it comes to fans so I need some advice when it comes to the type of fans (centrifugal? squirrel cage?) and the supplier where I can get it from. Quietness would also be great.

I heard EBM - PAPST is good.

Please advise if you can.
Thanks

[sparkydog]

Unless you have special requirements, if all you want to do is improve air quality, turnover is more important than filter efficiency. In other words, what you want is a Corsi–Rosenthal Box... which you can make with just about anything. A $15 no-name box fan will do nicely.

But you said "quiet" and "PWM". In that case, I'd probably go with several Noctua NF-A20s. ("Make a CRB" happens to be on my own to-do list. My current thinking is to use 6 NF-A20s as two sides of the box and two filters as the other two sides, with a solid top and bottom.)

[MarkT]

The filter will have a curve of back-pressure against flow rate, and fans will have a curve of flow-rate against back-pressure - plot these on the same axes to see the operating point for a particular fan and filter....

[Siwastaja]

Not a direct answer, but: unless you are space constrained, just increase filter area to the point that the pressure generated by even simple axial fan designs running at modest RPM is enough. Positives are energy saving, less noise, easier fan choice, longer time to filter clogging. Negatives are larger size and initially higher filter cost (offset by the fact you don't have to replace it as quickly).

[thm_w]

I want to buy a quality, name brand fan that has high enough pressure and flow rate for my purpose. Ideally one that I can control the rpm of using a microcontroller (via PWM perhaps).

You probably do not want this as they are going to be obscenely expensive.

You can purchase a decent in-line brushless duct fan from amazon, with included speed adjustment. The centrifugal style (much larger diameter) will have better static pressure, so more appropriate for a filter, but they tend to hum and vibrate quite a bit, I don't think many are brushless.

https://www.eevblog.com/forum/reviews/cfm-for-solder-fume-extractor/
https://www.eevblog.com/forum/chat/hepa-filters-and-fans/

But you said "quiet" and "PWM". In that case, I'd probably go with several Noctua NF-A20s. ("Make a CRB" happens to be on my own to-do list. My current thinking is to use 6 NF-A20s as two sides of the box and two filters as the other two sides, with a solid top and bottom.)

For the price, noctua won't make sense. You are talking about $260 in fans, which have very limited static pressure (1mm H2O).
Better off with a larger fan that is more pressure optimized, or noise isolation and a more powerful fan.

[engineheat]

Not a direct answer, but: unless you are space constrained, just increase filter area to the point that the pressure generated by even simple axial fan designs running at modest RPM is enough. Positives are energy saving, less noise, easier fan choice, longer time to filter clogging. Negatives are larger size and initially higher filter cost (offset by the fact you don't have to replace it as quickly).

That's a good idea. Is it also possible to put two fans in series to boost static pressure while keeping air flow the same? Does it work like that?

[thm_w]

Google has info:
https://techcompass.sanyodenki.com/en/training/cooling/fan_basic/004/index.html
https://forum.digikey.com/t/does-stacking-fans-double-the-cfm/10553

From my experience one suitable fan makes more sense then cobbling together inadequate ones. But, you might have a pile of scrap fans or something.

[Siwastaja]

That's a good idea. Is it also possible to put two fans in series to boost static pressure while keeping air flow the same? Does it work like that?

The second one would overspeed. I think you would get nearly the same effect by just using one and applying some overvoltage. In both cases, lifetime suffers. If you are limited in filter area, I would just use a centrifugal fan type instead of axial "computer fan", for more pressure.

[engineheat]

In order to select a fan for my project, I'd like to actually get a sense of the pressure drop across the HEPA filter I want to use. I have a 3M air purifier that uses the exact filter I want for my project. Can I use a manometer like shown below to measure the pressure drop across the filter?

https://www.amazon.com/Manometer-Professional-Pressure-Differential-Measuring/dp/B07CWQJD6D

This might require drilling and sealing the holes.
Thanks

[thm_w]

In order to select a fan for my project, I'd like to actually get a sense of the pressure drop across the HEPA filter I want to use. I have a 3M air purifier that uses the exact filter I want for my project. Can I use a manometer like shown below to measure the pressure drop across the filter?

https://www.amazon.com/Manometer-Professional-Pressure-Differential-Measuring/dp/B07CWQJD6D

This might require drilling and sealing the holes.
Thanks

I guess you could, but I wouldn't bother. Anemometer might be of more value (airflow CFM).
A new HEPA filter is going to be around ~0.25kPa. So either you'll be at or below that, and then maybe increase as its starts to clog up.

[aeberbach]

Can you locate the fan outside of the room where you won't hear it? The inline fans designed for hydroponics might be what you're looking for.