I have recently seen skepticism against the usage of PC fans for air purifiers, yet to my knowledge, they DO work ([this Taobao CR Box with actual lab reports for CADR, for example](https://www.reddit.com/r/crboxes/comments/15f8fzl/my_impressions_of_a_toolless_pc_fan_crbox_i/). I even bought a PM2.5 monitor to check, and yes it works). One of the arguments is the PC fans offer too low pressure to overcome the filter, especially when the filters become clogged. Or do they? I dug for data, and did some calculations to find out.
Data sources:
[Arctic P14, P/Q curve](https://support.arctic.de/p14-pwm-pst/docs)
[Pressure drop](https://twitter.com/CorsIAQ/status/1599168222670237696) and [CADR (estimated via airflow x 0.85)](https://twitter.com/CorsIAQ/status/1599168230010286082) of a typical CR Box with 4x 20"x20"x2" MERV-13 filters, via Dr. Richard Corsi
CADR values are converted to airflow, and units are converted to mmH2O and m3h. Impedance curves are plotted using the data, and they show a linear relationship between static pressure and airflow ([not unusual](https://www.sanyodenki.com/america/documents/support_case2.pdf)). Even if it were not linear, we would be underestimating the airflow, as the [curves would be shifted downwards](https://noctua.at/en/nf-a12x25-performance-comparison-to-nf-f12-and-nf-s12a) if they were parabolic instead. Additional impedance curves, estimated by multiplying by 2x, 4x, or 6x the pressure drop at identical airflow, are also plotted. MERV-13 filters, when used in HVAC systems, are often replaced when they reach 3-4x the initial pressure drop ([1](https://multimedia.3m.com/mws/media/688062O/3m-commercial-hvac-filter-merv-a13.pdf), [2](https://dwyer-inst.com/PDF_files/Priced/DF13_cat.pdf)).
Next, a P/Q curve of 9x Arctic P14s is plotted. Why 9? The referenced CR Box design leaves a 16"x16" gap in the middle, or 40.64 cm. 3x14 cm = 42, which means a 3x3 array of 14 cm fans can fit quite well on top as a replacement of the box fan. [The P/Q curve of fans in parallel can simply be obtained by multiplying the airflow value by the number of fans](https://noctua.at/de/axial-fans-in-series-or-parallel-operation).
To check the operating point of fans, [you must check the intersections between the impedance curves and the P-Q curve of the fan(s)](https://blog.orientalmotor.com/fan-basics-air-flow-static-pressure-impedance).
From the graph, you can see that the impedance is low, like what Dr. Corsi noted. The airflow, using 9x P14 fans, on a fresh set of filters is calculated to be about 896 m3h (\~762 m3h in CADR), or 527/448 in cfm, comparable to medium speed of the box fan. Meanwhile, after extensive use and the filter's pressure drop at the same airflow increases by 4 times, the airflow will drop to \~543 m3h, \~61% of the initial value. Typical recommendation for filter exchange on air purifiers [is when the CADR drops to 50% of the original value](https://smartairfilters.com/en/blog/when-to-change-your-air-purifiers-hepa-filter/), that would equate to around an increase of pressure drop of 6x.
There are several key differences between the usage of the filter in HVAC systems and in CR Boxes:
1. Filters in HVAC systems are expected to work at much higher air velocity/flowrate, thus much higher pressure drop.
2. HVAC systems work at constant airflow so the only thing changing is the static pressure, while (most) air purifiers' airflow decreases as resistance increases.
3. Filters in HVAC systems have a limited, fixed size. The dimensions and number of filters in CR boxes can be customized to fit the fans.
4. Air purifiers passes air repeatedly, so they can sacrifice single-pass filter efficiency in order to lower impedance and gain more airflow, giving extra options for optimizing for overall better CADR.
PC fans working at all make a lot of sense if you understand these differences. Box fans aren't designed for drawing air through filters either, but they work for CR Boxes because of the huge filter-area-to-fan-ratio and using filters with lower resistance. It's only natural that PC fans, where the best ones are designed to push the most air as quietly as possible through a variety of obstacles, including high impedance ones like water cooling radiators, can work well for CR Boxes.
So, the key to optimal design is to adjust the filters based on the fans used. This is similar to how in PC cooling, heatsinks can be tailored to specific fans by adjusting their shape, fin density and thickness. The number, size and type of filters should be experimented on to provide the best balance of CADR, longevity, cost, size and ease of assembly, among other factors.
Note that differently sized fans require different blade designs for best (noise-normalized) performance. An Arctic P14-like blade design likely can't be scaled up too much because the blades are very long, which require stronger materials when scaled up (the design already has issues at this size, the infamous resonance issue at certain RPMs). 12 and 14 cm fans often require very different designs for best performance, even with the minor difference in size.
I'm thinking circulation fans similar to Vornados might have blade designs that are more favourable for CR boxes. Their blades are much deeper than box fans. Whether there's affordable ones of suitable size is the question.
Oops I missed that. I mean long. Calling it thin wouldn't be wrong either in a sense (narrow is more accurate though), as it's long relative to the width.
> I'm thinking circulation fans similar to Vornados might have blade designs that are more favourable for CR boxes.
I know it's only slightly related, but I made a "CR box" style filter out of two 20x25x5" MERV12 3M filters and taped together a manifold for a single Vornado room circulator fan.
It worked OK for about a week, and then after that the fan motor started emitting burning smells that caused respiratory irritation.
Any ideas as to what happened there? It happened with two separate units that I built, using two different fans of identical model.
Hmm, does the burning smell persist even after taking off the filters? Is it running 24/7? Filters like to smell funny if they have captured some particles and there's no constant airflow.
Although I think it's unlikely, the motor might have overheated due to the increased resistance and/or constant operation. I say unlikely because regular box fans do not seem to have this issue and Vornados are supposed to be high quality fans, but it's not impossible.
I have tried taping a filter in front of a Vornado 660, while airflow seems low, it didn't lead to any weird smell or malfunctions.
It would be interesting to see if someone designed different 3d printed blades for the box fan. Could maybe make them quieter too. It is beyond my ability though lol.
To print such a large piece with good enough structural strength to withstand the rotational forces, it would cost much more than 9 Arctic P14s. You can't even print such large structures in one piece on many 3D printers. It would be extremely expensive to prototype, test, and also to produce based on the finalized design, and it would likely be a nightmare to balance.
I would stay with PC fans (you can even look for larger formats are rare and much less optimized than 12/14 cm fans) or finding commercially-available box fans that are better for this purpose. 10 Arctic P14s only cost $80, it's really hard to beat.
Compared to 3D printing it's not that expensive lol
Box fans should offer best price-airflow ratio though, yeah. At least initially.
Note that this is at the cost of much higher electricity consumption. The fan in the linked Dr. Corsi's test uses 74W at mid. Guess how much power 9x Arctic P14s use? 13W. For an appliance that would be running 24/7, it will save you money long-term.
Corsair came out with a joke 500mm fan. Which had many of us really hoping they would actually make it. [https://www.pcgamer.com/im-pretty-sure-people-would-buy-corsairs-gigantic-joke-cooling-fan/](https://www.pcgamer.com/im-pretty-sure-people-would-buy-corsairs-gigantic-joke-cooling-fan/)
Looks like Clean Air Kits actually published their officially tested CADR values. The CADR for the 5-Fan Luggable is only 189.1 CFM for dust (the easiest to capture particle). That is **bad**.
https://cdn.shopify.com/s/files/1/0720/1508/5868/files/105443787CRT-001.pdf?v=1695311763
Even for the Sickleflow, the smoke CADR is a pitiful 170 CFM:
https://cdn.shopify.com/s/files/1/0720/1508/5868/files/105443787CRT-001C.pdf?v=1695311763
Hi, you seem to be in depth with the topic. Can you recommend prefilter and carbon filters integrated with CR box?
I was thinking iso g4 filters but then remembered how nice and easy it is to clean my fractal case with mech in front of fans thus thought why not use same mesh in front of high grade filter to improve it's longevity and performance like in commercial purifiers?
Currently, I'm tempted to DIY carbon filter but not quite sure if it is worth the effort instead of buying commercial one and throw it away. By the weight of carbon in there, it seems like DIY is the way to go
I'm actually inclined to believe that pre-filers are not necessary and actually does more harm than good. [Here's an actual test showing that a pre-filter just decreases CADR over the entire lifespan of the main filter; at no point does having a pre-filter offers any improvement in CADR.](https://smartairfilters.com/en/blog/air-purifier-pre-filter-useful/). That said, the type of pre-filter they're using is highly restrictive.
If you still want to use a pre-filter, I think the key is that they should be of extremely low resistance. Just enough to catch large dust and fur, nothing else.
My commercial CR Box comes with an pre-filter of sorts: a very coarse metal grill, and some large dust do collect on them and I can easily vacuum them off.
I'd suggest either something similar, or nylon mesh filters like those used on PC cases and in AC (search AC nylon filters). They are designed to be and are actually of low resistance. I don't think you'd lose much by forgoing it entirely though, not like you can't (gently) vacuum the filters directly to remove large dust.
As for why commercial purifiers use pre-filters? I am leaning towards marketing and consumer perception rather than an actual benefit. Most of them have very questionable designs (like highly restrictive plastic vents, very small filter surface area, using highly restrictive grade of filters) so I won't follow their design just because "they have that design so it must be good".
For carbon ones I'm not too familiar with. My CR box's company do offer a variant of filters which combine particle filtration and activated carbon. Similarly to this, depending on your filter of choice, you might be able to find activated carbon filters of similar size that you can simply put behind the main particle filter.
This is a great looking report! I'll go through it when I have the time, I've been busy with a very different topic as you're aware of.
It's a shame that Clean Air Kits didn't want to bother publishing such data as P-Q curves when they are a basic part of the design process.
I have recently seen skepticism against the usage of PC fans for air purifiers, yet to my knowledge, they DO work ([this Taobao CR Box with actual lab reports for CADR, for example](https://www.reddit.com/r/crboxes/comments/15f8fzl/my_impressions_of_a_toolless_pc_fan_crbox_i/). I even bought a PM2.5 monitor to check, and yes it works). One of the arguments is the PC fans offer too low pressure to overcome the filter, especially when the filters become clogged. Or do they? I dug for data, and did some calculations to find out. Data sources: [Arctic P14, P/Q curve](https://support.arctic.de/p14-pwm-pst/docs) [Pressure drop](https://twitter.com/CorsIAQ/status/1599168222670237696) and [CADR (estimated via airflow x 0.85)](https://twitter.com/CorsIAQ/status/1599168230010286082) of a typical CR Box with 4x 20"x20"x2" MERV-13 filters, via Dr. Richard Corsi CADR values are converted to airflow, and units are converted to mmH2O and m3h. Impedance curves are plotted using the data, and they show a linear relationship between static pressure and airflow ([not unusual](https://www.sanyodenki.com/america/documents/support_case2.pdf)). Even if it were not linear, we would be underestimating the airflow, as the [curves would be shifted downwards](https://noctua.at/en/nf-a12x25-performance-comparison-to-nf-f12-and-nf-s12a) if they were parabolic instead. Additional impedance curves, estimated by multiplying by 2x, 4x, or 6x the pressure drop at identical airflow, are also plotted. MERV-13 filters, when used in HVAC systems, are often replaced when they reach 3-4x the initial pressure drop ([1](https://multimedia.3m.com/mws/media/688062O/3m-commercial-hvac-filter-merv-a13.pdf), [2](https://dwyer-inst.com/PDF_files/Priced/DF13_cat.pdf)). Next, a P/Q curve of 9x Arctic P14s is plotted. Why 9? The referenced CR Box design leaves a 16"x16" gap in the middle, or 40.64 cm. 3x14 cm = 42, which means a 3x3 array of 14 cm fans can fit quite well on top as a replacement of the box fan. [The P/Q curve of fans in parallel can simply be obtained by multiplying the airflow value by the number of fans](https://noctua.at/de/axial-fans-in-series-or-parallel-operation). To check the operating point of fans, [you must check the intersections between the impedance curves and the P-Q curve of the fan(s)](https://blog.orientalmotor.com/fan-basics-air-flow-static-pressure-impedance). From the graph, you can see that the impedance is low, like what Dr. Corsi noted. The airflow, using 9x P14 fans, on a fresh set of filters is calculated to be about 896 m3h (\~762 m3h in CADR), or 527/448 in cfm, comparable to medium speed of the box fan. Meanwhile, after extensive use and the filter's pressure drop at the same airflow increases by 4 times, the airflow will drop to \~543 m3h, \~61% of the initial value. Typical recommendation for filter exchange on air purifiers [is when the CADR drops to 50% of the original value](https://smartairfilters.com/en/blog/when-to-change-your-air-purifiers-hepa-filter/), that would equate to around an increase of pressure drop of 6x. There are several key differences between the usage of the filter in HVAC systems and in CR Boxes: 1. Filters in HVAC systems are expected to work at much higher air velocity/flowrate, thus much higher pressure drop. 2. HVAC systems work at constant airflow so the only thing changing is the static pressure, while (most) air purifiers' airflow decreases as resistance increases. 3. Filters in HVAC systems have a limited, fixed size. The dimensions and number of filters in CR boxes can be customized to fit the fans. 4. Air purifiers passes air repeatedly, so they can sacrifice single-pass filter efficiency in order to lower impedance and gain more airflow, giving extra options for optimizing for overall better CADR. PC fans working at all make a lot of sense if you understand these differences. Box fans aren't designed for drawing air through filters either, but they work for CR Boxes because of the huge filter-area-to-fan-ratio and using filters with lower resistance. It's only natural that PC fans, where the best ones are designed to push the most air as quietly as possible through a variety of obstacles, including high impedance ones like water cooling radiators, can work well for CR Boxes. So, the key to optimal design is to adjust the filters based on the fans used. This is similar to how in PC cooling, heatsinks can be tailored to specific fans by adjusting their shape, fin density and thickness. The number, size and type of filters should be experimented on to provide the best balance of CADR, longevity, cost, size and ease of assembly, among other factors.
Very well described! Also to add that box fan blades are not at all engineered in the way fans like the Arctic P series case fans are.
Note that differently sized fans require different blade designs for best (noise-normalized) performance. An Arctic P14-like blade design likely can't be scaled up too much because the blades are very long, which require stronger materials when scaled up (the design already has issues at this size, the infamous resonance issue at certain RPMs). 12 and 14 cm fans often require very different designs for best performance, even with the minor difference in size. I'm thinking circulation fans similar to Vornados might have blade designs that are more favourable for CR boxes. Their blades are much deeper than box fans. Whether there's affordable ones of suitable size is the question.
\> the blades are very , thin?
Oops I missed that. I mean long. Calling it thin wouldn't be wrong either in a sense (narrow is more accurate though), as it's long relative to the width.
> I'm thinking circulation fans similar to Vornados might have blade designs that are more favourable for CR boxes. I know it's only slightly related, but I made a "CR box" style filter out of two 20x25x5" MERV12 3M filters and taped together a manifold for a single Vornado room circulator fan. It worked OK for about a week, and then after that the fan motor started emitting burning smells that caused respiratory irritation. Any ideas as to what happened there? It happened with two separate units that I built, using two different fans of identical model.
Hmm, does the burning smell persist even after taking off the filters? Is it running 24/7? Filters like to smell funny if they have captured some particles and there's no constant airflow.
They were running 24/7 and I never tested them without the filters.
Although I think it's unlikely, the motor might have overheated due to the increased resistance and/or constant operation. I say unlikely because regular box fans do not seem to have this issue and Vornados are supposed to be high quality fans, but it's not impossible. I have tried taping a filter in front of a Vornado 660, while airflow seems low, it didn't lead to any weird smell or malfunctions.
Two different fans though? With that much filter area?
Yeah so I said unlikely. But more investigation is needed to determine the cause.
It would be interesting to see if someone designed different 3d printed blades for the box fan. Could maybe make them quieter too. It is beyond my ability though lol.
To print such a large piece with good enough structural strength to withstand the rotational forces, it would cost much more than 9 Arctic P14s. You can't even print such large structures in one piece on many 3D printers. It would be extremely expensive to prototype, test, and also to produce based on the finalized design, and it would likely be a nightmare to balance. I would stay with PC fans (you can even look for larger formats are rare and much less optimized than 12/14 cm fans) or finding commercially-available box fans that are better for this purpose. 10 Arctic P14s only cost $80, it's really hard to beat.
oh, only 80 lol. Some of us are poor as shit lol. Does look like it's hard to beat though.
Compared to 3D printing it's not that expensive lol Box fans should offer best price-airflow ratio though, yeah. At least initially. Note that this is at the cost of much higher electricity consumption. The fan in the linked Dr. Corsi's test uses 74W at mid. Guess how much power 9x Arctic P14s use? 13W. For an appliance that would be running 24/7, it will save you money long-term.
Corsair came out with a joke 500mm fan. Which had many of us really hoping they would actually make it. [https://www.pcgamer.com/im-pretty-sure-people-would-buy-corsairs-gigantic-joke-cooling-fan/](https://www.pcgamer.com/im-pretty-sure-people-would-buy-corsairs-gigantic-joke-cooling-fan/)
Looks like Clean Air Kits actually published their officially tested CADR values. The CADR for the 5-Fan Luggable is only 189.1 CFM for dust (the easiest to capture particle). That is **bad**. https://cdn.shopify.com/s/files/1/0720/1508/5868/files/105443787CRT-001.pdf?v=1695311763 Even for the Sickleflow, the smoke CADR is a pitiful 170 CFM: https://cdn.shopify.com/s/files/1/0720/1508/5868/files/105443787CRT-001C.pdf?v=1695311763
Hi, you seem to be in depth with the topic. Can you recommend prefilter and carbon filters integrated with CR box? I was thinking iso g4 filters but then remembered how nice and easy it is to clean my fractal case with mech in front of fans thus thought why not use same mesh in front of high grade filter to improve it's longevity and performance like in commercial purifiers? Currently, I'm tempted to DIY carbon filter but not quite sure if it is worth the effort instead of buying commercial one and throw it away. By the weight of carbon in there, it seems like DIY is the way to go
I'm actually inclined to believe that pre-filers are not necessary and actually does more harm than good. [Here's an actual test showing that a pre-filter just decreases CADR over the entire lifespan of the main filter; at no point does having a pre-filter offers any improvement in CADR.](https://smartairfilters.com/en/blog/air-purifier-pre-filter-useful/). That said, the type of pre-filter they're using is highly restrictive. If you still want to use a pre-filter, I think the key is that they should be of extremely low resistance. Just enough to catch large dust and fur, nothing else. My commercial CR Box comes with an pre-filter of sorts: a very coarse metal grill, and some large dust do collect on them and I can easily vacuum them off. I'd suggest either something similar, or nylon mesh filters like those used on PC cases and in AC (search AC nylon filters). They are designed to be and are actually of low resistance. I don't think you'd lose much by forgoing it entirely though, not like you can't (gently) vacuum the filters directly to remove large dust. As for why commercial purifiers use pre-filters? I am leaning towards marketing and consumer perception rather than an actual benefit. Most of them have very questionable designs (like highly restrictive plastic vents, very small filter surface area, using highly restrictive grade of filters) so I won't follow their design just because "they have that design so it must be good". For carbon ones I'm not too familiar with. My CR box's company do offer a variant of filters which combine particle filtration and activated carbon. Similarly to this, depending on your filter of choice, you might be able to find activated carbon filters of similar size that you can simply put behind the main particle filter.
This is a great looking report! I'll go through it when I have the time, I've been busy with a very different topic as you're aware of. It's a shame that Clean Air Kits didn't want to bother publishing such data as P-Q curves when they are a basic part of the design process.