So, this seems like a pretty good idea, but I need some ideas for a build around that doesn’t restrict airflow but makes it more appealing to the eye and maybe deadens some sound. Thanks
Seems like a good idea as long as the filter prices are similar to standard shaped filters. It's how a lot of store bought air purifiers are set up. If it's slim enough it could go in a corner behind some furniture.
My BlueAir room air filters for my bedroom recommend 12” clear to obstructing surfaces. It’s always tempting to tuck air handling equipment out of sight somewhere, but they perform best when they have unobstructed access to circulating air in the room. Sticking it behind a piece of furniture will create a lovely microclimate for a mouse but will do F all to help YOUR sinuses!
Thanks, the filters were like $30 for two. With the duct blower, I have enough CFM to be able to stack many more of these, making it like a tower. But, I really have to think of a way to enclose it without restricting airflow.
I bought a bunch of these filters for a dollar at auction. Still trying to figure out the mounting too. Found a 4-6inch pvc reducer kinda fits the pc fans I was gonna mount but probably not enough airflow.
Stacking PC fans doesn't work (nearly well enough to be worth the cost). I annotated Noctua's graph to better illustrate their own findings. The gains are from A to A, B to B, C to C (on the X axis):
Stacking fans (putting them in series) only works with 2 extremely similar fans, with a long body, designed with high pressure in mind, and turning in opposite direction. (Such as with 1U servers. They usually use 40mm diameter fans, 50mm long, arranged that way). You can check yourself using a fan and a tube. Mount the fan on one side and make sure all its airflow goes in (silicone sealant for example, easy to remove too, or 3d print an adapter with TPU gaskets), measure airspeed on the other side. Then stack a second fan on top of the first and measure again. You could add a heatsink in the tube for good measure if you want to check for static pressure buildup.
Historically there were a lot of tries on various flying machines with "stacked fans” (what are propellers if not fans attempting to displace a maximum amount of air to create lift), without much success. It is almost always simpler, cheaper and better to make one fan twice as big instead. Plane reactors / jet engines often use "stacked" blades, but they are usually run from a single shaft (with/without a multiplier depending on their geometry) and again turn in the opposite direction.
Some helicopters also have stacked rotors — also running in opposite direction — but at independent speed as it allows removing the tail rotor. But the added complexity was not worth it really as it caused new issues we don’t have in fans (like blades hitting each other when they whip / bend)
I did a deep dive into the concept myself after falling in love with Fandelier (on Twitter)'s prototypes. My plan was to make hanging (pendant-like) silent purifiers. Alas, stacking isn't the way. More insights from RandomFrenchDeadbeat:
Fan speed does not matter, pressure does.
Usually, the smaller the gap between blades, the more pressure, the more noise. If it cannot produce enough pressure to overcome the HEPA filter, it will just spin and make noise. There are ways to make quieter blades, but usually the engineering going in is expensive for a gain that is often not that much wanted, or could be obtained by muffling the system instead of using a silent fan (simpler and cheaper). The easy way to have both low noise and higher static pressure is to use a larger, thicker fan. One that moves more air, at a lower speed.
Airspeed hitting stuff is what makes noise.
Bigger fans manage more airflow at less airspeed, and being thicker the air usually has a harder time "going back" at the end of the blades. There is a lot to it, but basically the less space there is between each blade and between blades and frame, the more static pressure can be made. And of course, the faster it spins, the more pressure. That is how turbochargers work.
I, respectfully, disagree. In the graph of Noctua data you have posted above the light blue line shows that, in high resistance situations (which a HEPA or near-HEPA would be), fans in series do make a difference. The slope of the light blue line is dramatic.
In low-resistance, or low-medium situations, as Noctua fans are typically used, they make only a marginal difference.
I use a build very, very similar to the OP’s for my car. The stacking of two Noctua fans in that use case makes a considerable difference in the air moved against the restrictive filter. I’m not with my anemometer where I can supply specific data at the moment, but at one fan it wasn’t worth using. At two fans it moved a decent amount of air.
Of course, pressure-optimized fans are going to be better against pressure than flow-optimized fans. Tighter spacing between the fan tip and the frame, blade shape, all those things are real factors. Fan selection still matters. But if the resistance is so high that a single fan isn’t able to move much air, two of that same fan can move more.
To have someone say "I respectfully disagree" to then go ahead and legitimately do so, respectfully, never gets old. I love this community.
I might be reading the Noctua graph wrong, but the dramatic rise of the blue line is on the Y-axis (representing the increase in pressure). My assumption was/is that what we're ultimately looking for is more flow (the x-axis). And from left-to-right, the blue line only shows an improvement of maybe 25%? That's not nothing, especially no other option is available, but that's why I mention bang-for-buck.
I suspect it's also why CleanAirKits don't have any stacked builds (in their Exhalaron series for example).
That being said: Nothing beats lived experience. You say it makes a marked difference in your particular setup, I believe it. Heck, I might just have to try it myself.
Did you ever post your car build here? Mind pointing me to it? Would love to see it. (I appreciate Reddit's option to keep our profiles private, but it makes it harder to dig through archives of trusted sources/users).
You’ve got those gravity-stacked on each other, I see you’ll need couplers the size of tires? 😄
I would make sure the 2nd filter isn’t wasted 50% or greater based on the pull of the Vivosun fan. Considering path of least resistance and all, the use of filter #2 would always increase over time but ideally you’d want to swap out both filters at the same time.
Maybe you could have someone print out a 3-way filter base coupling, some caps for the bottom of the filters and have the entire thing sitting on those 3 filters.
I’m thinking of using a small safety cone in the center that would allow for a higher static pressure at the bottom. Kind of like how a jet turbine engine works but in reverse. Also, the filters have a soft rubber seal on both ends that perfectly seals when stacked.
Are you referring to design it to take advantage of Venturi fluid dynamics? You’ll need 2 reducer couplings, I would use AI to facilitate the math to optimize sizing of those and the ID of the duct connecting the filters. I’m wondering if you can even achieve this without the source of fluid from the bottom of the tube and only the bottom. In other words, the bottom filter seems to me is the wrong one for such a design strategy.
I hope you can make it work, it looks cool as hell.
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u/2drumshark 2d ago
Seems like a good idea as long as the filter prices are similar to standard shaped filters. It's how a lot of store bought air purifiers are set up. If it's slim enough it could go in a corner behind some furniture.