I’ve been reading up on Corsi-Rosenthal Boxes, a relatively quiet and cheap kind of air purifier. The idea is to use off the shelf filters, but instead of a loud high static pressure fan on a single filter, using multiple filters so that the air resistance is low enough for quieter low static pressure fans. They’re typically made with MERV filters, but those are not readily available in Europe, so instead I could use Ikea brand EPA filters, or perhaps automotive particulate filters.
One rough rule of thumb says use about 4x the fan area in filter area. I think they mean filter dimensions, not total unfolded filter area. If I use 3x Ikea Starkvind filters (37x29x4cm), that comes to about 4x 14cm PC fans or 5x 12cm. I assume the 14cm fan option is quieter for the same airflow so I’ll go with that.
I don’t have space in my room for a free standing box, and the only place where a wall mounted box makes sense is above my headboard where I was planning to put a plant shelf. So I’ve designed something that combine a CR box (with 3x Starkvind filters and 4x 14cm fans) with two small planters that fit 9x9cm square nursery pots:
This is a “traditional” CR box approach with the 4 fans mounted together at the top, and the planters just hanging on the wall separately. I think the box sticking out of the wall looks a little weird, but I think this would be the most efficient option. Still, I don’t like it, so here’s another design:
This one has the filters neatly mounted side by side, and the four fans on the top have two planter trays interspaced. This option is quite slim and has everything integrated. I think it would look nice with some vines trailing over the filters, although I should take care that they don’t grow into the fans.
One thing to keep in mind is that this is right over my head when I’m sleeping, so it needs to be very quiet. So another option is to design a “sound maze” by having the air flow around a baffle, and sound proofing the surfaces (drawn here in lavender):
To keep the design somewhat narrow, it’d use 5-6x 12cm fans instead of 4x 14cm. An added benefit is that the plant tray runs the whole width, and plants are less likely to get into the fans:
As a soundproofing material I’d use either cotton (low VOC) or sheep’s wool, which actually absorbs VOCs and is better with moisture. Perhaps use old wool blankets.
Quick budget calculations: €38.99 for the plywood, €35.97 for the filters, €24.79 for the fans, so about €100 in materials excluding sound proofing and a fan speed controller.
For fan speed control, I’m thinking of an ESP32 or Raspberry Pi controller, maybe also to log some sensor data and/or control lights in my room.
Some calculations. SergioCoscolin looked at a box with 2x Starkvind filters, and tested 3x Arctic P12 fans versus 5x the same fans. Results:
| Fans | CADR | CADR/fan | CADR/fan airflow |
| 3x P12 | 143 m³/h | 48 m³/h | 51% |
| 5x P12 | 173 m³/h | 35 m³/h | 37% |
Either 5 fans is running into diminishing returns for 2 filters, or perhaps the dual direction output is less efficient.
Let’s assume my design is a little less efficient because of the extra air path from the baffle, so let’s take the 70 m³/h per filter of the higher efficiency solution above, but assume a 40% CADR-per-fan-airflow. Since I’ve got 3 Starkvind filters, I’m aiming for 210 m³/h CADR (4 air chances per hour in my room), so 525 m³/h total fan airflow. That’s 5x P12 fans or 4x P14 fans on full speed.
I like the “dampened” design, but it’s quite big and I don’t like the 45 degree angles. Also, the filters in plain view will get dirty. Perhaps I could hide them. Here’s a simplified design with the filters behind a mesh covered front inlet slit:
The inside is also simplified, with only right angle plywood. I haven’t drawn in any damping material but I think it could still fit. The front panel would need to hinge open, to be able to access the filters and fans, and it also clamps the filter in. This version uses 4x 14cm fans.
Next up I should do some static pressure calculations. The designs with baffle don’t make much sense if the added static pressure requires many extra fans to compensate the loss in airflow. In that case I’d probably just use a more airflow-efficient design with better fans, or just run at a lower speed.
Here’s a simpler more airflow efficient design with the 4x 14cm fans just pointing straight out the top. Air comes in through a mesh in the bottom. The bottom panel hinges to be able to access and clamp the filters.
Ok, one more design. This is the above design expanded with a “hat” so I could include a baffle. The air comes in the bottom and goes out the front over the plants. I think this is the least flow impeding baffle design, and it has the added benefit of being able to put stuff on top of the thing.
Oops, I forgot to draw in a flow diverting corner piece in the top right in this side view. Anyway, I think this is a pleasing design, especially if I use some of the same felt I used on my headboard as the top layer damping material.
I guess I could use the same mesh material used for the bottom opening in the top too, to prevent the plants from growing into the fans. Maybe attach both of the mesh pieces with little magnets, like an insect screen.
I could install GU10 downlights into the bottom panel. There’s enough space, and I don’t need to worry about the fittings leaking air since it’s before the filter.
I’m happy with this design! I guess I’ll skip the static pressure calculations.
Note to self: don’t use PVC pond liner to make the plant tray watertight; use something lower VOC like EPDM or PE. I think I might still have some 250 µm PE sheet.