Corsi-Rosenthal frame and PC box fan structure

# High-Performance 3D-Printed Corsi-Rosenthal Box Air Purifier

## Overview

Build a powerful DIY air purifier that outperforms commercial units at a fraction of the cost. This 3D-printed Corsi-Rosenthal box uses 9 Arctic P14 Pro fans to achieve **819 CFM Clean Air Delivery Rate (CADR)** - that's 2x the flow of a Blueair Signature at half the price.

## Key Features

- **925 CFM airflow** at full speed (50 dB)
- **88.5% filtration efficiency** for respiratory particles
- **PWM speed control** - whisper quiet at 50% speed (<40 dB)
- **No glue or tape required** - fully modular snap-fit design
- **Compact storage** - disassembles in minutes
- **Total cost: ~$200** including fans and filters

## Performance Specs

| Setting | Airflow | Noise | Clean Air (CADR) |
|---------|---------|-------|------------------|
| 50% PWM | 462 CFM | <40 dB | 404 CFM |
| 75% PWM | 693 CFM | 44 dB | 606 CFM |
| 100% PWM | 925 CFM | 50 dB | 808 CFM |

## What You'll Need

### Electronics
- 9× Arctic P14 Pro PWM fans (~$90)
- 1× 12V 5A power supply (~$15) (I used this one: https://www.amazon.com/dp/B01GEA8PQA)
- 1× PWM fan controller (~$15) (I used this one: https://www.amazon.com/dp/B0FBRNFLDM)

### Filters
- 4× MERV-14 filters, 20"×20"×1" (~$60), I used the Filtrete 2500's which can be had at Costco for $55 (though at the time of writing there was a sale for $45)
- Replace every 6-12 months

### 3D Printing
- ~1.9 kg of filament
- Print time: ~24 hours total
- No supports needed for most parts

## Assembly Instructions

### Step 1: Print the Parts
- Print all parts from `corsi-rosenthal-box.3mf`
- Remove skirts and clean up prints
- Parts include: corner bases, edge pieces, fan plate, fan mount plate, controller cover

### Step 2: Build the Filter Box
1. Arrange 4 filters in a pinwheel configuration
2. Use corner bases at each corner joint
3. Add edge pieces to form the base
4. Add vertical edge pieces
5. Build the top rim using edge connectors and corner pieces

### Step 3: Assemble the Fan Array
1. Arrange the fans in 3 rows daisy chained
2. Arrange into a 3x3 array with each row's controller/power input on the same side, face up (ie inlet direction facing up)
3. Use included bolts and square brackets to connect corner fans to center fan
4. Use bottom fan brackets, using included screws from the fans to secure the corner and furthest end of each bracket to the corner and middle-edge fans
5. Flip structure around, add corner brackets to the top to complete the structure/frame. Be sure to route connectors for each row of fans toward/to the controller slot
6. Install controller, connecting to the fan leads
7. Add controller cover to clean up the look a bit and protect the cables from jostling.

### Step 4: Final Assembly
1. Place fan array on top of filter box
2. Plug in and test
3. Adjust PWM for desired speed/noise level

## Tips & Tricks

- **Quiet operation**: Run at 50% PWM for near-silent operation with 400+ CFM CADR
- **Easy storage**: Fans stack at 1.2" thick when controller is removed
- **Quick setup**: Reassembly from stored state takes <5 minutes
- **Filter seal**: The compression fit design achieves <<1% air bypass without any tape or glue

## Why It Works

Corsi-Rosenthal boxes use multiple MERV filters instead of a single HEPA filter. While HEPA filters catch 99.97% of particles, they restrict airflow significantly. I've calculated that the Filtrete 2500 MERV-14 filters catch 88.5% of viral particles but allow much more air through. By moving much more, you actually clean more air per minute despite the lower single-pass efficiency.

The Arctic P14 Pro fans are key - they're designed for radiators with high static pressure (5.2mm H₂O), perfect for pushing air through filters. The 9-fan array provides redundancy and allows individual fans to run slower and quieter while moving massive amounts of air.

## Test Results

Measured with digital anemometer and tachometer through various methods, all confirming:
- Confirmed 925 CFM minimum at 2500 RPM (100% PWM)
- Linear PWM scaling verified
- Filter face velocity: ~104 feet per minute

## Files Included

- `corsi-rosenthal-box.3mf` - All frame parts ready to print.
- Scad files for every part for easy manipulation for slightly different fans or longer/shorter filters etc.

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**Note**: This design has been tested at 925 CFM continuous operation. The 88.5% filtration efficiency is calculated based on MERV-14 specifications and respiratory aerosol size distributions. Your results may vary based on print quality and assembly.

Build one and breathe easier!