GhettoBlast
| Project GhettoBlast | |
|---|---|
| DIY air purifier based on HEPA filter from Smart Air Blast. | |
| Status | In progress |
| Contact | Juerd |
| Last Update | 2024-01-13 |
Why
We have two big air purifiers at RevSpace: one for the main space, one for the kitchen. Both are the Blast Mk II from Smart Air, which was the ideal device (at lesat when we bought these in 2022), in terms of CADR, noise, and CADR/€. And maybe even more importantly: you can switch them on/off with a simple external relay, in our case a Zigbee socket switch, so we can automatically turn them off when the space is closed. Most air purifiers require pressing one or more buttons after a power outage.
Unfortunately, our lounge has negligible ventilation and no air purifier. That means opening a window to curb the spread of respiratory disease. Open windows are nice in summer, but way too cold when it's freezing outside.
The lounge is much smaller than the other rooms in the space, and as such gets fewer occupants, so it could do with a less powerful air purifier. HEPA filters are expensive and can't be recycled, but that doesn't have to mean sending them to incinerator or landfill when their capacity degrades. They can be reused for smaller rooms!
What
An air purifier is just a filter and a fan and making one is a nice 10 minute DIY project. Or 10 hours, depending on how you decide to design it.
GhettoBlast is not a sound system, it's an air purifier! Basically, a wooden box to house a (used) Blast HEPA filter and some PC fans. It uses PC fans because they're silent, efficient, and easy to buy. Unlike the real Blast, GhettoBlast has the fans on the clean side of the HEPA filter. That will likely result in a lower CADR, but also more durably clean fans and internal wiring.
BOM
- 1x (used) Blast HEPA filter
- Plywood:
- 2x 1200x150x15
- 2x 570x150x15
- 2x 1170x75x15
- 4x 540x75x15
- 4x 600x30x15
- Screws
- Glue (I used polymer caulk)
- 9x fan
- MDF panels with cutouts for the fans
- 12 V, 1.5 A power supply (uses 1 A when the fans are on)
Get some quality plywood, not "powerplex". I've lost many hours dealing with the wood that has spots so soft that it won't hold screw threads.
How
The filter is a bit more than 75 mm, but that includes foam that compresses to make a seal. The filter rests on the inner frame of 75 mm wide slats that are screwed and glued to a 150 mm wide outer frame. Some extra strips on the fan side for stability, some extra strips on the rear to keep the filter in place and compressed.
Build log
- 2024-01-11/12: built frame
- 2024-01-12/13: lasercut front plates, attached fans, did preliminary tests
To do:
- Glue front plates to frame.
- Drill hole for 12V wires, caulk it air tight.
- Add fan gaskets and grills (measure air flow before/after!).
- Find a way to mount to wall or ceiling.
- Make/buy a fan controller with RPM warning to alert if a fan fails. MQTT?
Limitations
Lower CADR
The measured air flow from a real Blast Mk II with brand new HEPA filter was 102 m/s on average. With the used filter (>5500 h on high), it was approximately 20% less: 1.0 m/s. The GhettoBlast with its 9 fans and the same dirty filter does only 0.35 m/s. Assuming the manufacturer's air flow specification of 2000 m³/h for the original Blast is correct, that would mean the GhettoBlast does 700 m³/h. And if the CADR scales linearly (does it? I have no idea), that would mean it has a CADR of only 330 m³/h instead of 80% of 950 m³/h = 760 m³/h.
However, 330 m³/h is still quite usable. Based on the WHO recommended minimum ventilation rate of 36 m³/h that's still enough for 9 people, exactly the number of seats in the lounge.
By the way, if you want to build this with a higher CADR, the obvious fix would be to use more fans, and maybe to push air out through the filter instead of sucking it in through the filter.
9 fans = 9 SPOFs
If any of the fans gives up the ghost, that will kill much more than 1/9 of the air flow, as the remaining fans will draw a lot of air in through the now defunct fan instead of through the filter.