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1 month ago
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A shroud is often used with Corsi-Rosenthal boxes to increase efficiency. The shroud is a piece of cardboard, with a cutout 15’ circle in the middle that is stuck to the top.
Here’s the theory. When you don’t have the shroud, the fan airflow on the corners actually goes the opposite way. This creates eddies that prevent a nice flow profile. So, covering the corners reduces this back-flow and increases efficiency. To be honest, the fluid dynamics of this is still a little unclear to me, and I will do a separate post later.
We care a lot about efficiency because it would let us run the box at lower power whilst maintaining the same clean air delivery rate, which would make it more quiet. The loudness of DIY air purifiers are a big thing that deter people from using it regularly. Interestingly, I’ve caught my girlfriend using it when she cooks, probably because the loudness of the frying is there anyway.
On my blog, I take nothing for granted. So, I wanted to test the shroud in the real world. Firstly, I found a more efficient way to generate PM2.5 to around 200 micrograms / m^3 - I just burn 20 matches in rapid succession in a ceramic pot in a sealed room.
I then ran two tests on my box fan at the lowest setting.
First, I increased the PM2.5 concentration in my sealed room to 200 micrograms / m^3. I then tested the decay with my CR box without the shroud.
Once the PM2.5 concentration went back to background level, I made and installed the shroud, increased the room PM2.5 concentration again to 200 micrograms / m^3, then tested the decay curve again. The process to install it was built below.
I used my Temtop m2000 to run the tests. Here were the results:
So interestingly, I’ve almost doubled the clean air rate of my CR box fan by shrouding it. This is surprising, and raises some alarm bells.
At this point, I’m pretty interested to see if I can
Use shrouding on cheaper fans to now get this thing really quiet and really cheap.
Use vacuum filters instead of the more expensive MERV 13 ones.
Figure out how to nebulise salts with actual particle counters.
I also want to figure out a way to make the filters replaceable, perhaps with a cardboard frame.
I also need to buy more equipment, like an anemometer to better characterise the airflows.
