Thread: Improvised Ambient Air Filter

Because of the current fires on the US West coast there's a shortage of room air purifiers and so a lot of discussion on using a cheap box fan and furnace filter ('return air filter' in Aus.) as a substitute. There were some posts on another forum linking Youtube videos showing "testing" of the concept. They all made the usual mistake of focusing only on filter efficiency, completely ignoring flow rate. A box fan has very little static pressure and so it's flow rate is highly sensitive to filter resistance. The proper figure of merit for an ambient air cleaner is flow rate times filter efficiency.

In the US the industry standard for testing air purifiers is ANSI/AHAM AC-1. One of the outputs of the test is Clean Air Delivery Rate (CADR) which is, in essence, flow times filter efficiency. It's measured by recording particle levels over time and using the slope of the log of particle count to get the exponential decay rate. CADR is then that decay rate minus the natural decay rate times room volume.

As it turns out, I happened to have two cheap box fans and two MERV 12 filters in my shop. In addition, I just finished upgrading the datalogging capability of my particle counters (PMSA003 and PMS7003). So, it was kind of inevitable that I would start a newspaper fire in my shop and see how fast I could clear the smoke.

Taking hint from the AHAM spec I added a small circulating fan to keep things well mixed. I let a small piece of newsprint smoulder for about thirty seconds. I waited twenty minutes to start the filtering fans so I could get a value for the natural precipitation rate. I then started the fans.

SmokeTestCounts091520_b.jpg

The natural decay rate is .0096 /min and the forced rate .0567 /min for a difference of .0471 /min. My shop is about 5800 cubic feet so roughly a CADR of 273 CFM. That's for two fans, so one cheap fan with a MERV 12 is about the equivalent of a small air purifier. Actually, I did the test using both the MERV 12 and a MERV 2 prefilter on each fan. Without the prefilter the flow is about 10% higher.