Boxfan Ambient Air Filter Hack Revisited #6: CFM and Efficiency - Part B, the Filter

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Blog entry by clagwell posted 06-28-2020 05:00 PM 614 reads 0 times favorited 0 comments Add to Favorites Watch
« Part 5: CFM and Efficiency - Part A, the Fan Part 6 of Boxfan Ambient Air Filter Hack Revisited series no next part

In the last post we looked at the effect of CFM on the length of time it takes to reduce the dust concentration in a room. That applies to either venting outside and bringing in clean air or passing the air through a perfect filter. Lacking a perfect filter we have to ask what effect the filter capture efficiency has on the air clearing rate?

Let’s start with the example from the previous post and assume 1,000 particles per cubic foot to start. At 500 CFM we’re sending 500,000 particles per minute through the filter. If the filter is, say, 80% efficient then we’re removing 0.80×500,000 particles per minute = 400,000 particles per minute from the room. A 0.80×500 CFM = 400 CFM fan with a perfect filter will also remove 400,000 particles per minute. The net effect in the room is the same.

We can easily generalize that and say that filter efficiency essentially decreases the CFM value used to calculate clearance rate. The effective CFM is the fan CFM times the filter efficiency. So we modify the equation from the last post:

T10 = 2.3 x Vol/(CFM x Efficiency)

One complication is that filter efficiency varies with particle size. That means the clearance rate also varies with particle size. It takes longer to clear out the smaller particles. We need to look at a different clearance rate for each particle size.

We can try for some improvement by selecting a higher efficiency filter but, along with the efficiency is a higher pressure drop. An axial fan doesn’t generate lot of pressure so a small amount of additional pressure drop in the filter can make a big difference in flow. That makes filter selection kind of tricky. I think my dissatisfaction with the setup I described in Part 1 was due to a poor choice of filters. The MERV 13 simply had too much pressure drop for my fan.

For the next section I’ll use the flow data I have and combine it with MERV specs to calculate some clearance times.

-- Dave, Tippecanoe County, IN --- Is there a corollary to Beranek.s Law that applies to dust collection?

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