Thread by @DavidElfstrom

#boxfanfilter optimizing:
Fan shroud sizing for Lasko 20" Comfort Fan /1

Drove a screw into the middle /2

Made concentric circles with dry-erase marker /3

Used alternating colors to make it easier to see on the grid /4

Flow capture hood mounted on inlet side, with a 3M Filtrete 1900 (MERV-13) taped to inlet. There is some extra resistance from the capture hood, so actual flow will be a bit higher. Three speeds. 117 V
74 W, -259 CFM, 0.99 PF
59 W, -216 CFM, 0.93 PF
48 W, -166 CFM, 0.87 PF
/5

17.5" diameter: -328 CFM.
27% more flow. Same power /6

16.5" diameter: 338 CFM.
31% more flow. /7

15.5" diameter: -348 CFM.
34% more flow. /8

14.5" diameter: -351 CFM.
36% more flow. /9

13.5" diameter: -341 CFM.
32% more flow. Whoops, too far! /10

Final result: No change in power or power factor. Use between 14.5-15.5", so a 15" internal diameter shroud for the Lasko 20" air circulating box fan.
Speed 3: 74 W. 36% higher flow
Speed 2: 59 W. 30% higher flow
Speed 1: 48 W. 31% higher flow
/11

The fan is noticably quieter with the shroud. I have since removed the tape and will make a cleaner version and measure the noise difference with a calibrated microphone later. /12

Forgot to mention it's a 3M Filtrete 1900, 20x20x1.
If we assume 75% PSE the effective clean air delivery rate is 0.75x 350 = 263 CFM CADR, with 74W power, for an efficiency of 3.6 CADR/W, good for Energy Star (must be > 3.0). Further tests to come!
/13

Optimization so far, with 3M Filtrete 1900 20x20x1

Lasko with 15" shroud: 350 CFM, 74 W

Utilitech with 13.5" shroud: 350 CFM, 52 W!

Actual flow will be slightly higher when measured at outlet with backpressure compensation feature in the flow capture hood. /14

The mains voltage fluctuates (goes higher at night when less demand) and has a large impact on fan power and flow. These numbers are without shroud, with 20x20x1 MERV-13:
115V: 255 CFM, 70 W
117V: 259 CFM, 74 W
120V: 268 CFM, 77 W /15