David Elfstrom Profile picture
Aug 22, 2021 26 tweets 10 min read Read on X
FINAL OPTIMIZATION! Box fan single filter with 400 CFM measured at 120V. Estimated clean air delivery rate is 248 CFM CADR. Power is just 53 W, placing it well into Energy Star level for air cleaners. Room size: 384 sqft (8' ceilings). Cost: 🇨🇦$55 plus some tape. /1 Image
The filter is a single MERV-13 3M Filtrete 1900 which has the lowest pressure drop of the whole range. I also tested a Tex-Air 20x20x2 MERV-13 and there was no significant difference, it was 0.7% less flow, within measurement variation. /2 Image
Lifting the filter away from the back of the fan adds 8% more flow, and if cut a circular hole with 21 cm radius instead of a matching square, it adds another 3% more flow. /3 Image
Not only does the back box add 11% flow, it makes filter changes super easy.

And it provides a channel where some sound attenuation material could be placed, if desired. /4 Image
Measurements were done with a flow capture hood with homemade version of an anti-swirl flow straightener. Backpressure compensation mode was used and 16 samples were taken of each configuration. Voltage was true RMS, and power was kill-a-watt meter. /5 ImageImage
Sound is 55 dB(A) with calibrated microphone at 1m from the side... /6 Image
And it's a bit louder at 1m from the back (where the filter is) at 58 dB(A) /7 Image
Materials:
A 20" box fan as shown below. Some names are Utilitech, Genesis, Hurricane, For Living.
The box it comes in.
A 3M Filtrete MPR 1900 (MERV-13) 20x20x1. Alternative: 20x20x2 MERV-13 Filter from Tex-Air Filters, or other brands. /8 Image of the generic box fa...
Tape over the front to create a shroud. To maximize flow, tape to the ring at 13.5" (34.5 cm). This increases flow by 47%‼️
You can use a colorful duct tape. (I used painter's tape for temporary tests, you should use something better) /9 Image
OPTIONAL: Saw a channel for the power cord so the back will be flat, improving contact. /10 ImageImage
You saved the box, right? Seal it shut and rotate it for a good fit on the back (this one is slightly rectangular, works better on its side) /11 ImageImage
For the back, center the filter and trace it with a marker. Freehand or measure out an offset frame and cut it out. ProTip: Use a new blade! Your cuts will be easier, cleaner, and safer. /12 ImageImage
For the back shroud, trace around the case. Offset an inner frame edge and cut there, OR for an additional 3% more flow, identify the center and draw a circle with a compass, radius 21 cm and cut that out instead. /13 Image
Tape the spacer box on to the back of the fan, forming a complete seal with no cracks. You might want to add some foam in the channel formed by the box to reduce noise (I have not tried that yet) The spacer box increases flow by 8% /14 Image
Center the filter over the back and tape on all sides. You're done!

Stay tuned for more options and optimization using a 2-filter wedge, 4-sided box or 5- sided cube, plus measurements at lower speeds. /15 Image
A Corsi-Rosenthal cube will definitely have more flow for the same level of noise.

But for cost-effectiveness this single-filter optimized design can have a huge impact on health, with widespread deployment. /16
🤔Does it strain the motor?
No! The shroud unloads the motor, lowers the power, and produces more flow. The extra resistance from the filter changes the point on the fan curve, resulting in less flow than without, but the same power consumption. /17
🤔What about fire risk?
It's fine. These are designed to be safe even if they fall flat on their front or back. And just to be sure, check out this safety report from scientists at Underwriter's Laboratories Inc! PDF: chemicalinsights.org/wp-content/upl… /18 Image
🤔So is this better than a filter cube?
No! But it *is* far better than the same two materials taped together. The filter taped directly to the back of the fan is 245 CFM.
By optimizing the design, the flow is 400 CFM, a 63% improvement in performance! 🤯 /19
🤔Is this good for my classroom?🧑‍🏫
You need flow, as much as possible, and low noise. For that you should build one or two Corsi-Rosenthal boxes. Check out the mega-thread by @LazarusLong13, and I'll be doing further tests on them soon! /20
🤔How often should I change the filter?
It depends on the air it's filtering (forest fire smoke?) and run time. Suggest once every three months for this single-filter design.
A Corsi-Rosenthal box made from four or five filters might last a whole year. /21
🤔Will you patent it?
No! If anything, I hope fan manufacturers optimize their products. Lasko even makes a box fan with filter holder (model FF305) but didn't add a shroud, and ships with a low quality filter! Result: 42 CADR, versus 248 CADR. Drives me bonkers. /22 Image
🤔What if I have a different fan?
The Lasko Comfort fan has a different optimal fan shroud size (15" opening).
Or, use tissue paper as nicely illustrated by @PragmaV. Good STEM activity for students! /23 Image
Final full measurements at all speeds including noise for the single filter optimized design /24 Image
Compare with the next size up: A two-filter wedge. Doubling the filter area (+200%) increases flow by 35% and slightly reduces noise. /25 Image
Here's a link to the next size up, the wedge, and how to construct it. /26

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More from @DavidElfstrom

May 31
Vancouver Coastal Health has released an updated Ventilation and Indoor Air Quality resource for Schools and Childcare Facilities () but their CO2 page needs some edits. vch.ca/en/document-li…
States high levels of CO2 and other contaminants ... can contribute to [list of five health effects, such as headaches, fatigue, difficultly concentrating], says CO2 should be below 1000 ppm, then says (incorrectly) that health effects from CO2 occur at levels above 5000 ppm. No, those effects can occur below 5000 ppm.  It also states the CO2 concentrations do not indicate a risk of infectious disease transmission in a space. No, ASHRAE's position statement on CO2 states that elevated CO2 concentrations CAN reflect higher risk. Obviously, because it indicates poor ventilation, which IS an i...
Vancouver Coastal Health "CO2 concentrations do not indicate a risk of infectious disease transmission in a space". No.
ASHRAE's position document on indoor CO2 says "higher CO2 conc correspond to lower ventilation and potentially increased risk of airborne transmission"
Vancouver Coastal Health "Note that health effects from CO2 occur at levels above 5000 ppm". Did WorkSafeBC interfere? Because that's contradicted by your Health Canada reference in the sentence immediately before it.
Read 10 tweets
Mar 28
Three years ago on this day ASHRAE stated that Airborne transmission of SARS-CoV-2 is significant.

Today, WHO finally released a model () of #CovidIsAirborne risk by experts in the field. /1 iris.who.int/handle/10665/3…

Landing page for the WHO document titled: Indoor airborne risk assessment in the context of SARS-CoV-2: description of airborne transmission mechanism and method to develop a new standardized model for risk assessment
Four years ago on this day WHO said : "FACT: #COVID is NOT airborne."

Today, WHO finally released a model () of #CovidIsAirborne risk by experts in the field that warned of airborne even earlier, but were shut out. /2
iris.who.int/handle/10665/3…
WHO Tweet March 28, 2020 FACT: #COVID19 is NOT airborne.   The #coronavirus is mainly transmitted through droplets generated when an infected person coughs, sneezes or speaks.  To protect yourself: -keep 1m distance from others -disinfect surfaces frequently -wash/rub your hands -avoid touching your eyes / nose / mouth  Cover page of Indoor airborne risk assessment in the context of SARS-CoV-2
This document has been a long time coming. As described by @jljcolorado, Lidia Morawska, co-chair of the group that published the new WHO airborne model, was previously cut off by John Conly when making the case that #COVIDisAirborne to WHO. /3
Acknowledgements: WHO ARIA Technical Advisory Group:  First name: Lidia Morawska (Co-Chair) (Queensland University of Technology, Australia) (many other names) John Conly (University of Calgary, Canada)
Read 4 tweets
Feb 23
Air purifier manufacturers say HEPA should always be the filter of choice, and their product's proprietary filter delivers. Which HEPA? ISO 35H at 99.95% or ISO 40H at 99.99%? Why not ISO 50U? That's 10x better at 99.999%. Why stop there? Go for ISO 70U at 99.99999%! /1
The answer is, single-pass filtration efficiency DOESN'T MATTER except in specific cases like Powered Air Purifying Respirators (PAPR), clean rooms, operating theaters, or nuclear laboratory exhaust—HEPA's original purpose. /2
For portable/in-room air cleaners, all that matters is the Clean Air Delivery Rate (CADR) for a target particle size and type, within acceptable for sound power and frequency characteristics for the people in the room. /3
Read 13 tweets
Jan 3
Four years into this and we can't keep duct-taping in-room filter solutions for clean air. It's just filter(s) and a fan. We need open-source, optimized design, certifiable product, efficient, repairable using commodity filters and commodity components. /1
We need air cleaners assembled and distributed by not-for-profit community-based social enterprise. No more lock-in to proprietary filters. Verified replacement commodity filtration performance for safety. /2
Low income with donated CR boxes will pay over time in electrical costs for the duct-taped solution for clean air.
Power utilization for Smoke CADR, same filters:
Conventional CR Box: 4 CADR/W. (77 W)
PC fan array air cleaner: 24 CADR/W. (8 W)
/3
Read 4 tweets
Aug 28, 2023
1/ Levoit Core 400S versus Austin Air HM400 in a challenge to see which portable air cleaner removes submicron salt particle aerosols the fastest! Which do you think will win, and by how much? Poll in next tweet below... A large indoor tent with taped seams on a concrete floor is shown. On the left is a bench with instruments. A webcam is mounted on the top of a chair facing the instruments. In the middle are two air purifiers: The cylindrical Levoit Core 400S and the larger boxy Austin Air HM400. In the back right corner is a circular floor fan pointed diagonally upward into the middle. At the front right is a chair holding what looks like a tiny pitcher with a spout. A long thin clear plastic tube is connected to the side of this device, and exits the frame. Power cords are on the floor.
2/ Which has a higher CADR (Clean Air Delivery Rate):
Levoit Core 400S, or Austin Air HM400?
See if you can find the manufacturer's claims for both, and then come back and vote:
[sarcasm] Not only is the Austin Air bigger and far heavier, it also draws way more power, is much louder, and more expensive. It couldn't possibly be *worse* than the Levoit, right? Right?
Read 13 tweets
Jun 12, 2023
When to change the filter on DIY #CorsiRosenthalBox? Holder et al (onlinelibrary.wiley.com/doi/10.1111/in…) showed that performance degradation depends on contaminant: Dust reduces airflow. Smoke kills electrostatic properties. @JimRosenthal4 @joeyfox85 @CorsIAQ Table for MERV 13 single-fi...
@JimRosenthal4 @joeyfox85 @CorsIAQ Note the test substance for the CADR PM2.5 performance measurement was simulated wildfire smoke.
The same would likely apply to a commercial HEPA air cleaner. Monitor for clearing time for a fixed release of contaminant with a simple PM meter.
Read 6 tweets

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