Thread by @joeyfox85

CALCULATING WHAT A HEPA FILTER DOES

Portable air purifiers (HEPA filters) are not magic machines that you turn on in the corner of the room and they make COVID go away.

They usually are advertised with a square footage, but that's not accurate. Here's what it means:

1/10

I'll use the Austin Healthmate HM400 as an example. It says it's designed for a 1500 sq ft room. Assume 9 ft ceilings. Total volume = 13500 ft^3.

The advertised CFM (cubic ft/min) is 400, but it's really 250 with a filter: cdn.shopify.com/s/files/1/1002…

2/10

austinair.ca/products/healt…

Using their advertised CFM, 13500ft^3/400 CFM = 33.8 minutes/air change = 1.8 Air Changes/Hour (ACH).

The true clean air delivery rate (CADR) is 250 CFM: 60*250/13500=1.1 ACH

The claim is it "effectively cleans an area of 1500 sq. ft". That's not what it does.

3/10

It provides 1.1 ACH for an area of 1500 sq ft when running on high speed. It also doesn't filter all the air in the room well when it's put in a corner. It should be placed near the center of the room.

4/10

What happens on low speed? Can't find the data on low speed with filter, but without the filter, it is 75 CFM. That gives you 0.3 ACH.

Typical classroom of 750 sq ft gives you 2.2 ACH on high and 0.6 ACH on low.

5/10

Another way to calculate - ignore the area and look at the people. 25 people in a room - 250 CFM = 10 CFM/person. 75 CFM = 3 CFM/person.

This adds to the existing system. If you have ventilation providing 3 ACH and your HEPA filter has 2 ACH, you now have 5 ACH.

6/10

If your existing system has 10 CFM/person and this adds 10 CFM/person, you now have 20 CFM/person.

What do these numbers mean? How to interpret?

7/10

For total CFM:
10 CFM/person=poor
15 CFM/person =typical
20 CFM/person=good
25 CFM/person = really good
30 CFM/person = good for smoking conditions

8/10

For ACH, see 👇

https://twitter.com/j_g_allen/status/1339576175472095233?s=20

Note: Modern day ventilation calculation as per ASHRAE 62.1 combines the expected occupancy and square footage of the area. For classrooms, it ends up around 15 CFM/person.

9/10

Ultimately, ignore the "sq ft" they advertise. Find the clean air delivery rate (CADR). Use the volume of the room to get the ACH=60*CADR/volume or divide the CADR by the amount of people to see what the change is in CFM/person.

10/10