Clean Air Filter Design Lab
Build a filter by stacking layers of real media, then set the dirty inlet air and the fan strength. Watch the capture efficiency compound, the pressure drop add up, and the airflow fall, and learn why the clean air delivery rate, not the capture percentage, is the honest measure of an air cleaner.
Guided Experiment: Does stacking more filters always clean more air?
Predict what happens to the clean air delivery rate (CADR) as you add more and finer filter layers. Will more layers always move more clean air, or is there a point where it gets worse?
Write your hypothesis in the Lab Report panel, then click Next.
Air Through the Filter
Controls
- 1.Coarse pre-filter (foam mesh)η 20%
- 2.MERV 13η 85%
Around 80 µg/m³ is unhealthy outdoor air. Below about 12 µg/m³ counts as good air quality.
A weaker fan pushes less air, so a high resistance filter delivers even less clean air.
Results
replace about every 130 days.
| Medium | η (%) | ΔP (Pa) | Flow | CADR | $ |
|---|---|---|---|---|---|
| MERV 13best CADR | 85.0 | 55 | 218 | 184.9 | 22 |
| Pleated MERV 8 | 50.0 | 25 | 263 | 131.3 | 12 |
| HEPA | 99.7 | 120 | 120 | 119.6 | 45 |
| Polyester pad | 35.0 | 12 | 282 | 98.7 | 5 |
| Coarse pre-filter (foam mesh) | 20.0 | 8 | 288 | 57.6 | 4 |
| Activated carbon | 10.0 | 40 | 240 | 24.0 | 18 |
HEPA captures the most per pass, but its high resistance can drop its CADR below a MERV 13 because the airflow falls. Clean air delivery, not capture percentage, is the real measure.
Data Table
(0 rows)| # | Filter stack | Inlet (µg/m³) | Efficiency (%) | Outlet (µg/m³) | Pressure drop (Pa) | CADR (m³/h) | Cost ($) | Life (days) |
|---|
Reference Guide
How Layers Compound
Each filter layer catches a fraction of the particles and lets the rest, the penetration, pass on to the next layer. The penetrations multiply, so the total capture of a stack is one minus the product of the penetrations.
A 50 percent layer behind another 50 percent layer does not give 100 percent. It gives 75 percent, because the second layer only works on what got through the first. Stacking finer media raises capture, but with shrinking returns.
Pressure Drop, Airflow, and CADR
Every layer resists airflow, and those resistances add up like resistors in series. A fan can only push so hard, so the more resistance you stack, the less air actually moves through the filter.
The clean air delivery rate (CADR) is the airflow Q times the capture efficiency. A near perfect HEPA filter can deliver less clean air than a MERV 13, because its high resistance starves the airflow. Capture percentage alone hides this tradeoff.
Pre-filters and Carbon
A cheap coarse pre-filter on the dirty side captures the big dust first. It adds little capture and little resistance, but it stops the large particles from clogging the expensive fine layer, so the whole filter lasts much longer.
Activated carbon is different. It is made to adsorb gases and odors, not particles, so it captures very little PM2.5 while still adding resistance. Use it for smells and fumes, not for clearing smoke or dust from the air.
Good filter design is layered on purpose. Coarse first to protect and extend life, a fine layer to do the real particle capture, and only the resistance the fan can afford.
Reading the Air Quality
The lab reports the outlet PM2.5 in micrograms per cubic metre and labels it with the US air quality category. Below about 12 µg/m³ is good, up to 35 is moderate, and higher levels become unhealthy first for sensitive groups, then for everyone.
Fine PM2.5 particles are small enough to reach deep into the lungs, which is why clearing them matters. A good filter design moves a lot of air to a good or moderate outlet level, rather than scrubbing a tiny trickle of air to perfection.
