The Science of Filtration
Learning Objectives
Students will be able to:
- Explain how filters capture particles through multiple mechanisms
- Interpret MERV ratings and compare filter effectiveness
- Describe the relationship between filter efficiency and airflow resistance
- Justify filter selection based on application requirements
Key Concepts
Three Capture Mechanisms
Interception
Particles following airflow touch fiber and stick
Impaction
Large particles can't turn with air, crash into fibers
Diffusion
Tiny particles zigzag randomly, eventually hit fibers
MERV Ratings
| MERV | Captures | Common Use |
|---|---|---|
| 1-4 | Large dust, pollen | Basic residential |
| 5-8 | Mold spores, dust mite debris | Better residential |
| 9-12 | Fine dust, legionella | Commercial buildings |
| 13-16 | Bacteria, smoke, sneeze droplets | Hospitals, CR boxes |
| 17-20 | Viruses, carbon dust | Clean rooms (HEPA) |
Lesson Activities
Filter Examination (15 min)
Students examine different filter samples (if available) or high-quality images. Compare fiber density, thickness, and structure between MERV ratings. Students predict which will capture more particles and why.
Capture Mechanism Modeling (15 min)
Using marbles of different sizes and a mesh/grid, students model how particle size affects capture. Large marbles (large particles) are easily caught; medium marbles may pass through or be caught; very small marbles might slip through unless moving slowly.
The Trade-off Discussion (10 min)
Key concept: Higher MERV = more particles captured BUT also more resistance to airflow. This means fans must work harder, use more energy, and may move less air. Students discuss: Why not always use the highest MERV rating?
Key Takeaway
MERV-13 filters are the "sweet spot" for CR boxes: they capture most harmful particles (including virus-carrying aerosols) while still allowing adequate airflow through a box fan. Higher isn't always better if it means less air actually moves through the filter.