CADR and Air Changes
Learning Objectives
Students will be able to:
- Define CADR and explain how it is measured
- Convert CADR to equivalent air changes per hour
- Size air cleaners for specific room volumes
- Compare ventilation and filtration approaches
- Perform cost-effectiveness calculations for air cleaning
The Big Question
"How do we quantify air cleaner performance, and how much filtration do we need for a given space?"
Clean Air Delivery Rate (CADR)
CADR = Qfilter x Efficiency
CADR represents the volumetric flow rate of particle-free air delivered by an air cleaner.
- Units: m3/h or CFM (cubic feet per minute)
- Conversion: 1 CFM = 1.7 m3/h
- AHAM standard: Tested for smoke, dust, and pollen particles
Equivalent Air Changes per Hour
CADR can be expressed as equivalent ventilation air changes:
eACH = CADR / V
Where V is room volume. This allows direct comparison with ventilation rates.
| CADR (m3/h) | Room 30 m3 | Room 100 m3 | Room 300 m3 |
|---|---|---|---|
| 150 | 5.0 eACH | 1.5 eACH | 0.5 eACH |
| 300 | 10 eACH | 3.0 eACH | 1.0 eACH |
| 600 | 20 eACH | 6.0 eACH | 2.0 eACH |
Sizing Air Cleaners
Target eACH Guidelines
| Application | Target eACH | Notes |
|---|---|---|
| Residential (typical) | 2-4 | General improvement |
| Classrooms | 4-6 | CDC/ASHRAE guidance |
| Healthcare waiting | 6-12 | Higher for infection control |
| Isolation rooms | 12+ | ASHRAE 170 standard |
Sizing formula: Required CADR = Target eACH x Room Volume
Activity: Air Cleaner Analysis
Problem Set
- Sizing: A classroom is 200 m3. What CADR is needed for 5 eACH?
- Multiple units: If available air cleaners have CADR = 300 m3/h, how many are needed?
- Cost analysis: Each unit costs $150 and uses 50W. For 2000 operating hours/year at $0.15/kWh, what is the annual operating cost?
- Comparison: Installing a ventilation upgrade to add 3 ACH would cost $5000 and add $500/year in energy costs. Compare 5-year total costs.
Key Takeaway
CADR provides a standardized metric for comparing air cleaner performance. Converting CADR to equivalent air changes allows direct comparison with ventilation and helps size air cleaners for specific rooms. Cost-effectiveness analysis must consider both capital and operating costs to make informed decisions about air quality interventions.