3
Explain

Meet the CR Box

Duration
45 minutes
Type
Explain
Standards
MS-ETS1-2, 7.G.B.6

Learning Objectives

Students will be able to:

What is a CR Box?

The Corsi-Rosenthal box (named after Dr. Richard Corsi and Jim Rosenthal) is a DIY air purifier made from a box fan and furnace filters arranged in a cube. It was developed during the COVID-19 pandemic as an affordable, effective way to clean indoor air.

Components

  • 1 box fan (20")
  • 4-5 MERV-13 filters (20"x20"x2")
  • Cardboard for top
  • Duct tape

How It Works

  • Fan pulls air through filter walls
  • Large filter area = low resistance
  • Particles captured by fibers
  • Clean air exhausted through top

Key Calculations

Clean Air Delivery Rate (CADR)

CADR = Airflow (CFM) × Filter Efficiency

Example: A box fan moves 1,200 CFM. With MERV-13 filters (approximately 85% efficient for PM2.5):

CADR = 1,200 × 0.85 = 1,020 CFM of clean air

Air Changes per Hour (ACH)

ACH = (CADR × 60) ÷ Room Volume

Example: Classroom is 30 ft × 30 ft × 10 ft = 9,000 cubic feet

ACH = (1,020 × 60) ÷ 9,000 = 6.8 air changes per hour

Recommendation: 4-6 ACH for general spaces; 6+ for high-risk settings

CR Box vs. Commercial HEPA

Factor CR Box Commercial HEPA
Cost $60-100 $300-800
CADR 600-1,000+ CFM 200-400 CFM typical
Noise Higher Lower
Appearance DIY look Professional
Filter efficiency MERV-13 (~85%) HEPA (99.97%)

Key insight: CR boxes deliver more clean air per dollar, making them ideal for spaces where cost is a constraint.

Practice Problems

  1. A box fan moves 1,500 CFM. With MERV-13 filters, what's the CADR?
  2. A bedroom is 12 ft × 14 ft × 8 ft. If a CR box has CADR of 800 CFM, what's the ACH?
  3. A hospital recommends 12 ACH. How many CR boxes (CADR = 600 each) would a 20 ft × 25 ft × 9 ft waiting room need?
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