Air Filtration Design for HVAC Engineers

Air Filtration Design for HVAC Engineers

Filtration protects occupants from particulates, extends equipment life, and maintains indoor air quality. Proper filter selection balances capture efficiency, pressure drop, and cost.

MERV Ratings

Minimum Efficiency Reporting Value (ASHRAE 52.2):

Common selections:

• Office: MERV 8-11
• School: MERV 11-13
• Hospital: MERV 14-15 + HEPA in critical areas
• Cleanroom: HEPA (MERV 17-20)

Filter Pressure Drop

Clean filter:

$$\Delta P_{clean} = 0.1 - 0.8 \text{ “w.g.}$$

(Higher MERV = higher pressure drop)

Dirty filter (replace when):

$$\Delta P_{dirty} = 2 \times \Delta P_{clean}$$ (typically 1.0-1.5 “w.g.)

Pressure drop increases exponentially as dust loads

Energy cost of pressure drop:

$$Cost = \frac{CFM \times \Delta P \times hours \times $/kWh}{6,356 \times \eta_{fan}}$$

Filter Sizing

Face velocity:

$$v_f = \frac{CFM}{A_{filter}}$$

Typical face velocities:

• Pleated filters (MERV 8-13): 300-500 FPM
• HEPA filters: 250-300 FPM
• Bag filters: 400-600 FPM

Lower face velocity:

• Reduces pressure drop
• Increases filter life
• Requires larger filter bank (higher first cost)

Filter Service Life

Dust holding capacity: Grams of synthetic dust per filter area

Service life estimation:

$$t_{service} = \frac{Capacity_{grams}}{Dust_{loading,g/h}}$$

Typical service life:

• MERV 8 pleated: 3-6 months
• MERV 13 pleated: 6-12 months
• HEPA: 1-3 years

Replacement triggers:

• Pressure drop exceeds limit
• Time-based schedule
• Visual inspection (if accessible)

Gaseous Filtration

Activated carbon:

• Removes odors, VOCs
• Does NOT remove particles
• Pressure drop: 0.2-0.5 “w.g.
• Service life: 6-24 months (depends on contaminant concentration)

Potassium permanganate:

• Removes odors, ethylene (food storage)

Filtration System Design

Two-stage filtration (common):

1. Pre-filter (MERV 8): Captures large particles, protects final filter
2. Final filter (MERV 13-16): Fine particle removal

Benefits of two-stage:

• Extends final filter life
• Lower overall pressure drop
• Better protection

HEPA filter bank design:

• Scan test ports (verify no bypass)
• Gel-sealed frames
• Bag-in-bag-out for hazardous materials

Practical Applications

1. ASHRAE 62.1 compliance: MERV 6 minimum (13 recommended)
2. Energy code (90.1): Consider pressure drop in fan power budget
3. Wildfire smoke: Upgrade to MERV 13+ during events

Related Technical Guides:

• Ventilation Rate Calculations
• Indoor Air Quality Management
• Fan Selection & Performance

References:

• ASHRAE Standard 52.2: Method of Testing General Ventilation Air-Cleaning Devices for Removal Efficiency by Particle Size
• ASHRAE Handbook of HVAC Systems and Equipment, Chapter 29: Air Cleaners for Particulate Contaminants
• NAFA Guide to Air Filtration, 5th Edition