Filter Types: Panel, Pleated, Bag, Cartridge, HEPA

Filter Construction Categories

Air filters are classified by construction method, which directly influences filtration efficiency, pressure drop, dust-holding capacity, and service life. Understanding construction differences enables proper selection for specific applications and operating conditions.

Panel filters represent the simplest filter construction, consisting of a flat media pack held within a rigid frame.

Construction Characteristics

Media Types:

Fiberglass mat (low-efficiency applications)
Polyester synthetic fiber
Electrostatically charged synthetic media
Metal mesh (washable/reusable types)

Frame Materials:

Cardboard (disposable, MERV 1-4)
Galvanized steel (industrial applications)
Aluminum (corrosion resistance)
Molded plastic (moisture resistance)

Typical Dimensions:

Standard: 20" × 20", 20" × 25", 24" × 24"
Thickness: 1" to 2"
Limited media area equals low dust capacity

Performance Range

Parameter	Low-Efficiency	Medium-Efficiency
MERV Rating	1-4	5-8
Initial Δp	0.05-0.10 in. w.g.	0.15-0.30 in. w.g.
Dust Capacity	50-150 g	150-300 g
Particle Removal (0.3-1.0 μm)	<20%	20-35%
Service Life	1-3 months	2-4 months

Applications

Residential HVAC systems (MERV 1-8)
Pre-filtration in multi-stage systems
Light commercial applications with low particulate loads
Equipment protection (coil protection)

Pleated Filters

Pleated construction increases media area within the same face dimensions by folding the media into accordion-like pleats. This configuration provides superior dust-holding capacity and service life compared to panel filters.

Construction Features

Pleat Geometry:

Pleat height: 0.5" to 3" (depth determines media area)
Pleat spacing: 12-24 pleats per foot
Media support: Metal wire or plastic grid prevents pleat collapse

Media Options:

Synthetic fiberglass blends (MERV 8-13)
Electrostatically charged media (enhanced efficiency)
Cotton-polyester blends (MERV 7-10)

Performance Characteristics

graph TD
    A[Air Stream] --> B{Pleated Media}
    B -->|Inertial Impaction| C[Large Particles >10 μm]
    B -->|Interception| D[Medium Particles 1-10 μm]
    B -->|Diffusion| E[Small Particles <1 μm]
    B -->|Electrostatic| F[Submicron Particles]
    C --> G[Clean Air]
    D --> G
    E --> G
    F --> G

Comparative Performance:

Depth	Media Area Ratio	MERV Range	Typical Δp	Dust Capacity
1"	3-4× panel	7-10	0.20-0.35 in. w.g.	250-400 g
2"	6-8× panel	8-11	0.25-0.45 in. w.g.	400-600 g
4"	12-16× panel	11-14	0.30-0.60 in. w.g.	800-1200 g

Applications

Commercial HVAC systems (standard efficiency)
Residential high-efficiency upgrades
General office and retail environments
Healthcare non-critical areas

Bag Filters

Bag filters utilize pockets of media extending downstream from the filter frame, maximizing media area and dust-holding capacity. This design provides the highest dust capacity among sub-HEPA filters.

Construction Details

Bag Configuration:

Number of pockets: 3 to 12 per filter
Pocket depth: 12" to 36"
Header attachment: Welded, stitched, or sonic-bonded seams
Support: Internal wire supports maintain bag shape under airflow

Media Selection:

Synthetic microfiber (MERV 13-15)
Multi-layer lofted media
Progressive density media (surface loading to depth loading)

Performance Specifications

Pockets	Depth	Media Area	MERV	Initial Δp	Dust Capacity
6	24"	45-55 ft²	13-14	0.50-0.70 in. w.g.	1500-2500 g
8	24"	60-70 ft²	14-15	0.55-0.75 in. w.g.	2000-3000 g
6	36"	65-80 ft²	13-14	0.45-0.65 in. w.g.	2500-3500 g

Collection Mechanisms

flowchart LR
    A[Particulate-Laden Air] --> B[Bag Entry]
    B --> C{Filtration Zone}
    C -->|Surface Filtration| D[Large Particles]
    C -->|Depth Filtration| E[Fine Particles]
    D --> F[Dust Cake Formation]
    E --> F
    F --> G[Increased Efficiency]
    C --> H[Clean Air Exit]

Applications

Hospital non-critical areas (MERV 13-14)
Pharmaceutical manufacturing (pre-filtration)
Clean room pre-filtration stages
Commercial buildings requiring high IAQ
Data centers and electronics manufacturing

Cartridge Filters

Cartridge filters employ cylindrical or conical media configurations, primarily used in industrial dust collection and specialized HVAC applications.

Design Variations

Cylindrical Cartridges:

Media: Cellulose, polyester, or spunbond
Pleat count: 150-300 pleats
Filtration area: 20-150 ft² per cartridge
End caps: Metal or plastic with gasket seal

Panel-Style Cartridges:

Rigid frame with extended pleats
Used in compact filter housings
MERV 11-15 typical range

Performance and Maintenance

Operating Characteristics:

Initial pressure drop: 0.3-0.8 in. w.g.
Replacement threshold: 2.0-3.0 in. w.g.
Cleaning: Some designs allow pulse-jet cleaning
Service life: 6-24 months depending on loading

Applications

Industrial ventilation systems
Dust collection pre-filtration
Process air filtration
High-volume low-pressure applications

HEPA Filters

High-Efficiency Particulate Air (HEPA) filters represent the highest efficiency class for mechanical filtration, removing ≥99.97% of particles at 0.3 μm (most penetrating particle size).

Construction Standards

Media Composition:

Microglass fiber paper
Submicron fiber diameter (0.5-2.0 μm)
Depth: 0.3-0.5 mm compressed
Binder content: 5-15% for structural integrity

Assembly Requirements:

Separator material: Aluminum corrugated, hot-melt adhesive
Frame: Particle board, metal, or plastic
Seal: Polyurethane or neoprene gasket
Quality assurance: 100% factory tested per MIL-STD-282 or equivalent

Filtration Physics

graph TD
    A[HEPA Filtration Mechanisms] --> B[Interception]
    A --> C[Impaction]
    A --> D[Diffusion]
    B --> E[Particles 0.3-1.0 μm]
    C --> F[Particles >1.0 μm]
    D --> G[Particles <0.3 μm]
    E --> H[>99.97% Removal]
    F --> H
    G --> H
    style H fill:#90EE90

Performance Specifications

Standard	Efficiency at MPPS	Typical Δp	Velocity	Application
HEPA (Type A)	99.97% @ 0.3 μm	0.8-1.2 in. w.g.	250 fpm	Healthcare critical
HEPA (Type B)	99.97% @ 0.3 μm	1.0-1.5 in. w.g.	300 fpm	Pharmaceutical
ULPA	99.999% @ 0.12 μm	1.2-2.0 in. w.g.	250 fpm	Semiconductor

Critical Applications

Hospital operating rooms and isolation rooms
Pharmaceutical clean rooms (ISO Class 5-7)
Semiconductor manufacturing
Biosafety laboratories (BSL-3, BSL-4)
Nuclear facilities

Selection Criteria Matrix

Factor	Panel	Pleated	Bag	Cartridge	HEPA
Initial Cost	$	$$	$$$	$$-$$$	$$$$
Operating Cost	High	Medium	Low	Medium	High
Efficiency Range	MERV 1-8	MERV 7-14	MERV 13-15	MERV 11-15	MERV 17-20
Dust Capacity	Very Low	Medium	Very High	High	Low
Space Required	Minimal	Minimal	Moderate	Moderate	Significant
Maintenance Frequency	Monthly	Quarterly	Semi-Annual	Quarterly	Annual
Pressure Drop	Low	Medium	Medium-High	Medium	High

Design Considerations

Airflow Requirements:

Face velocity limits prevent media damage
Panel/Pleated: 300-500 fpm maximum
Bag: 250-350 fpm recommended
HEPA: 250 fpm standard (critical applications)

Filter Housing:

Access door sizing for filter replacement
Manometer ports for pressure drop monitoring
Gasket contact surface flatness (HEPA: ≤0.062")
Bypass prevention (100% airflow through media)

System Integration:

Pre-filtration extends final filter life
MERV 7-8 before MERV 13-15 recommended
MERV 11-13 before HEPA mandatory
Pressure drop monitoring for timely replacement

References

ASHRAE Handbook—HVAC Systems and Equipment, Chapter 29: Air Cleaners for Particulate Contaminants
ASHRAE Standard 52.2: Method of Testing General Ventilation Air-Cleaning Devices for Removal Efficiency by Particle Size
ISO 16890: Air filters for general ventilation
MIL-STD-282: Filter Units, Protective Clothing, Gas Mask Components and Related Products: Performance-Test Methods

HVAC Systems Encyclopedia

Filter Types: Panel, Pleated, Bag, Cartridge, HEPA

Filter Construction Categories

Panel Filters

Construction Characteristics

Performance Range

Applications

Pleated Filters

Construction Features

Performance Characteristics

Applications

Bag Filters

Construction Details

Performance Specifications

Collection Mechanisms

Applications

Cartridge Filters

Design Variations

Performance and Maintenance

Applications

HEPA Filters

Construction Standards

Filtration Physics

Performance Specifications

Critical Applications

Selection Criteria Matrix

Design Considerations

References