Custom Pocket Filters for an Industrial AHU Retrofit

Illustrative Technical Case Study | Industrial Manufacturing | HVAC Filtration

An industrial manufacturing facility operating an older air handling unit (AHU) faces a common maintenance challenge: replacing pocket filters when standard catalog dimensions no longer match the existing filter housing.

The existing filtration system remains operational, but replacement filters must accommodate the original frame dimensions, limited installation depth, and required airflow conditions.

This technical case study explores a custom pocket filter replacement approach, focusing on three critical requirements: non-standard frame compatibility, optimized pocket configuration, and controlled airflow resistance.

This illustrative case study examines a representative industrial AHU retrofit scenario. Project dimensions and performance targets are examples for engineering discussion, not results from a documented customer order.

 

Project Overview

Project Detail	Example Requirement
Industry	Industrial Manufacturing
Region	Central Europe (Illustrative Scenario)
Application	Existing AHU Filter Replacement
Filter Type	Custom Pocket / Bag Filter
Example Frame Dimensions	580 × 580 × 550 mm
Proposed Filter Media	Fiberglass Pocket Filter Media
Filtration Requirement	F7 Reference Grade (EN 779)
Design Airflow	2,000 m³/h per filter
Initial Pressure Drop Target	≤130 Pa at design airflow
Proposed Configuration	6 Pockets
Frame Material	Galvanized Steel
Main Objective	Fit the existing AHU without major housing modifications

The dimensions and performance targets above are illustrative. Final filter efficiency, airflow resistance, and dimensional compatibility must be verified for the selected configuration. EN 779 is a superseded classification standard; current purchasing specifications should use an appropriate ISO 16890 classification where required.

 

1. Project Background: Replacing Pocket Filters in an Aging Industrial AHU

Industrial manufacturing facilities depend on air handling units to provide ventilation, maintain indoor air conditions, and protect downstream HVAC components from airborne particles.

Pocket filters are commonly installed in these systems because their multi-pocket structure provides a relatively large filtration surface within a limited installation space.

However, older AHUs may use filter mounting frames that do not match current standard replacement dimensions.

In this illustrative scenario, a manufacturing facility in Central Europe is planning a scheduled replacement of its existing bag filters.

The AHU remains in service, but the filter rack uses a non-standard mounting arrangement. Commonly available pocket filters do not provide an exact match for the existing frame dimensions.

Replacing the entire filter rack would require additional modification work and could increase maintenance downtime.

The facility therefore needs to evaluate a custom replacement filter that can be installed in the existing housing while meeting the specified filtration and airflow requirements.

The central engineering question:

How can a custom pocket filter be designed to fit an existing industrial AHU while maintaining appropriate filtration efficiency, airflow resistance, and structural stability?

 

2. Technical Challenge: Frame Compatibility, Pocket Design and Airflow

Three requirements shape the proposed replacement design.

Challenge 1 — Non-Standard Filter Frame Dimensions

The existing AHU requires a filter configuration with a nominal envelope of 580 × 580 × 550 mm.

Standard replacement pocket filters may use different external dimensions, frame profiles, or pocket depths.

Even small dimensional differences can affect how a filter fits into the mounting rack.

A filter that does not match the existing housing may create installation difficulties, inadequate sealing, or unwanted air bypass.

The replacement specification must therefore consider:

• External frame width and height

• Available installation depth

• Frame profile and thickness

• Mounting and retention method

• Sealing contact surfaces

• Clearance for pocket expansion

• Airflow direction

Design requirement: Develop a filter frame configuration compatible with the existing AHU rack, based on confirmed installation measurements.

Challenge 2 — Pocket Count and Filtration Area

Pocket filters use multiple fabric bags to provide a larger effective filtration area than many flat filter arrangements.

However, the number of pockets alone does not determine filter performance.

Pocket width, length, spacing, media structure, and airflow distribution all influence the effective filtration area and resistance.

Increasing the number of pockets without adequate spacing can restrict airflow between adjacent bags.

Likewise, longer pockets may not be suitable when the AHU has limited installation depth or insufficient clearance downstream.

For this scenario, a six-pocket design is proposed as an initial configuration for evaluation.

Design requirement: Review pocket count, pocket depth, and media arrangement to provide suitable airflow distribution within the available installation space.

Challenge 3 — Filtration Efficiency and Pressure Drop

The example AHU requires a design airflow of 2,000 m³/h per filter.

The illustrative initial pressure-drop target is no more than 130 Pa at this airflow.

Selecting a pocket filter with higher particle removal efficiency can affect airflow resistance, depending on media properties and filter construction.

The replacement must balance filtration requirements with the available fan capacity and system operating conditions.

For this scenario, an F7 reference grade under the historical EN 779 classification is used to describe the intended filtration level.

Any final purchasing specification should identify the required performance classification under the applicable standard, such as ISO 16890.

Design requirement: Select a suitable filter media and pocket configuration that meets the agreed efficiency requirement and initial resistance limit under verified operating conditions.

 

3. The Proposed Clean-Link Custom Pocket Filter Solution

For this scenario, a custom galvanized-steel-frame pocket filter with fiberglass filtration media provides a design approach worth evaluating.

Rather than modifying the existing AHU housing to accommodate a standard filter, the proposed replacement is specified around the available installation dimensions and operating requirements.

A. Custom Galvanized Steel Frame

The proposed filter uses a galvanized steel frame designed to match the required installation envelope.

Frame construction would be reviewed according to the filter rack dimensions, mounting method, and structural requirements.

The design review includes:

• External frame dimensions

• Frame depth and profile

• Corner construction

• Pocket attachment method

• Installation clearances

• Frame-to-housing sealing arrangement

Final dimensional tolerances would be confirmed through an approved technical drawing.

For similar non-standard filter requirements, explore Clean-Link's custom air filter manufacturing capabilities, including custom frame dimensions, media options, and project-specific configurations.

B. Six-Pocket Filtration Configuration

A six-pocket configuration is proposed for initial engineering evaluation.

Pocket geometry would be reviewed according to the available depth, design airflow, and filtration media characteristics.

Important design factors include:

• Number of pockets

• Pocket length and width

• Spacing between pockets

• Effective filtration area

• Airflow distribution

• Pocket stability under operating airflow

The arrangement should allow the pockets to expand appropriately without excessive contact with adjacent pockets or surrounding equipment.

A six-pocket configuration is not automatically the most efficient solution. The final arrangement must be evaluated against the required airflow and pressure-drop limits.

C. Fiberglass Filter Media Selection

Fiberglass pocket filter media is proposed for the selected filtration stage.

Clean-Link's existing Fiberglass Pocket/Bag Filter range provides published configurations that can be used as technical references during specification review.

For example, one catalog configuration includes:

Parameter	Clean-Link Reference Product
Filter Type	Fiberglass Pocket/Bag Filter
Reference Dimensions	595 × 595 × 550 mm
Reference Pocket Count	6
Reference Grade	F7 (EN 779)
Rated Airflow	2,000 m³/h
Published Initial Resistance	100 Pa
Published Filtration Area	3.86 m²

These figures describe a published standard product configuration, not the proposed custom 580 × 580 × 550 mm filter. They are reference data only and cannot establish the performance of the modified design.

The proposed custom configuration would require separate review of the media area, frame geometry, and airflow resistance.

D. Frame Sealing and Installation

For an existing AHU, the sealing interface between the filter frame and rack is an important part of the overall installation.

The replacement design should consider the mounting arrangement, frame contact surface, and any required gasket or sealing material.

A correctly manufactured filter still requires proper installation.

The proposed design must therefore be evaluated for both frame compatibility and suitable sealing against the existing housing.

 

4. Manufacturing and Quality Verification

Custom pocket filter manufacturing requires attention to dimensional accuracy, pocket construction, media attachment, and product-specific performance requirements.

For this type of retrofit, the manufacturing plan would focus on the characteristics most relevant to the existing AHU.

Learn more about Clean-Link's air filter manufacturing facilities, production processes, and quality-control capabilities.

Frame and Dimension Verification

The finished filter frame would be checked against the approved drawing.

Inspection should include external dimensions, frame construction, and relevant mounting features.

Pocket Assembly Inspection

The pocket configuration would be evaluated for consistent construction and appropriate attachment to the frame.

Relevant inspection points may include:

• Pocket quantity and dimensions

• Pocket seam or joining quality

• Media-to-frame attachment

• Visible media damage

• Pocket alignment and spacing

• Frame rigidity

• Product identification and labeling

Airflow and Pressure Drop Verification

The custom filter should be evaluated under the agreed test conditions.

For this example, the design target is an initial pressure drop of no more than 130 Pa at 2,000 m³/h.

Whether the proposed design achieves this target must be determined through applicable testing.

Filtration Efficiency Verification

The selected media and finished filter should meet the agreed efficiency requirements.

Since EN 779 is no longer the current general ventilation filter classification standard, buyers should confirm the appropriate ISO 16890 requirements and any required documentation before placing an order.

Installation and Commissioning Checks

After installation, the facility should verify that the replacement filters fit correctly and operate within the AHU's required conditions.

Relevant checks may include:

• Frame fit and retention

• Sealing condition

• Pocket expansion and clearance

• Operating airflow

• Filter differential pressure

• Visible deformation or interference

The commissioning procedure should follow the relevant equipment and system requirements.

 

5. Expected Project Value and Acceptance Criteria

The objective of the proposed retrofit approach is to establish a suitable pocket filter replacement specification while preserving the existing AHU housing.

Several practical benefits may be achievable if the replacement design meets the agreed requirements.

Reduced Need for AHU Modification

A custom frame designed around the existing rack may help avoid unnecessary changes to the filter housing.

This could simplify maintenance planning and reduce the scope of retrofit work.

Improved Replacement Compatibility

A confirmed frame specification and pocket configuration can help reduce uncertainty when ordering replacement filters for older equipment.

Performance-Based Filter Selection

Defining filtration efficiency, design airflow, and initial pressure-drop requirements creates a clearer basis for comparing replacement options.

Consistent Repeat Purchasing

Once a custom configuration is approved, its drawings and agreed specifications can support subsequent manufacturing and procurement.

This can be particularly useful for facilities operating multiple AHUs with similar replacement requirements.

What Would Define Project Success?

For this illustrative scenario, acceptance would require confirmation that:

1. The finished frame meets the approved dimensional specifications.

2. The pocket configuration fits within the available installation depth.

3. The filter satisfies the agreed filtration efficiency requirements.

4. Initial pressure drop meets the specified limit at the design airflow.

5. The filter is installed securely without unacceptable bypass or pocket interference.

6. The approved specifications are documented for future replacement orders.

These are proposed acceptance criteria, not measured results from a completed customer installation.

 

6. What This Case Demonstrates

Replacing pocket filters in an older industrial AHU requires more than finding a product with similar nominal dimensions.

Filter frame compatibility, pocket configuration, filtration efficiency, and operating pressure drop must be evaluated together.

A standard filter with the correct efficiency grade may still be unsuitable if its frame does not match the rack or its pockets cannot expand within the available space.

Similarly, changing pocket count or media grade without evaluating resistance may affect system airflow.

For industrial HVAC buyers, a complete custom pocket filter specification should therefore address:

• Existing AHU housing dimensions

• Frame design and sealing requirements

• Pocket count and pocket depth

• Filter media and efficiency classification

• Design airflow

• Initial and final pressure-drop requirements

• Manufacturing and inspection criteria

• Future replacement and purchasing needs

These considerations can help equipment operators, HVAC contractors, and industrial procurement teams assess custom pocket filter replacement options more systematically.

 

7. Custom Pocket & Bag Filter Manufacturing with Clean-Link

Clean-Link manufactures pocket and bag filters for commercial HVAC systems, industrial ventilation, and air handling units.

Our product range includes synthetic fiber pocket filters, fiberglass pocket filters, hot melt pocket filters, and selected specialized configurations.

Depending on product type and manufacturing feasibility, customization options may include:

• Non-standard frame dimensions

• Custom pocket counts and depths

• Synthetic or fiberglass filter media

• Specified filtration efficiency requirements

• Galvanized steel or aluminum frames

• Project-specific airflow requirements

• OEM labeling and packaging

• Bulk manufacturing and repeat supply

 

Related Products and Manufacturing Capabilities

Pocket & Bag Filters

Explore Clean-Link's range of industrial and commercial HVAC pocket filters.

Air Filter Manufacturing Facilities

Explore Clean-Link's production equipment, manufacturing processes, and quality-control capabilities.

 

Request a Custom Pocket Filter Quote

Need replacement pocket filters for an existing industrial AHU?

Whether you are replacing discontinued filter sizes, adapting to a non-standard mounting rack, or sourcing pocket filters for repeat maintenance orders, Clean-Link can review your requirements and discuss suitable manufacturing options.

To request a quotation, please provide:

• Filter dimensions (width × height × depth)

• Existing AHU or filter housing details

• Required filtration efficiency

• Rated airflow and pressure-drop requirements

• Preferred pocket count and depth, if known

• Frame material and sealing requirements

• Estimated order quantity

• Existing filter photographs, drawings, or product labels

• Delivery destination

Request a Custom Pocket Filter Quote →

Clean-Link — Custom Air Filtration Manufacturing for HVAC, Industrial and OEM Applications.