AHU bag filter replacement procurement: How to measure the size and installation interface of the old part?

1. How Should You Measure the Existing Filter?

Measure the Outer Frame, Not the Pocket Opening

Shut down the unit and remove the existing filter according to site maintenance procedures. Use a steel ruler to measure the overall outer-frame width (W) and height (H), recording all dimensions in millimetres. If the frame is distorted, take measurements at several points and check them against the mounting channel and original drawings.

Do not use nominal dimensions such as “600 × 600” directly for ordering. Some common pocket filters have actual outer-frame dimensions of 592 × 592 mm.

Record Pocket Length and Frame Depth Separately

Extend the pockets naturally without stretching them, and measure from the pocket opening to the closed end. Separately measure the depth of the rigid front frame in the airflow direction.

When requesting a quotation, clarify whether the stated pocket length includes the frame to avoid different interpretations of the overall depth.

Record the following parameters:

2. Which Mounting Interfaces Should Be Checked?

Guide Rails and Retaining Clips

For side-access installations, measure the guide-channel width, insertion clearance and clear maintenance-door opening.

For clamped installations, check the retaining-clip positions, clamping travel and frame edge against which the clamps bear. Different holding frames may accommodate different header-frame depths, so width and height alone do not establish compatibility.

Sealing Surface and Downstream Clearance

Confirm whether the gasket is fitted to the filter or to the unit’s holding frame, and check that it makes continuous contact around the perimeter. Poor sealing allows air to bypass the filter media, reducing installed filtration performance.

Also check the clearance between the fully extended pockets and the coil, fan or next filter stage. Prevent pocket ends from contacting equipment and allow sufficient maintenance space to withdraw the existing filter.

3. How Can Performance Be Confirmed Once the Dimensions Match?

Procurement specifications should include the filtration class, operating airflow per filter, initial pressure drop at that airflow, and the system’s allowable change-out differential pressure.

The same number of pockets does not guarantee equivalent performance. Filter media, effective filtration area and pocket geometry also affect pressure drop.

For filters installed in parallel, estimate the airflow per filter from the total airflow and its distribution across the modules, then check the manufacturer’s airflow–pressure drop curve. Before upgrading the filtration class, verify the available fan pressure margin.

4. What Are Common Issues During Replacement?

Can the Pocket Length Be Reduced?

A shorter-pocket design can be considered when installation space is limited. However, reducing pocket length may decrease filtration area, so airflow capacity, pressure drop and dust-holding capacity must be reassessed.

Can the Frame Material Be Changed If the Outer Dimensions Are the Same?

Check the new frame’s depth, rigidity, retaining-clip compatibility and sealing arrangement. Matching overall dimensions alone is not sufficient.

5. What Information Should Be Prepared Before Procurement?

Provide the existing filter’s nameplate, front and side photographs, close-up photographs of the mounting interface, dimensional records, operating airflow, filtration class and required quantity.

For a first-time replacement or bulk order, confirm the dimensional drawing before purchase and arrange a sample trial fit where necessary.

6. BoYang Pocket Filter Replacement Solutions

When requesting a quotation from BoYang, submit the information above and specify your requirements for pocket quantity, pocket length, filter media and frame configuration.

Confirming the mounting interface and performance parameters before ordering helps reduce installation problems and insufficient operating airflow.