Is the Pressure Drop of the New Filter Too High?
Filters with different filtration grades, media areas, and structural designs can have different initial pressure drops even when they have the same external dimensions. For example, when a G4 panel filter is replaced with an F7 or F8 bag filter, or a standard medium-efficiency filter is replaced with a high-efficiency filter, the supply airflow may decrease if the existing fan does not have sufficient available static pressure.
The pressure drop of the new filter should be compared with the manufacturer's airflow-pressure drop curve at the actual operating airflow. It is not sufficient to compare only the rated pressure drop, because the actual system airflow may be higher than the rated airflow of the filter.
What Are the Common Problems with Filter Selection and Installation?
1. Mismatch Between Filter Size and Airflow
The face velocity of a filter can be calculated using the following formula:
V = Q ÷ (3600 × A)
Where Q is the airflow rate and A is the effective filter face area.
If the filter is undersized or the media area is insufficient, the face velocity and pressure drop will increase. Even with the same frame dimensions, the rated airflow can vary depending on the number of bags, bag depth, and number of pleats.
2. Incorrect Installation Direction
The arrow on the filter should point in the same direction as the airflow. If a bag filter is installed in the wrong direction, the filter bags may not expand properly. Incorrect installation of V-bank or HEPA filters may also affect structural support and airflow distribution.
3. Filter Bags Touching Each Other or Airflow Passages Being Blocked
Insufficient installation space, inadequate spacing between filter bags, or uneven airflow can cause the bags to touch each other. Packaging materials left on the filter, deformed protective screens, or a diffuser installed too close to the filter can also increase local pressure drop.
4. Improper Gasket Installation
A gasket that is too thick, a deformed frame, or misaligned clamping devices may prevent the filter from being fully seated in the mounting frame, reducing the effective airflow area.
Insufficient sealing can cause bypass leakage. In this case, the measured airflow may appear abnormal, while the actual filtration efficiency cannot be guaranteed.
The Problem May Not Be the Filter Itself
When replacing a filter, other system problems may occur at the same time, such as a partially closed air damper, loose fan belt, reversed fan rotation, an incorrectly restored VFD frequency, a blocked coil, or a collapsed flexible connection.
For systems with constant-static-pressure control, the static pressure setpoint may also need to be recalibrated after filter replacement.
In addition, an incorrect airflow meter range, an unsuitable measurement location, or a differential pressure gauge that has not been zeroed can lead to an incorrect diagnosis of insufficient airflow.
How Should the System Be Checked On Site?
It is recommended to perform the following checks in order:
Verify the filter model, dimensions, filtration grade, and rated airflow;
Check the airflow direction, packaging, filter bag expansion, and installation sealing;
Measure the pressure drop across the filter;
Measure the actual airflow, fan speed, and VFD frequency;
Check the pressure drop across the pre-filter, medium-efficiency filter, coil, and HEPA filter separately;
Inspect air dampers, belts, ductwork, and supply air outlets;
Compare the measurements with previous operating data and the manufacturer's performance curves.
How Can Insufficient Airflow Be Corrected?
If the initial pressure drop of the new filter is too high, consider a filter with a larger media area, deeper bags, or a low-pressure-drop design.
Where installation space permits, increasing the number of filters can reduce face velocity and pressure drop. If bag filters cannot expand properly, the number of bags, spacing, or housing depth should be adjusted.
The VFD frequency should only be increased after confirming that the fan has sufficient available static pressure. Simply increasing fan speed to compensate for an incorrect filter selection can increase energy consumption and shorten filter service life.
Frequently Asked Questions
Can a filter with the same dimensions be used as a direct replacement?
Not necessarily. Rated airflow, initial pressure drop, filtration grade, and filter media area should also be compared.
Is a higher pressure drop after installing a new filter normal?
It should be evaluated based on the manufacturer's data at the rated airflow. If the measured pressure drop is significantly higher than the specified curve, check the filter selection, installation condition, and whether the filter media has become damp.
What parameters should be provided when purchasing filters?
Provide the filter dimensions, design airflow, filtration grade, allowable initial pressure drop, housing depth, available fan static pressure, and operating environment.
Boyang Air Filter can select panel, bag, V-bank, and HEPA filters according to the actual airflow requirements of air handling units, cleanrooms, and industrial ventilation systems. By optimizing filter media area and operating pressure drop, we help reduce airflow loss after filter replacement.
