How Is HEPA Filter Face Velocity Calculated?
The formula for calculating face velocity is:
V = Q ÷ (3600 × A)
Where:
V: Face velocity, m/s;
Q: Actual airflow through the filter, m³/h;
A: Effective face area of the filter, m².
For example, if a HEPA filter has an effective face size of 0.61 m × 0.61 m and an actual airflow of 1,000 m³/h:
V = 1000 ÷ (3600 × 0.61 × 0.61) ≈ 0.75 m/s
The effective face area provided by the manufacturer should be used whenever possible instead of the nominal dimensions that include the outer frame. A wide frame or special mounting structure may create a difference between these two areas.
How Should Filter Size and Airflow Be Matched?
When the target face velocity and design airflow are known, the required area can be calculated as:
A = Q ÷ (3600 × V)
For example, if the system design airflow is 1,200 m³/h and the target face velocity is 0.70 m/s, the required effective area is approximately:
A = 1200 ÷ (3600 × 0.70) ≈ 0.48 m²
A filter with an effective face area of at least 0.48 m² should therefore be selected. Alternatively, two smaller filters can be installed in parallel to divide the airflow.
Common HEPA filter dimensions include 305 × 305 mm, 305 × 610 mm, and 610 × 610 mm. However, filters with the same external dimensions do not necessarily have the same rated airflow. Pleat height, number of pleats, separator design, filter depth, and frame construction all affect dust-holding capacity, pressure drop, and rated airflow.
Is a Lower Face Velocity Always Better?
No. Reducing face velocity generally lowers pressure drop, reduces energy consumption, and extends filter life, but it also requires a larger filter and more installation space.
Excessively high face velocity may increase the initial pressure drop and fan energy consumption and shorten the time required for the filter to reach its final pressure drop.
The following three concepts must also be distinguished:
Filter Face Velocity: Airflow divided by the effective face area of the filter;
Media Velocity: Air velocity through the total unfolded filter media area;
Cleanroom Cross-Sectional Velocity or Supply-Air Outlet Velocity: Used to evaluate indoor airflow distribution.
These values are based on different reference areas and must not be used interchangeably. Parameters such as 0.3–0.5 m/s used in cleanroom design should not be directly treated as the allowable face velocity for every HEPA filter.
What Other Parameters Should Be Checked During Selection?
In addition to filter size and airflow, purchasers should confirm:
Filter class and applicable testing standard;
Initial pressure drop at rated airflow;
Recommended final pressure drop and replacement conditions;
Separator or separatorless construction;
Filter depth, unfolded media area, and dust-holding capacity;
Galvanized steel, aluminum, stainless steel, or plastic frame;
Gasket seal, gel seal, or knife-edge sealing structure;
Whether individual scan testing and test reports are required.
If the design airflow is close to the filter’s rated maximum, the available fan static pressure at the final pressure drop should also be verified. The fan should not be selected solely according to the initial pressure drop of a new filter.
How Should HEPA Filters Be Selected for Different Projects?
Terminal filtration in clean manufacturing facilities must balance installation height, noise, and uniform air distribution.
FFU systems should match the filter according to the airflow of each unit and the module coverage area.
Pharmaceutical facilities, hospitals, and biological laboratories should also focus on the sealing method, on-site leak testing, and safe replacement requirements.
Large-airflow AHUs can use deep-pleat separator HEPA filters or multiple filters installed in combination.
Frequently Asked Questions
Can HEPA filters with the same 610 × 610 mm dimensions be directly interchanged?
Not necessarily. Filter depth, rated airflow, initial pressure drop, efficiency class, sealing structure, and actual installation dimensions must also be compared.
If airflow increases by 20%, will filter resistance increase by only 20%?
Usually not. The relationship is not simply linear. Pressure drop may increase more significantly as airflow rises, so the manufacturer’s airflow–pressure drop curve should be used.
What information should be provided when requesting a quotation?
It is recommended to provide the filter dimensions, design airflow, filtration class, allowable initial pressure drop, frame material, sealing method, installation location, and leak-testing requirements.
Yueboyang can match HEPA filter dimensions, construction, and rated airflow for cleanroom terminals, FFUs, AHUs, and laboratory exhaust systems, helping reduce the risk of excessive pressure drop or insufficient airflow.
