How to calculate the air volume and face velocity of ULPA filter?

Air volume, face wind speed, and effective area—these three numbers often need to be converted to each other in projects. The formula itself is not complicated, but when using it, it is easy to have problems with 'which area to use' and 'how to unify the units'.

First remember the basic relationship: air volume (m³/h) = surface wind speed (m/s) × effective area (m²) × 3600 . In turn, face wind speed = air volume ÷ effective area ÷ 3600. The 3600 in the formula is the conversion factor between hours and seconds.

Which one to use for effective area?

The 'area' in the formula refers to the effective ventilation area through which the airflow actually passes , not the length times the width of the outer frame.

The outer frame area includes the portion occupied by the frame, sealing edges and internal support structures, which are not ventilated. The effective ventilation area must deduct these occupations and is usually smaller than the outer frame area. Products with different structures have different deduction ratios.

If you estimate the face wind speed based on the outer frame area when selecting a model, the result will be too high , and you may misjudge whether the actual wind speed meets the standard. The definition of effective area will generally be indicated in the product information, which shall be subject to the technical data of the corresponding model. For specific differences, please refer to the relevant instructions for effective area calculation.

HEPA high efficiency filter and The effective area of ​​ULPA ultra-high efficiency filter is defined in the same way and the verification method is the same.

What range should the face wind speed take?

The reasonable range of face velocity depends on the type of filter and its installation location.

At the end of the clean room HEPA/ULPA , the common surface wind speed is between 0.35 and 0.5 meters per second. This range is to take into account both filtration efficiency and filter element life - if the wind speed is too high, it will increase resistance and accelerate dust accumulation, while if the wind speed is too low, the air supply volume will be insufficient.

The wind speed in the working area of ​​a unidirectional flow clean room is usually between 0.3 and 0.6 meters per second, and the specific value is determined according to the project design requirements.

For the filter on the FFU , the face wind speed is generally around 0.35 to 0.45 meters per second, depending on the unit design and target wind speed. Supporting the FFU fan filter unit , the wind speed on the filter element surface and the wind speed distribution on the outlet surface of the unit should be looked at together. When using

Device-side applications such as The EFU equipment-end fan filter unit has a compact internal space, and the surface wind speed may be unevenly distributed due to structural obstruction. When measuring, multiple points must be averaged.

The rated air volume of ULPA is usually lower than that of HEPA of the same size. Under the same dimensions, the air volume processed by ULPA may be only 50% to 80% of that of HEPA, because the filter material is denser and the resistance is higher. If you directly apply the original HEPA air volume data when selecting a model, it may exceed the applicable scope of ULPA.

Don’t get confused with unit conversions

The most error-prone place in on-site measurement is the unit.

Length is expressed in millimeters, and area must be calculated in meters. 600 mm is 0.6 meters, not 600.

Air volume unit. The difference between cubic meters per hour (m³/h) and cubic meters per second (m³/s) is 3600 times. When using formulas, make sure the input and output are consistent.

Unit of surface wind speed. Usually meters per second (m/s) are used. If the anemometer reading is meters per minute, divide it by 60 first.

A calculation example. The effective area is 0.5 m², the average surface wind speed is 0.4 m/s, and the air volume = 0.4 × 0.5 × 3600 = 720 m³/h. Conversely, the air volume is 720 m³/h, the area is 0.5 m², and the surface wind speed = 720 ÷ 0.5 ÷ 3600 = 0.4 m/s.

This is the arithmetic result in the example, not the product promise value. In actual projects, this number needs to be checked with other measurement data.

How to measure face wind speed

Face wind speed is an average, not a single point reading.

Measuring point location. When directly measuring the wind speed on the filter surface, the distance between the measuring point and the air outlet surface is usually 150 to 300 mm. The test surface is divided into equal-area grids, and a measuring point is set in the center of each grid.

Measure time. Read the average value after stabilization at each measuring point to avoid misjudgments caused by instantaneous fluctuations.

Multiple averages. When the wind speed distribution on the air outlet surface is uneven, a single point reading cannot represent the entire unit. If the wind speed deviation at different positions of the same filter exceeds 20%, you need to check whether the filter paper is damaged or whether the installation is tilted. If the surface air velocity deviation of multiple filters in the same room exceeds 30%, it may be due to uneven airflow distribution in the static pressure box.

Record conditions. The fan gear position, damper opening and operating mode during measurement should be recorded together. Readings under varying operating conditions are not comparable.

How are air volume and ventilation rate related?

After the air volume of a single filter is calculated, the number of air changes in the room can be further calculated.

Number of air changes = total air supply volume ÷ room volume. The total air supply volume is the sum of the air volumes of multiple filters, and the room volume is the area multiplied by the net height.

For example: a filter has an effective area of ​​0.6 m², a face wind speed of 0.45 m/s, and a single air volume = 3600 × 0.6 × 0.45 = 972 m³/h. If 4 similar filters are installed in the room, the total air supply volume will be about 3888 m³/h, the room area will be 50 m², the net height will be 2.8 m, and the air exchange rate will be about 27.8 times/hour.

This is an example, not a product promise. In actual projects, the number of air changes is subject to actual measurement, and the estimated value is only used as a reference.

Project judgment example

The original H14 filter of a certain equipment has an effective area of ​​0.8 m², a surface wind speed of about 0.45 m/s, and a single air volume of about 1296 m³/h.

Consider upgrading to U15. The effective area of ​​the U15 filter element provided by the supplier is also 0.8 m², but the resistance is higher. If the fan is not adjusted, the face wind speed may drop to about 0.35 m/s, and the air volume of a single unit is about 1008 m³/h, a decrease of about 22%.

In this case, it is necessary to check whether the air volume requirements of the target area can still be met. If the air volume is insufficient, treatment directions include: increasing the fan speed (within the allowable range), selecting a U15 structure with a larger effective area, or re-evaluating the target surface wind speed. You cannot directly select U15 based on the original HEPA air volume data, as the air volume capabilities of the two are different. involving Products of different grades of HEPA/ULPA air filters have the same air volume calculation method, but the rated value should be based on the data of the corresponding model.

Selection and Acceptance Checklist

About Aili Industrial’s air volume accounting cooperation

The basis for air volume accounting is accurate product data and site conditions.

Aili Industrial's manufacturing system covers filters, motors, fans, control systems and complete machine assembly, and can provide HEPA high efficiency filter、ULPA ultra-high efficiency filter and supporting air supply equipment. The effective area and test conditions will be noted in the product information. The filter and FFU fan filter unit、When the EFU equipment-end fan filter unit is matched, the air volume, surface air speed and fan working point can be calculated together during the planning stage.

When enquiring, provide the design air volume, target surface wind speed, installation space and operating conditions to help confirm appropriate specifications. Specific parameters are subject to the technical data of the corresponding model.