How to balance the air volume of two HEPA units connected in parallel?

When two HEPAs of the same model are used in parallel, many people assume that they will naturally share the air volume equally. In actual operation, this assumption often does not hold.

Air volume distribution depends on the resistance of each branch, not the type of filter. The length of the connecting air duct, the number of elbows, and the position of the air valve will all make the resistance of the two branches different. The air volume is larger on the side with less resistance, and smaller on the side with greater resistance. The models are the same, but the actual air volume installed may be quite different.

First check the difference in air volume distribution

It is found that the air volume of the two units is different, so check from these aspects first.

The product itself : Check the model, rated air volume and resistance data of the two filters to confirm whether they are really of the same specifications. Sometimes products with similar appearance have different internal structures and resistance curves. involving HEPA/ULPA air filters are mixed. The difference will be more obvious when products of different grades of

Connect the air duct : check the length of the two branches, the number of elbows and the opening of the air valve. Even if the design is symmetrical, construction deviations can cause differences in resistance.

Flow field inside the box : If two filters are installed in the same box, the arrangement of the air inlets may bias the airflow to one side. In this case, the source of uneven air volume lies in the box structure, not the filter itself. Supporting The system of the FFU fan filter unit , the air inlet space of the unit and the internal flow guide structure will also affect the distribution.

Pay attention to the uniformity of wind speed when adjusting

A common means to level the air volume is to adjust the branch air valve.

Make adjustments step by step, changing only one setting at a time, and record the actual measured air volume of each filter. You can't just look at whether the total air supply volume meets the standard, but also make sure that the air volume of a single unit does not exceed its applicable range - if one unit operates at an excess air volume for a long time, the filter element load and resistance will increase faster.

At the same time, pay attention to the uniformity of wind speed. Even if the air volume is leveled, if the wind speed on the outlet surface is unevenly distributed, there will still be problems with the actual coverage effect. Measuring the surface wind speed one by one can better reflect the true state than just looking at the total air volume. For device-side applications such as The EFU equipment-side fan filter unit has a compact internal space, and the air distribution is greatly affected by the structure. The operating status of the equipment needs to be taken into account when leveling.

Leave running benchmark

After the system is leveled, record this set of data: fan gear, air valve opening of each branch, air volume and pressure difference of a single filter.

This initial set of data is the basis for subsequent judgments. After running for a period of time, if the pressure difference of a certain unit rises rapidly, comparing the baseline data can tell whether the dust accumulation speed is different or the position of the air valve has changed. Without benchmark data, we can only judge based on our feelings, which is prone to misjudgment.

For systems that require regular replacement of filter elements, it is recommended to save the baseline data corresponding to the product number and installation location. When replacing the next time, after the new filter element is installed, it can be retested under the same conditions to confirm whether it has returned to its original state.

An example of leveling

Assume that two HEPA units are connected in parallel. During operation, it is found that the air volume on the left side is larger and the air volume on the right side is smaller.

First record the damper opening, pressure difference and surface wind speed of the two circuits. After confirming that the models and specifications of the two filters are consistent, gradually close the left air valve while monitoring changes in the air volume on the right side. After each adjustment, wait for the system to stabilize before reading to avoid misjudgments caused by fluctuations in readings. After adjusting until the air volume on both sides is close to the design value, record the damper opening and actual measured data.

If the deviation is still large after adjusting the air valve, you need to further check whether the branch air duct is blocked, whether there is any obstruction inside the box, or the resistance of the two filters itself is different.

The design phase can reduce subsequent hassles

Uneven air volume in parallel systems is often left over from the design or installation stage.

When designing, try to keep the resistance of each branch as close as possible, or reserve adjustment means. When installing, pay attention to the symmetry of the air duct connection to avoid one side being significantly more curved than the other. The position of the box air inlet and the flow guide structure should also consider the uniformity of air flow distribution during the design stage.

If there are different types of terminals in the system at the same time, such as FFU and high-efficiency air supply inlets, the air volume distribution will be more complicated. It is recommended to consider the resistance and adjustment method of each terminal at the design stage.

About Aili Industrial’s parallel system cooperation

Aili Industry also produces filters and clean air supply equipment, and can check the designed air volume based on the fan, filter element resistance and box structure. filter with FFU fan filter unit、The fan filter unit at the EFU equipment end is matched in the same manufacturing system, and the consistency of the filter element resistance can be checked before leaving the factory, reducing uneven air volume caused by product differences when connected in parallel.

For parallel systems, it is recommended to provide the branch layout diagram, the designed air volume and adjustment method at each end, so as to facilitate the judgment of the air volume distribution plan. Actual configuration is confirmed with project drawings and on-site measurements.