How does the filter's rated air volume match the actual operating air volume?

When purchasing a filter, a rated air volume is usually marked on the sample. For example, the rated air volume of a certain model of high-efficiency filter is 1000m³/h. However, when the project is actually running, the air volume passing through the filter may be higher or lower than this value. If you only select based on the rated air volume, without considering the system resistance and fan operating point, problems such as excessive resistance, insufficient air volume, or shortened filter life may occur.

The rated air volume describes the filter's capacity under specified test or design conditions. The actual operating air volume depends on the system resistance, fan capacity and operating settings. The two need to match, you can't just look at one number. The following will start from the rated working conditions, resistance correspondence, replacement judgment, and complete machine matching.

First confirm the performance conditions corresponding to the rated air volume

The rated air volume of the filter usually corresponds to specific test conditions, including test air volume, efficiency, initial resistance and final resistance. Filters of different grades may have different resistances at the same rated air volume. H13 and H14 may have different resistances at the same air volume; ULPA is usually higher than HEPA of the same size.

When reading the sample, you should confirm:

• The efficiency level corresponding to the rated air volume;

• The initial resistance and final resistance under this air volume;

• Test methods and standard systems;

• The rated air volume is the design recommendation value or the maximum allowable value.

If you are still unclear about the caliber of the efficiency report, you can read How to read the test particle size in the filter efficiency report? , first distinguish the test conditions and reporting objects.

The actual operating air volume is determined by the system

The actual operating air volume is not determined by the filter alone, but is the result of the combined effect of filter resistance, fan operating point and system resistance. On the given fan curve, the system resistance increases and the air volume decreases; after the filter accumulates dust, the resistance increases and the air volume further decreases.

If you only select based on the rated air volume, without considering the actual resistance of the system, two situations may occur: first, the actual air volume is lower than the rated value, and the terminal wind speed is insufficient; second, the actual air volume is higher than the rated value, the wind speed facing the filter is too high, and the resistance increases. The impact of excessive wind speed on the face of the filter can be referred to What will be the impact if the wind speed facing the filter is too high?。

Use the working point to check the rated air volume and operating air volume

When selecting, you should first determine the target air volume and system resistance required by the project, and then look for the resistance and efficiency of the filter at that working point. Specific steps:

• Determine the target air volume: calculated based on protection range, air flow pattern or ventilation demand;

• Calculate system resistance: including filters, boxes, air ducts and return air channels;

• Find the fan curve: determine the available pressure at the target air volume;

• Check filter resistance: Confirm whether the resistance is acceptable at the target air volume;

• Confirm the adjustment margin: consider the increase in resistance after dust accumulation on the filter.

Please refer to the working point reading method Why should we look at the complete performance curve when selecting FFU? , the target air volume calculation can refer to How to convert face wind speed and air volume? and How to convert the clean area area into air supply demand?。

Same size does not mean they can be replaced

When replacing filters, having the same dimensions does not mean they can be replaced. Also need to check:

• Whether the efficiency level meets the target particle size requirements;

• Whether the rated air volume is close to the actual operating air volume;

• Whether the initial resistance and final resistance are within the fan capacity;

• Whether the sealing method matches the installation interface;

• Does the filter material structure and thickness affect installation?

If you only replace the parts with the same size, the air volume may decrease, the resistance may increase, or the seal may not be tight after replacement. For specific judgment, please refer to How to choose between H13, H14 and U15? and Does higher filtration efficiency necessarily lead to improved site ratings?。

Insufficient filter area changes operating point

If the filter area is insufficient, the headwind speed and resistance will increase under the same air volume. The fan may require higher pressure or speed to maintain flow. When selecting, you should check the windward size and filtration area, and do not judge based on the outer dimensions alone. For details, please refer to How does insufficient filter area affect fan selection?。

If the filtration area is sufficient but the system resistance is too high, the air duct path, return air channel and connecting components should be checked instead of simply increasing the fan speed.

Operating range under initial resistance and final resistance

From new to old filters, the resistance will gradually increase. When selecting the model, the operating points at the initial resistance and final resistance should be confirmed to ensure that the air volume can meet the requirements during the entire use cycle.

If the air volume under the final resistance is insufficient, it may be necessary to:

• Choose models with larger filter area or lower resistance;

• Increase fan capacity or adjust operating point;

• Shorten the replacement cycle;

• Optimize system resistance paths.

For the judgment method of initial resistance and final resistance, please refer to What are the initial resistance and final resistance used to judge?。

Sealing and installation leak detection cannot be omitted

After the filter is replaced, sealing and installation leak detection are equally important. A loose frame seal can cause bypass leaks, allowing unfiltered air to enter the clean side. At this time, even if the rated air volume and efficiency are qualified, the expected results will not be achieved on site. For relevant inspections, please refer to What should be checked when cleaning equipment is accepted?。

Coordination with fans and complete machines

The matching of the filter's rated air volume and the actual operating air volume ultimately depends on the fan and complete machine. Please refer to the room background air supply FFU clean air supply products , please refer to the local protection of the equipment side. EFU equipment side fan filter unit . The logic of division of labor between the two lies in The division of labor between device-side EFU and room FFU is explained in detail.

If multiple devices require partition duty and troubleshooting, they can be combined FFU group control system provides unified management. When it comes to ceiling installation, please refer to How does the FFU appearance size match the ceiling module? , confirm the replacement path and maintenance space.

Project judgment example

When replacing the filter in a certain project, a model with a lower rated air volume was selected based on the same external dimensions. After installation, the working point of the fan shifted, the actual air volume was lower than the target value, and the air speed at the workstation was insufficient. Adjustment method: Check the filter resistance and fan curve according to the target working point, replace it with a model that matches the rated air volume, and remeasure the wind speed. The wind speed at the workstation recovered after adjustment.

In another project, after replacing a high-efficiency filter, the rated air volume was qualified, but the final resistance was high, and the air volume dropped significantly after the filter was used for a period of time. Adjustment method: Check the working points under the initial resistance and final resistance, select a model with a larger filter area, and adjust the replacement cycle. After adjustment, the air volume remains stable throughout the entire use cycle.

These two situations illustrate that the rated air volume of the filter needs to match the actual operating air volume, and the selection should be based on a comprehensive judgment based on the system resistance and the fan operating point.

Key points for selection and acceptance inspection

Aili industrial equipment supporting support

The gap between the rated air volume of the filter and the actual operating air volume is the easiest pitfall when selecting and replacing the filter. Aili Industrial also handles air filters, fans, motors, control systems and complete machine assembly. It can compare the filter resistance, efficiency and fan operating points under different air volumes in the same set of data to help the project determine whether the air volume is enough after replacing the filter, whether the replacement cycle needs to be adjusted under the final resistance, and whether the sealing interface needs to be changed simultaneously. Specific filter parameters can be viewed HEPA and ULPA filters , the complete machine package can be viewed FFU clean air supply products and EFU equipment side fan filter unit。