How to calculate the removal efficiency of chemical filters?

The formula of removal efficiency itself is not complicated: efficiency = (inlet concentration − outlet concentration) ÷ inlet concentration × 100%.

For example, if the inlet concentration is 100 and the outlet concentration is 20, the efficiency is 80%. This calculation seems simple, but in actual use it is easy to have problems in several places: whether the data is measured under the same working conditions, whether the units are consistent, how to deal with fluctuations in the inlet concentration, and how to express when the outlet is lower than the detection limit. If these problems are not solved, the calculated efficiency values ​​may be meaningless. For specific report interpretation methods, please refer to Discussion of why chemical filter performance reports list specific target gases .

Use corresponding concentration data

Efficiency is directly related to the two concentration data, and the two data must be able to correspond.

The inlet and outlet should represent the same air flow and time conditions. If the inlet concentration is changing rapidly and the outlet sampling lags for a long time, the two sets of data may not reflect the same state. In this case, the calculated efficiency may have a large deviation, or even abnormally negative values ​​(the outlet concentration is higher than the inlet).

The units must be consistent. The concentration units of the inlet and outlet must be the same, such as mg/m³ or ppm. If one is the mass concentration and the other is the volume concentration, they need to be converted first and then calculated.

Sampling methods should be comparable. The detection methods, instrument types, and sampling flow rates used at the inlet and outlet should be as consistent as possible or cross-validated. Differences in readings between different types of instruments can affect efficiency calculations. For specific sampling methods, please refer to Discussion of sampling points for chemical filter performance verification .

When the entrance fluctuates significantly, a suitable synchronization or time matching solution is required. If the pollution source is intermittent, the sampling time should cover representative periods, and the corresponding production status and ventilation conditions should be recorded.

Describe low concentration measurement boundaries

When the inlet concentration is lower, the efficiency calculation is more sensitive to measurement error.

As the inlet approaches the detection limit, small measurement differences can significantly change the scale. For example, if the inlet concentration is very low and the outlet concentration is also very low, both values ​​have a certain measurement uncertainty, and the calculated efficiency may fluctuate greatly. In this case, the efficiency value has limited reference value.

Exports below the detection limit should be explained according to the method. Being below the detection limit does not mean that the concentration is zero, it just means that it cannot be detected by the current method. The report should indicate what the detection limit is and whether the export data is 'not detected' or 'below the detection limit'. The concentration below the detection limit cannot be written directly as absolute zero concentration.

Background control of the sampling process is also important. Sampling lines, containers, and operations may introduce trace amounts of contamination that can affect low-concentration measurement results. For specific detection methods, please refer to Discussion of what baseline testing needs to be done before trialling a chemical filter .

Separate efficiency from capacity

Removal efficiency describes the performance at a certain moment, and capacity describes how long it can last.

The removal efficiency at a certain point in time does not represent the performance over the entire life cycle. When a new filter material is first put into operation, the efficiency is usually higher. As the medium is consumed, the outlet concentration may gradually increase and the efficiency gradually decreases. It is impossible to judge how long the filter material can be used by just looking at the initial efficiency.

Long-term applicability also depends on the penetration trend, cumulative load and replacement criteria. The penetration curve can reflect the change of outlet concentration over time, and the cumulative load can estimate the media consumption. Only when these data and efficiency are used together can a complete judgment be formed. For specific life assessment methods, please refer to Discussion of how long chemical filters last .

High-efficiency filter media does not necessarily have a long service life. Some media have high initial efficiency but limited capacity and penetrate quickly; some media have slightly lower initial efficiency but large capacity and more stable long-term performance. Both dimensions should be considered when selecting.

Calculation records must be complete

The efficiency calculation results must be able to support subsequent review and comparison.

Record the gas name and test conditions. Efficiency is specific to a specific gas and cannot be separated from the gas type. The test air volume, temperature and humidity, and inlet concentration range should also be recorded.

Record sampling time and method. When it was collected, what method was used, and where the sampling points were, this information is critical when interpreting the data.

Record the calculation process. Inlet concentration, outlet concentration, unit, calculation formula, retain the original data. If subsequent review is required, the basis can be found in the records.

Document usage phases. Is it new filter material or data after running for a period of time? Different stages of use have different meanings of efficiency. involving When selecting chemical filter product series , it is recommended to provide efficiency data and corresponding working conditions to the supplier for verification.

Project judgment example

A project tested the removal efficiency of a chemical filter. The inlet concentration was 100 mg/m³ and the outlet concentration was 20 mg/m³. The calculated efficiency was 80%.

Further verification revealed that: the inlet sampling was during the equipment start-up phase, the outlet sampling was during the stable operation phase, and the working conditions at the two time points were inconsistent. Efficiency data will only be comparable after pair sampling again under stable operating conditions.

This case illustrates: The premise of efficiency calculation is the alignment of working conditions. If the conditions in the data collection stage are inconsistent, the reference value of the calculation results will be compromised. involving FFU with chemical filter or For the acceptance of MAU box-type chemical filters , it is recommended to clarify the sampling conditions and methods during the planning stage.

Key points for selection and acceptance inspection

About Aili Industrial’s testing and selection support

The calculation of removal efficiency requires that the data collection conditions are controllable and the records are complete. If this part is clarified during the project planning stage, subsequent verification will be much smoother.

Aili Industrial has independent manufacturing capabilities in filters, motors, fans, control systems and sheet metal processing, and can provide chemical filter product series and supporting air supply equipment. For projects that need to evaluate the filtration effect, we can assist in checking the detection plan and filter material configuration based on the target gas, sampling method, system conditions and acceptance requirements. When the filter is matched with MAU, FFU, EFU and other equipment, the sampling point location and maintenance method can be confirmed uniformly.

Related products:Chemical filter product series、MAU box chemical filter、FFU comes with chemical filter . Providing target gases, detection requirements and system conditions when inquiring will help confirm a suitable solution. The specific selection is subject to the technical documents and test results confirmed by the project.