ISO 29461-1:2021 Air Intake Filter Testing for Gas Turbines and Compressors

22/04/2026

Air intake filtration plays an important role in protecting gas turbines, compressors and other rotary machinery from airborne contamination.

Particles entering the machine can contribute to compressor fouling, erosion, reduced operating efficiency and increased maintenance requirements. At the same time, the filtration system must supply the required airflow without creating excessive pressure loss.

ISO 29461-1:2021 provides a standardized laboratory framework for evaluating the static performance of individual air intake filter elements used in these applications.

What Does ISO 29461-1 Cover?

ISO 29461-1 applies to particulate air filters used in the air intake systems of rotary machinery, including:

Stationary gas turbines
Industrial compressors
Stationary internal combustion engines
Power-generation and other industrial air intake systems

The standard primarily covers individual static, or barrier-type, filter elements. It does not normally evaluate the performance of an entire filtration system after installation in the field, unless the complete assembly meets the qualification requirements of the test arrangement.

ISO 29461-1 applies and extends established filter test methods from two other international standard series:

Lower-efficiency filters are tested according to the ISO 16890 series.
EPA and HEPA filters with an efficiency of 85% or higher at the most penetrating particle size, or MPPS, are tested according to the ISO 29463 series.

ISO 29461-1 therefore should not be considered an entirely separate filtration classification method. It connects ISO 16890 and ISO 29463 test procedures with the specific requirements of rotary machinery air intake applications.

Applicable Airflow Range

ISO 29461-1 covers filter elements operating at airflow rates from:

0.24 to 2.36 m³/s, equivalent to 850 to 8,500 m³/h

This extended airflow range is important because industrial air intake filters may operate at higher airflow rates than conventional general ventilation filters.

Testing at these airflow rates requires careful control of:

Airflow stability
Airflow measurement accuracy
Pressure drop measurement
Aerosol concentration and uniformity
Upstream and downstream sampling
Filter sealing
Duct and test rig structural stability

The selected test airflow should correspond to the intended operating condition and the requirements of the applicable test procedure.

Main Performance Parameters

The exact test programme depends on the filter efficiency, application and required report. Typical evaluations may include the following.

1. Airflow Resistance

The pressure drop across the filter is measured at specified airflow rates.

This measurement shows how much resistance the filter introduces into the air intake path. Pressure drop is particularly important in rotary machinery applications because it can affect the amount of air available to the machine and the energy required to move air through the filtration system.

Testing may include:

Initial pressure drop at the rated airflow
Resistance at multiple airflow points
Resistance development during dust loading
Final resistance at the defined test endpoint

A complete pressure-drop curve provides more useful information than a single measurement point.

2. Fractional Efficiency and ePM Performance

For lower-efficiency filters, particle efficiency is evaluated using procedures from the ISO 16890 series.

Fractional efficiency is determined across the specified particle size range by comparing upstream and downstream particle concentrations. The measurement results are then used to calculate particulate matter efficiencies such as:

ePM1
ePM2.5
ePM10

These results describe the performance of the filter for different particulate matter fractions.

Where conditioning according to ISO 16890-4 is required, the purpose is to determine the minimum fractional efficiency and evaluate the influence of electrostatic filtration mechanisms. This conditioning process should not be confused with a high-humidity or environmental endurance test.

3. MPPS Efficiency for EPA and HEPA Filters

Higher-efficiency filter elements cannot be evaluated only by their ePM performance.

For EPA and HEPA filters with an efficiency of 85% or higher at MPPS, ISO 29461-1 refers to the ISO 29463 series. The filter is evaluated at or around its most penetrating particle size, where penetration is highest and filtration efficiency is lowest.

Depending on the applicable test method and reporting requirements, the evaluation may include:

Determination of MPPS
Overall efficiency at MPPS
Overall penetration
Filter classification according to ISO 29463
Airflow resistance at the specified test condition

The particle measurement instruments and aerosol generation system must be selected according to the expected efficiency and particle size range.

4. Dust Loading and Resistance Development

Dust loading can be used to evaluate how the filter behaves as a defined quantity of standardized test dust is introduced.

Depending on the selected procedure, the test may evaluate:

Gravimetric efficiency
Mass of dust captured
Dust holding behaviour
Increase in airflow resistance
Filter performance at defined loading stages
Performance at the specified final pressure drop

Dust loading provides standardized comparative data. However, it does not reproduce every condition encountered in an actual gas turbine or compressor installation.

The composition and concentration of atmospheric particles, humidity, water droplets, salt, operating schedule, installation quality and maintenance strategy can all influence actual service performance.

For this reason, ISO 29461-1 test results alone cannot be used to calculate the exact efficiency or service lifetime of a filter in the field.

Typical Testing Process

A test programme should be defined according to the filter type and the required standard results. A typical process may include:

Recording the filter identification, dimensions and rated airflow
Inspecting the filter and installing it in the test section
Verifying the seal between the filter and the mounting frame
Measuring initial pressure drop at the specified airflow points
Performing fractional-efficiency testing according to ISO 16890
Performing MPPS efficiency testing according to ISO 29463 when applicable
Conducting conditioning or dust-loading procedures when required
Recording resistance and other performance changes at the specified stages
Completing the required calculations and classifications
Generating a traceable test report

Not every filter requires every test listed above. The applicable sequence depends on the filter efficiency, intended application and agreed test scope.

ISO 29461-1 and ISO 16890: How Are They Related?

ISO 16890 is intended for air filters used in general ventilation. Its primary result is a PM-based classification, such as ISO ePM1, ISO ePM2.5, ISO ePM10 or ISO Coarse.

ISO 29461-1 is intended specifically for air intake filters protecting rotary machinery. For lower-efficiency filters, it uses and extends ISO 16890 procedures to address the airflow range and application requirements of industrial air intake filtration.

The main differences can be summarized as follows:

Aspect	ISO 16890	ISO 29461-1
Main application	General ventilation	Rotary machinery air intake
Primary purpose	PM-based filter classification	Static performance evaluation for machinery protection
Lower-efficiency filters	Tested directly according to ISO 16890	ISO 16890 procedures are applied and extended
EPA and HEPA filters	Outside the principal ePM classification scope	ISO 29463 procedures are applied
Airflow range	900 to 5,400 m³/h for a nominal 610 × 610 mm test rig	850 to 8,500 m³/h
Main test object	Individual general ventilation filter elements	Individual static air intake filter elements
Field lifetime prediction	Not directly provided	Not directly provided

A filter intended for a gas turbine air intake system may therefore require results according to both ISO 16890 and ISO 29461-1.

For a more detailed explanation, see:
ISO 29461-1 vs. ISO 16890: What Is the Difference?

Testing Multistage Filtration Systems

Rotary machinery air intake systems commonly use multiple filtration stages, which may include:

Coarse prefilters
Fine filters
EPA filters
HEPA filters

ISO 29461-1 primarily evaluates individual filter elements rather than the complete system as installed at the operating site.

For multistage systems using fine filters, cumulative efficiency can be calculated using methods described in ISO 16890-1. However, calculated cumulative efficiency should not be treated as a substitute for evaluating installation leakage, sealing quality or the performance of the complete field-installed system.

Challenges in Air Intake Filter Testing

Accurate testing at high airflow requires more than a large fan and a particle counter.

The complete test system must maintain controlled and repeatable conditions throughout the test. Important technical considerations include:

Stable airflow over a wide operating range
Accurate flow measurement
Adequate upstream aerosol mixing
Representative upstream and downstream sampling
Suitable particle measurement instruments
Controlled switching between sampling positions
Stable and repeatable dust feeding
Accurate differential pressure measurement
Reliable sealing around different filter sizes
Traceable data acquisition and calculation
Correct test sequencing and report generation

Measurement-system verification is also essential. Background concentration, instrument zero, upstream and downstream correlation, sampling delay and dilution ratio can all influence the final efficiency result.

Choosing an ISO 29461-1 Test System

The required system configuration depends on more than the maximum airflow.

Before selecting or designing a test system, the following information should be defined:

Filter dimensions and installation method
Rated and maximum test airflow
Expected filter efficiency
Required particle size range
ISO 16890 testing requirements
ISO 29463 or EN 1822 testing requirements
Dust-loading requirements
Final pressure-drop condition
Required test reports
Future expansion requirements

A system designed only for conventional HVAC filter testing may not provide the airflow capacity, pressure capability or particle measurement range required for a complete rotary machinery air intake filter programme.

SCPUR Air Intake Filter Testing Solutions

SCPUR develops automated filter test systems for gas turbine and industrial air intake filter applications.

Our systems can be configured to integrate relevant procedures from:

ISO 29461-1
ISO 16890
ISO 29463
EN 1822
EN 779, where historical comparison is required

Depending on the application, the system can incorporate airflow and pressure control, fractional-efficiency measurement, MPPS testing, dust loading, gravimetric measurement, automatic test sequences and report generation.

The appropriate configuration is selected according to the filter size, efficiency range, airflow requirement and testing objectives.

Conclusion

ISO 29461-1:2021 provides a standardized method for evaluating the static performance of air intake filter elements used to protect gas turbines, compressors and other rotary machinery.

Its most important feature is its connection with other established filtration standards:

ISO 16890 is applied to lower-efficiency filters.
ISO 29463 is applied to EPA and HEPA filters.
ISO 29461-1 extends these procedures for rotary machinery air intake applications and airflow rates up to 8,500 m³/h.

The standard provides controlled and comparable laboratory data for efficiency, pressure drop and other relevant filter characteristics. These results support filter development, product comparison and technical selection, but they should not be interpreted as a direct prediction of field lifetime.

Looking for a test solution for gas turbine or compressor air intake filters?

Contact SCPUR to discuss your filter dimensions, airflow range, efficiency level and required test standards.