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ISO 29042-6:2010 — Safety of Machinery — Emission of Airborne Hazardous Substances — Part 6: Test Bench Method for Separation Efficiency (Ducted Outlet)
A comprehensive guide to the test bench method for measuring separation efficiency of air cleaning systems with ducted outlet according to ISO 29042-6
Introduction to ISO 29042-6
ISO 29042-6:2010 specifies a test bench method for measuring the separation efficiency by mass of air cleaning systems with ducted outlet. Unlike Part 5 which addresses unducted systems, Part 6 covers systems where the cleaned air is discharged through a duct to the outside environment or to a central ventilation system. This configuration is common in industrial applications where contaminated air is filtered before being released to the atmosphere or to a shared exhaust system.
For ducted outlet systems, the measurement of separation efficiency is technically simpler than for unducted systems because the outlet airflow is contained in a duct, allowing direct sampling of both inlet and outlet concentrations. This enables more accurate mass balance calculations and generally results in lower measurement uncertainty compared to unducted outlet testing.
The standard applies to air cleaning systems integrated into machinery with ducted outlets, including baghouse filters, cartridge collectors, wet scrubbers, electrostatic precipitators, and cyclone separators. It covers both the initial efficiency (clean condition) and the efficiency under loaded conditions that represent actual operating conditions.
Measurement Methodology
Test Bench Configuration
ISO 29042-6 specifies a test bench with inlet and outlet ducts equipped with sampling ports for aerosol concentration measurement. The test aerosol is generated upstream of the air cleaning system, and simultaneous concentration measurements are made at the inlet and outlet. The separation efficiency is calculated from the ratio of outlet to inlet concentrations, with appropriate corrections for any air leakage or bypass flows. The ducted configuration allows precise measurement of airflow rates, temperature, and humidity at both inlet and outlet.
| Parameter | Specification | Engineering Significance |
|---|---|---|
| Inlet sampling | Upstream of air cleaning device | Represents challenge concentration |
| Outlet sampling | Downstream, after sufficient mixing length | Represents penetrated concentration |
| Mixing length | Minimum 10 duct diameters downstream | Ensures uniform concentration profile |
| Isokinetic sampling | Required for particles above 1 micron | Prevents particle size bias |
| Pressure drop | Measured across the system | Critical for fan selection and energy use |
| Loading condition | Clean and loaded states tested | Characterizes performance over life |
A common error in ducted outlet efficiency testing is inadequate mixing length between the air cleaning device outlet and the sampling point. Short mixing lengths can result in stratified flow where particle concentration varies across the duct cross-section. ISO 29042-6 specifies a minimum of 10 duct diameters of straight duct with flow straighteners to ensure uniform concentration profiles for representative sampling.
Test Aerosol and Loading Protocol
The standard specifies standardized test aerosols for efficiency determination, including ISO 12103-1 test dusts and other well-characterized particulate materials. The loading protocol defines the dust feed rate, the duration of loading, and the intervals at which efficiency measurements are made. For systems that use liquid filtration (wet scrubbers), the standard references appropriate test methods for droplet and mist collection efficiency.
Engineering Considerations for Ducted Air Cleaning Systems
The design of ducted air cleaning systems involves trade-offs between separation efficiency, pressure drop, energy consumption, and maintenance requirements. ISO 29042-6 testing provides the quantitative data needed to optimize these parameters. For fabric filters, the filtration velocity (air-to-cloth ratio) is the primary design parameter — higher velocities reduce capital cost but increase pressure drop and may reduce efficiency. For electrostatic precipitators, the specific collecting area and applied voltage determine performance.
An important consideration for ducted systems is the potential for particle re-entrainment from collected dust. ISO 29042-6 testing under loaded conditions can reveal whether re-entrainment occurs, which would manifest as decreasing efficiency with increasing load time for certain types of collectors. The standard also addresses the measurement of pressure drop, which directly affects fan energy consumption and should be minimized consistent with achieving the required efficiency.
For ducted air cleaning systems, the most cost-effective approach to achieving high separation efficiency is often a multi-stage design. A cyclone or inertial pre-separator removes the bulk of coarse particles, followed by a fabric filter or electrostatic stage for fine particle collection. ISO 29042-6 testing of each stage individually enables optimization of the overall system for both efficiency and energy consumption.
Frequently Asked Questions
Q1: How does ISO 29042-6 differ from ISO 29042-5?
The primary difference is the outlet configuration. Part 5 addresses unducted outlets where cleaned air is discharged directly into the workplace, requiring special collection of penetrating emissions. Part 6 addresses ducted outlets where outlet sampling is straightforward, but greater attention must be paid to mixing and isokinetic sampling conditions. The ducted configuration generally allows more accurate efficiency measurement.
The primary difference is the outlet configuration. Part 5 addresses unducted outlets where cleaned air is discharged directly into the workplace, requiring special collection of penetrating emissions. Part 6 addresses ducted outlets where outlet sampling is straightforward, but greater attention must be paid to mixing and isokinetic sampling conditions. The ducted configuration generally allows more accurate efficiency measurement.
Q2: What is the typical measurement uncertainty for ducted outlet testing?
With proper execution, ducted outlet efficiency measurements can achieve uncertainties of +/-2-5% for the separation efficiency value (k=2). This is significantly better than unducted testing (typically +/-5-10%) due to the more controlled sampling conditions.
With proper execution, ducted outlet efficiency measurements can achieve uncertainties of +/-2-5% for the separation efficiency value (k=2). This is significantly better than unducted testing (typically +/-5-10%) due to the more controlled sampling conditions.
Q3: Can the method be used for high-temperature or corrosive exhaust streams?
Yes, but with appropriate modifications. The standard provides guidance on using heated sampling lines, corrosion-resistant materials, and dilution probes for high-temperature or reactive exhaust streams. These modifications must not affect the particle collection characteristics of the sampling system.
Yes, but with appropriate modifications. The standard provides guidance on using heated sampling lines, corrosion-resistant materials, and dilution probes for high-temperature or reactive exhaust streams. These modifications must not affect the particle collection characteristics of the sampling system.
Q4: How does the duct diameter affect the measurement?
Duct diameter affects the velocity profile and mixing characteristics. For large ducts (over 1 m diameter), multiple traverse sampling points are required to obtain representative concentration measurements. The standard provides guidance on the number and location of sampling points based on duct dimensions and flow conditions.
Duct diameter affects the velocity profile and mixing characteristics. For large ducts (over 1 m diameter), multiple traverse sampling points are required to obtain representative concentration measurements. The standard provides guidance on the number and location of sampling points based on duct dimensions and flow conditions.
For ducted outlet systems, the integration of continuous monitoring instrumentation (opacity monitors, particle counters, or triboelectric probes) is becoming increasingly common. These instruments provide real-time indication of separation efficiency and can trigger alarms if efficiency drops below acceptable levels. ISO 29042-6 type testing provides the baseline efficiency data against which continuous monitor readings can be calibrated, enabling effective condition-based maintenance programs that alert operators to developing problems before significant emission increases occur.
The determination of fractional separation efficiency as a function of particle size requires the use of particle size measurement instruments capable of detecting particles across the relevant size range, typically from 0.1 to 100 micrometers. Optical particle counters, aerodynamic particle spectrometers, and cascade impactors each have specific size ranges and resolution characteristics that influence the accuracy of the efficiency curve. ISO 29042-6 provides guidance on instrument selection based on the expected particle size distribution of the pollutant, ensuring that the measured separation efficiency is representative of the actual performance under field conditions.
For ducted separation systems incorporating multiple stages of filtration, the overall separation efficiency is the product of the individual stage efficiencies. ISO 29042-6 type testing can be applied to each stage individually to characterize the performance of the complete system. Engineers designing multi-stage systems should consider the particle loading distribution across stages, with pre-separators removing coarse particles to extend the life of high-efficiency final filters. The standardized test data enables optimization of the stage configuration to achieve the required outlet emission concentration while minimizing total system cost and energy consumption.
