Recent Advances in Particulate Air Filter Testing: Quality Assurance Framework
June 28, 2011
In the last ten years, there has been increased interest in health related aspects of indoor air quality. The application of general ventilation air cleaners and filters has subtly changed from the protection of equipment and prevention of soiling, to protection of people from particulate matter (Ensor et. al, 1994). Providing particle size fractional efficiency filter performance data of known quality is an important developing requirement. ASHRAE Draft Standard 52.2P, currently being subjected to public review, contains a number of provisions for quality assurance (OA) (ASHRAE, 1996). The provisions for OA in the draft standard grew out of the underlying development of the method. Before conducting the research for ASHRAE, Research Triangle Institute (RTI) conducted a number of studies for US EPA where a closely prescribed system of QA was required (Hanley and Smith, 1993), (Hanley et al, 1994). In the ASHRAE research project RP761, RTI applied many of these principles in performing research to support the writing of a new standard. (Hanley, 1992), (Hanley et al, 1995).
Data Quality Framework
In Figure 1, the data quality framework is shown for developing the filter tests. This process will be outlined through the various steps.
Establish Test Objectives
A successful filter test can be defined as one that provides an accurate measurement under meaningful test conditions. To maximize the likelihood of success, the measurement of filtration efficiency requires careful consideration of a number of interrelated factors. Perhaps the most important factor is a clear statement of the objective of the test. This includes specifying:
- Use of the results
- Particle diameter size range that is to be covered
- Upper (and lower) limits on measured efficiency (e.g., 99.99)
- Type and size ofthe filter to be tested
- Face velocity
- System temperature, pressure, and volumetric flow rate
- Appropriate challenge aerosol
- Required accuracy of the efficiency
- Required accuracy of the particle diameters
The accuracy of the results are often difficult to assess given the experimental difficulties of particle research. Calibration aerosols can be used to verify instrumentation performance. The repeatability of the measurements can be specified and usually reported as the coefficient of variation. Keep in mind that particle size dependent efficiency is dependent on a number of interrelated parameters with the likelihood that the error will be amplified (Beers, 1957).
These factors will determine the physical design of the test apparatus, the selection of aerosol instrumentation, the means of aerosol generation, the aerosol sampling strategy, and the number of replicate tests required. These objectives should be established with input from both the “customer” (i.e., whoever requested the tests and will use the results) and the person in charge of conducting the tests. Once defined, the objectives must be reviewed throughout the program to ensure that testing will meet the objectives as illustrated below.
Working within this quality assurance oriented framework helps to ensure that accurate filtration efficiency measurements are obtained and that the test conditions are appropriate to the actual use condition of the filter.
Build or Prepare Test Equipment
- Test Duct Selection-system Optimization
A test system is nominally very simple, with a duct, fan and test section. However to respond to the test requirements, details need to be carefully examined. Developing a test method to provide the required data at the lowest cost may not be straight forward. Some considerations are:
- Test duct/test section
- Fan performance and size
- Flow measurement and control
- Pressure measurement
- System gas composition, e.g. filtered, conditioned
- Integration of needs into one system
- Relationship to other standard test methods, e.g. ASHRAE 52.11992
- Simultaneous vs. Sequential Measurements
- Sequential sampling using one aerosol analyzer to sequentially sample upstream and downstream of the filter
- Simultaneous sampling using two aerosol analyzers with one positioned upstream and one downstream of the filter
- Selection of Aerosol Measuring Instruments
Selection of the appropriate aerosol analyzer is generally determined by the particle size range (Table 1) that must be covered, and the nature of challenge aerosol. In most instances, it is critically important that the aerosol instrument be able to accurately measure the particle size of the challenge aerosol. Most instruments will correctly measure aerosol particles that are spherical, unit density, and non absorbing (such as polystryne latex, PSL). Instruments based on light scattering (e.g., white light and laser based optical particle counters), however, have reduced sizing accuracy for non-spherical particles and for absorbing particles. Their output must be evaluated cautiously when used to sample aerosols such as ambient aerosol, resuspended dusts (e.g., AC Fine), and highly non-spherical particle shapes such as flaked material. The sizing errors associated with such aerosols depend upon their departure from spherical and non-absorbing.Table 1. Overview of the trade-off between aerosol generation and instrument selection for filter testing.
- Selection of Aerosol Material
In addition to being compatible with accurate measurements by the aerosol analyzer, the aerosol must be chosen with several other considerations in mind. The aerosol substance must be compatible with the filter medium being tested. Some filters are made of materials that can be damaged by oil-type aerosols. For example, some types of membrane filter media can be deformed by DOP, thereby altering the pore size, and hence, the filtration efficiency. Filters composed of electrostatically charged fibers may lose the effectiveness of the charge more quickly with oil aerosols that coat the fibers than for solid aerosol particles. For low efficiency filters, solid particles can have significantly higher levels of penetration due to bounce and reentrainment, whereas liquid particles will “stick” upon impact with the filter fibers.Conductive aerosols, such as carbon black, can quickly short out electrostatic precipitators. The aerosol material must also be compatible with aerosolization as well as safety and health issues. Thus, the selected aerosol must be compatible with:
- Requirements of aerosol analyzer for accurate size measurement
- Filter medium
- Reasonable simulation of an actual use condition
- Effective aerosolization requirements
- Safety and health concerns
- Quality Assurance (OA) Testing of the Aerosol Analyzer
Whatever aerosol instrument is chosen, its calibration should be verified prior to use. If a large number of tests are to be performed, routine calibration checks should be performed throughout the test series. The QA checks should include:
- Checking the sizing accuracy of the instrument. This is often performed with a monodisperse PSL or Vibrating Orifice Aerosol Generator generated aerosol
- Checking the zero count of the instrument. This is often performed by placing a HEPA filter on the instrument’s sample inlet
- If two aerosol instruments are used to allow simultaneous sampling upstream and downstream, the correlation of the two instruments must be measured frequently.
- When possible, the coincidence level of the instrument for the challenge aerosol should be measured. Aerosol instruments often have an upper limit of concentration that can be measured without significant error due to “coincidence,” i.e., the presence of more than 1 particle in the instrument’s view volume at a time. The coincidence level can be determined experimentally by controlled dilution tests, or by conducting the filter tests over a range of challenge concentrations. At a minimum, the aerosol concentration should be kept well below the manufacturer’s specification (e.g., 1 /10 of the specified coincidence level). Manufacturer specifications are often based on a 10% coincidence error, but this amount of error is unacceptable for some types of filter tests.
- Checking of the challenge aerosol concentration to ensure that the concentration is sufficient to provide statistically valid measurements in each size range.
Perform Control Testing of the Test Rig
The purpose of conducting control tests is to demonstrate that the test rig and sampling procedures are capable of providing reliable fractional penetration measurements on filters. Such tests are called for in a number of standardized test methods and are an important part of any quantitative test method. This approach was described by IES(1992) and Hanley (1992), and detailed test methods are found in the draft ASHRAE 52.2P standard (ASHRAE, 1996).
Aerosol-related qualification tests include:
- 0% penetration test
- 100% penetration test
- Representativeness of the upstream sample
- Representativeness of the downstream samples
- Sample Line losses
- Penetration Tests
Unlike many other test parameters, there are no filter “standards” that can be purchased and tested to quantify the accuracy of the fractional efficiency measurement. However, the ability to measure 0 percent penetration can be evaluated by using a HEPA or ULPA filter, and the ability to measure 100 percent penetration can be evaluated by performing the fractional penetration tests with no filter in the device section.The purpose of the 0% penetration test is to ensure that instrument response times, sample line lag, or other factors do not limit the measurement of 0% penetration . For example, when sampling sequentially between upstream and downstream, it is necessary to allow a certain amount of time (“purge time”) after switching from one sample to the other (e.g., from upstream to downstream) to allow the new sample to be transported to the aerosol instrument without cross contamination from the prior sample. The 0% penetration test will demonstrate whether the purge interval is sufficient. (It should be noted that, by tailoring the test conditions to high efficiency products, these efficiencies can be quantified.)The 100% penetration test is a stringent test of the adequacy of the overall duct, sampling, measurement, and aerosol generation system. The test is performed as a normal penetration test, except that no filter is used. A perfect system would yield a measured penetration of 1 at all particle sizes. Deviations from 1 can occur due to particle losses in the duct, differences in the degree of aerosol uniformity (i.e., mixing) at the upstream and downstream probes, and differences in particle transport efficiency in the upstream and downstream sample lines.
- Representativeness of the Upstream Sample
In most filter tests, the upstream sample is acquired using a single center-of-duct probe, located a short distance upstream of the filter. Depending upon the uniformity (i.e., well-mixed) of the upstream aerosol, this may or may not be a representative sample. If it is not representative, then the upstream aerosol concentrations will be in error and can lead to an inaccurate efficiency determination.In order for the center-of-duct probe to acquire a representative sample, the upstream aerosol must be well-mixed. When dealing with particles less than a few micrometers diameter in relatively small diameter ducts with most of the test air being provided by the aerosol generator, well mixed conditions are not too difficult to achieve. On larger ASH RAE-type test ducts (24″ x 24″ cross section operating at up to 3,000 cfm), provisions must be taken to ensure mixing of the injected challenge aerosol stream with the test airflow. This is often accomplished by mixing baffles between the aerosol injection point and the test filter. Also, for particle diameters greater than a few micrometers, gravitational settling and inertial effects can lead to a nonuniform distribution across the duct and should be considered when designing the test duct. For example, if relatively low air velocities are involved, a vertically-oriented duct may be needed to prevent gravitational settling of larger-sized aerosol particles.The uniformity of the challenge aerosol concentration across the duct can be determined by measuring the concentration over, for example, an equal area nine-point sample grid immediately upstream of the test filter.
- Representativeness of the Downstream Sample
The main concern with the downstream sample is to ensure that all aerosol that penetrates the air cleaner (media or frame) is detected by the downstream sampler; i.e., that the downstream sample is representative. For example, when testing a high-efficiency bag filter, we want to know that the downstream probe will detect a leak in a corner of one of the bags.One means of assessing the degree to which the downstream conditions are well-mixed begins by installing a high efficiency filter in the test rig. Then an aerosol is injected immediately downstream of the filter at preselected injection points located around the perimeter of the test duct and one at the center of the duct. In this test, the point of aerosol injection is traversed and the downstream sampling probe remains stationary in its normal center-of-duct sampling location. The injection points should include points close to the test duct walls. This is necessary because leakage associated with the frames of air cleaners, and bypass flow in electronic air cleaners, occurs in the close-to-the-wall areas. If the downstream duct is well-mixed, the downstream concentration will be independent of the injection point.
- Sample line losses
One of the more important factors to consider when designing a test apparatus for aerosol filtration testing is minimizing aerosol transport losses through the test rig I s ducting and sample lines. If particle losses are high, it will be difficult to achieve a sufficient concentration of large particles for an accurate test, and the differences in the upstream and downstream measurements will, for many low efficiency filters, be dominated by the test duct and sample line particle losses, rather than by collection by the air cleaning device being tested. This was one of the major tasks in the research supporting the new standard (Hanley, Smith and Censor, 1995).The design of the sample lines leading from the test duct to the instrument must be considered carefully in order to avoid excessive particle loss. Particle losses associated with the sample lines include inlet losses due to non-isokinetic sampling; losses to the walls of the sample, due to diffusion; electrostatic charging, and gravitational settling; and losses in bends, resulting from centrifugal force and eddies.Losses at the sample inlet can be minimized by sampling isokinetically. Electrostatic effects are avoided by passing the aerosol through an aerosol neutralizer, and using conductive and grounded ducting and sample lines. When possible, bends should be avoided, and when needed, they should have a gradual curvature. Gravitational settling can be reduced by minimizing the effective horizontal length of the sample lines.The degree of aerosol loss associated with settling, inertial losses, and diffusion can be estimated by the use of various equations (Fissan and Schwientek, 1987) (Hinds, 1982). The equations fall into two categories: those applicable to laminar flow and those applicable to turbulent flows. Laminar flow is generally defined as flow having a Reynolds Number (Re) less than about 2,300. At higher Re values, the flow is generally defined as turbulent. While each sampling system is different, it is often beneficial to have the Reynolds number of the flow be in the high laminar range.To reduce the effect of sample line losses on the computed filtration efficiency, it is often beneficial to design the upstream and downstream sample lines to be identical. In this way, the fraction of particles lost in each line will be approximately the same, and thereby “cancel” each other out.
Can Objectives Be Met?
In this step, the objectives and the test data from the rig are compared, and judgments are made concerning the development of the test rig. As an example, during the RP-761 contract, it was clear that the use of ambient aerosol could not provide sufficient particles over the desired particle range (0.3 to 10 ?m) to provide adequate measurements of efficiency. Therefore, the development of a method to generate artificial aerosol was pursued (returning to the second box) to overcome this test limitation.
The successful measurement of the aerosol filtration efficiency of a filter requires careful planning and attention to data quality concerns. Obtaining a valid measurement is usually not as straightforward as it first appears. A valid measurement requires consideration of the overall test objective, compatibility of the challenge aerosol with the aerosol instrument and the test filter, designing a test rig and sampling system that minimize particle losses, and conducting control tests to demonstrate the ability of the test rig to provide reliable data. The approach taken in ASHRAE 52.2P (1996) to implement extensive QA will lead to a robust test method with less reliance required for extensive round robin testing. In addition, an emphasis on system quantification testing will reduce the total effort required to build and qualify a test rig. The quality assurance advances in filter testing will have a significant impact on the filtration industry. The filer test data will have known quality. Therefore this data will improve confidence in making decisions from tests.
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