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
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
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.
Summary
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.
References
Fissan H., and G. Schwientek. 1987. “Sampling and Transport of Aerosols.” TSI Journal of Particle Instrumentation 2(2).
Hinds, W.C. 1982. Aerosol Technology: Properties, Behavior, and Measurement of Airborne Particles, John Wiley & Sons, Inc.
ASHRAE (1996) Proposed Standard 52.2P “Method of Testing General Ventilation Air-cleaning Devices for Removal Efficiency by Particle Size” ASHRAE Manager of Standards, 1791 Tullie Circle, NE, Atlanta, GA 30329-2305.
Ensor, D. S., Krafthefer, B. C., and T. C. Ottney (1994) “Changing Requirements for Air Filtration Test Standards” ASHRAE Journal, Vol. 36, No. 6, 52.
Hanley, J. T., D. D. Smith and D. S. Ensor (1995) “A Fractional Aerosol Filtration Efficiency Test Method for Ventilation Air Cleaners” AHSRAE Transactions V. 101 Pl. 1, 3842 (RP-671) ASHRAE 1791 Tullie Circle NE, Atlanta, GA 30329-2305.
Hanley, J. T., and D. D. Smith (1993) “Fractional Filtration Efficiency of Air Cleaners” Project Task Report; Test Rig Design and System Qualification Tests”, EPA Cooperative Agreement No. CR -817083, September.
Hanley, J. T., D. D. Smith and D. S. Ensor (1995) “A Fractional Aerosol Filtration Efficiency of In Duct Ventilation Air Cleaners.” Indoor Air Vol. 4, 169-178.
IES (1192) IES-RP-CC007.1, “Testing ULPA Filters” (Recommended Practice). Institute of Environmental Sciences, Mount Prospect, IL.
ASHRAE Standard 52.1-1992 “Gravimetric and Dust-Spot Procedures for Testing Air-Cleaning Devices Used in General Ventilation for Removing Particulate Matter.” 1992.
J. T. Hanley. “Define a Fractional Efficiency Test Method That is Compatible with Particulate Removal Air Cleaners Used in General Ventilation” (761-RP): Final Phase 1 Report. February 1992.
J. T. Hanley. “Fractional Efficiency of Air Cleaners: Interim Project Report, Test Rig Design and System Qualification Tests.” EPA Cooperative Agreement No. CR-817083. April 1993.
Yardley Beers. “Introduction to the Theory of Error.” Addison-Wesley Publishing Company, Inc. Reading, MA. 1957.