ASHRAE Standard 52.2 (“Method of Testing General Ventilation Air Cleaning Devices for Removal Efficiency by Particle Size”) will require the air filter industry and the user community to start thinking differently about filter efficiency. The Standard has been through many revisions since the first ASHRAE research project in 1992 that developed a new method of testing. This article will review the major differences between the test methods of ASHRAE 52.1 and ASHRAE 52.2 to help those in the industry begin educating those in the user community.

In The Beginning

The genesis of Standard 52.2 came from at least two sources, the first being the continued awareness that 52.1 ASHRAE Dust Spot Efficiency and ASHRAE Synthetic Dust Arrestance were confusing and less than accurate. The “efficiency” and “Arrestance” numbers in 52.1 were both stated in a percentage, and certain manufacturers advertised arrestance as efficiency. Also, the United States EPA had known for many years that the distribution of particles in smaller ranges Ð 1 to 3 micrometers - were more respirable than larger particles, and ASHRAE 52.1 filter testing did not show at what efficiency smaller particles were removed. Research Triangle Institute’s Jim Hanley received the ASHRAE research grant to explore a new test method, and he developed the protocol for the test method of what is now Standard 52.2.

Questions, Answers, Insights

Question #1: Does ASHRAE 52.2 replace ASHRAE 52.1 as the air filter test standard? Answer: No. ASHRAE 52.2 specifically directs filters with less than a specific efficiency to be tested using 52.1. In addition, 52.1 still utilizes a dust-holding capacity category not found in 52.2. John Sabelli of Intertek Testing Service is currently coordinating an ASHRAE group working to revise Standard 52.1, and your suggestions to him would be welcomed.

Question #2: From the enduser’s viewpoint, will ASHRAE 52.2 be:

  • a) More easily understood.
  • b) More confusing.
  • c) Understood as much 52.1.

Answer: B. ASHRAE 52.2 will need in-depth study by filter people and detailed explanation to endusers so that they fully understand the protocol of the testing and can interpret the result.

Question #3: The biggest difference between 52.2 and 52.1 is:

  • a) The test tunnel.
  • b) The test air.
  • c) The challenge aerosol.
  • d) Efficiency determination.
  • e) The pre-conditioning step.
  • f) The MERV.
  • g) The airflow test rate.
  • h) The end point.
  • i) The test instrumentation.
  • j) The test report.

Answer: All of the above. A clear understanding of 52.2 is a necessity, since all of the parameters above (and more) have changed the way we test filters.

The test tunnel is U-shaped to allow for shorter runs in the particle counter line, or it can be straight using two particle counters (see Standard 52.2, Section I for explanation).

The test air is conditioned air from the laboratory, as opposed to outdoor air. The lab air is building air and should conform to ASHRAE Standards 55 and 62 regarding temperature and relative humidity. In addition, all of the particles in the ambient test air are removed through HEPA filtration prior to the introduction of the challenge aerosol.

The challenge aerosol is potassium chloride as opposed to ASHRAE Synthetic Test Dust. This allows for testing of specific-sized particles several times during the test without undue contamination to the particle counter.

Efficiency determination for 52.2 is the minimum, not the average as in 52.1. This is a major adjustment, as the test shows a filter operating at its lowest level of efficiency.

The pre-conditioning step was a late addition to allow standard cotton-poly media and microfiberglass media to be tested with electrically enhanced (electret) synthetics. Since the purpose of the test is to determine the minimum efficiency of the filter, and since some electret media shows a decline after being in service, the pre-conditioning step was initiated to bring both media closer to their minimum efficiencies.

The Minimum Efficiency Reporting Value (MERV) attempts to present a single-number system of testing the efficiency of the filter. European testing standards have, for many years, reported a single number for filter efficiency, and the MERV is trying to achieve this goal. The first attempts have highlighted areas of concern, and it is clear that the graphs must be included to get an accurate interpretation of the performance of the test filter. The 52.2 standing committee will be working to resolve this and other items of 52.2 that have arisen from initial application of the standard.

This Standard specifies airflow test rates, as opposed to 52.1, which allowed the manufacturer/fabricator to determine flow rates.

Pressure drop end points are specified by this test as opposed to the manufacturer/fabricator specifying the end point.

Test instrumentation for Standard 52.2 represents the introduction of cleanroom technology (i.e., particle counting) to filters used in general ventilation. Particle counters have been used for many years, however their use was limited to cleanrooms because of the sheer numbers of particles in ambient air. With more sophisticated technology in the manufacture of counters, this roadblock to accuracy was diminished and counters have now been used in general IAQ applications for over 10 years.

The use of particle counters in 52.2 allows users to determine which particles and what removal rate they want to specify for their filter requirements. This ability to measure “particle size efficiency” is the biggest difference in the two standards.

The test report has some of the same information as 52.1. Page two of the 52.2 report will be significantly more important that 52.1 reports, in that it will show the efficiency curves in the three ranges for all five dust loading. This graph should be required as part of the test report shown to any specifier/user.



Looking Ahead

There are several new and/or potentially unfamiliar terms used in Standard 52.2 with which one should be familiar. Optical particle counter (OPC), polystyrene latex spheres (PSL), particle size efficiency (PSE), and potassium chloride (KCl) are just a few.

Finally, there are test items of 52.1 that are absent in 52.2, such as dust-holding capacity, that many believe should be part of the standard. At an ASHRAE forum entitled, “What Does the Market Require for New Filter Test Methods?” held in Dallas, other areas of interest were brought forward, including a new ASHRAE test dust (already in progress as an ASHRAE research project), a vibration test, a complete filter bank system test, a severe service test, and many others.

In summary, all of these points highlight the fact that 52.2 still has some refinements to be made, along with the thought that there may be a Standard 52.3 not too far down the road. ASHRAE Technical Committee (TC) 2.4, the committee that deals with particulate contaminants and filter test standards, meets at each ASHRAE meeting; engineers, professionals, and users are welcome and encouraged to participate in the formation of new research projects, standards, and guidelines. ES