Working group aims to develop uniformity in FFU testing parameters and reporting data
By Monroe A. Britt, Clarcor Air Filtration Products, Inc., and Chair, Working Group CC036
Fan filter units (FFUs) are being used more frequently for air recirculation systems in cleanrooms and minienvironments. These units are usually easily installed and allow for control of local air cleanliness and/or air velocities within a large cleanroom or in groupings of cleanrooms, which may be used for various functions and require different filtration efficiencies (i.e., HEPA or ULPA). In certain circumstances, these units also provide the most energy-efficient means of maintaining air control and cleanliness within a given workspace. Fan filter units can also be easily added or moved if the cleanroom requires minor changes or major modifications.
An FFU is a self-contained module consisting of a small fan, a HEPA or ULPA filter, and some means of controlling the speed of the fan, thereby controlling the airflow. These units are typically designed to be installed directly in the cleanroom ceiling grid and provide a downward airflow with velocities ranging from 60 to 100 fpm. These units can also be designed to meet specific requirements for minienvironments and tool enclosures. Some FFUs allow for roomside replacement of the filter. Also, the fan’s speed control may be mounted on the top side of the unit, the roomside of the unit, or it may be remotely located.
FFUs are manufactured by several companies and performance data are provided by each company in their unique formats and contain individual operational claims. Usually, test methods used to determine the airflow, power requirements, and sound data are not stated in the manufacturer’s literature. The lack of uniformity in test methods and in the presentation of performance data has made it difficult to compare the true performance of FFUs from different manufacturers.
IEST Working Group (WG) CC036 was formed to produce a recommended practice (RP) to provide a means for cleanroom industries to identify or develop uniform test methods for significant performance parameters of FFUs and to provide a reporting data format that allows a clear comparison of the performance of FFUs provided by various companies. During the formative meetings of this new WG, members identified airflow volume, external available pressure, power requirements, sound power levels, airflow uniformity, and FFU housing vibration as the parameters that were most important for inclusion in this new RP. Various methods of testing for each of these parameters have been fully discussed in several meetings. Approximately fifty percent of attendees of these meeting have been first-time attendees and, consequently, there have been wide discussions of various testing techniques and protocols. It is believed that, at this time, the WG has received sufficient guidance and information on testing methods from the contamination control industry and is ready to formalize the procedures. This RP will recommend only the test methods for measuring the individual performance parameters and will not contain recommended performance values.
The first major parameter to be measured is the airflow volume exiting the filter surface of the FFU. The WG has chosen to measure flow volume, rather than measuring the air velocity exiting the filter surface and converting it to volume. One method discussed was the use of airflow hoods, which are used widely in the cleanroom industry, to measure the airflow exiting ceiling grid filters. The method selected, however, was the use of an airflow chamber, which is widely described in the existing standard test methods of the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE; www.ashrae.org) and the Air Movement & Control Association International (AMCA; www.amca.org). An airflow chamber consists of a fan for moving the air through an air-control damper; airflow distribution screens; an American Society of Mechanical Engineers (ASME; www.asme.org) airflow-measuring orifice; and the FFU under test. The FFU is the last stage of the test apparatus and the airflow is exhausted into the test room at atmospheric conditions. This technology is well recognized and will provide accurate data. The airflow volume at various fan speed settings can be measured using the airflow chamber. The maximum possible airflow can be determined, as well as the minimum airflow with the fan speed reduced to its lowest setting.
During the airflow volume tests, the available external pressure can also be determined for any selected FFU airflow. The static pressure at the inlet to the FFU is monitored and modulated by adjustment of the chamber damper while measuring the airflow through the ASME orifice. This test method allows the FFU user to know the amount of static pressure that is available for use in cleanroom/minienvironment pressurization, for airflow through additional duct work and through prefilters, and for the increase in pressure drop of the HEPA/ULPA filter as the filter collects contaminants.
The electrical power will also be measured during the airflow and external pressure tests. The power parameters at each test condition will be measured and reported. Important parameters include line voltage, current, watts, and power factor. These values, in conjunction with the airflow and pressure data, allow potential users to readily determine the performance capabilities and the energy merits of FFUs from different manufacturers.
The WG has agreed that sound power levels are required and should be measured and reported instead of sound pressure levels. The FFU power levels can be used in conjunction with sound data from other equipment and the physical characteristics of the cleanroom to predict the expected sound pressure levels in the work environment. Sound power levels will be measured in 1/3-octave bands, using the sound intensity method as described in ANSI S12.12-1992. The FFU will be tested in an area with minimum background noise. The airflow volume of the FFU, the external available pressure, and electrical parameters will be reported with the sound power levels.
The velocity distribution of the airflow exiting the filter surface is important in many cleanroom applications. The velocity distribution below the filter will be measured and reported. The filter surface will be divided into 6-inch by 6-inch grids and the velocities will be measured 6 inches below the surface of the filter by suitable hot-wire anemometers. Measurements will be taken at each grid point. The purpose of these measurements is to determine the true velocity distribution, not the average over several locations.
Another extremely important parameter is the degree of vibration that is transmitted from the FFU fan/motor through the housing to the FFU mounting surface. Frequently, a soft gasket is used between the typical ceiling grid and the FFU housing to serve as an air seal. This gasket can help diminish the vibration passed to the mounting grid. More discussions within the WG are required before the proper test method is finalized for this parameter. Several approaches have been suggested and it is expected that the group can reach consensus at the next meeting of WG CC036 to be held at 8:00 a.m. on November 5, 2006, at the IEST Fall Conference (November 5-9, 2006, at the Hilton Garden Inn, Hoffman Estates, Illinois). More information is available at www.iest.org.
Attendance and active participation are encouraged for all interested parties. It is anticipated that the new RP can be ready for acceptance/vote by the IEST fall meeting.
Monroe Britt is manager of research and technology for Clarcor Air Filtration Products (CLC Air). He holds a bachelor’s degree in mechanical engineering from Georgia Tech and has been associated with essentially all phases of air filtration and air filter testing for more than thirty years. CLC Air’s air filter product areas include cleanrooms, commercial and industrials markets, dust control, gas turbine intakes and residential use. Mr. Britt can be contacted at (502) 810-5742 or via e-mail at [email protected].