NOISE AND VIBRATION CHARECTERISTICS OF CLEANROOM FAN-FILTER UNITS
Mei Q. Wu and Colin G. Gordon
BIOGRAHPY: Mei Wu has been working in the field of acoustics and noise control, since 1981. She has experience in both research and consulting. As a consultant she has provided services in architectural acoustics, mechanical system noise and vibration control and environmental impact assessment. She has worked on problems in existing buildings suffering from excessive mechanical system noise, poor acoustical separation between spaces and code violations involving interior and exterior (environmental) noise. Her R&D experience encompasses active noise cancellation, specialty silencer development, digital signal processing software development and work with finite element and boundary element modeling.
KEYWORDS: fan-filter unit, fan-powered HEPA, noise, vibration, recirculation air system, cleanroom.
ABSTRACT: Fan-filter units are an increasingly popular alternative to packaged air handlers or fan-tower recirculation air systems. They are used in mini-environments and general cleanrooms, especially in retrofit cleanrooms with limited height. This paper presents noise and vibration data measured from 9 fan-filter units. It also presents measurement methods and performance criteria for fan-filter units.
Fan-filter units (FFU’s) are used increasingly by the microelectronics industry to provide clean, recirculating air for the fabrication of integrated circuits. There may be several hundred FFU's in a large cleanroom, covering 100% of the ceiling area. The units are located on the ceiling suspension system. There is no space for external noise or vibration mitigation. The critical step to ensure that a cleanroom will meet the specified noise and vibration criteria is to select a unit with acceptable noise and vibration characteristics.
FAN-FILTER UNIT NOISE
An FFU usually consists of one direct-drive blower with a small discharge plenum and a HEPA or ULPA filter. Different FFU’s have very different noise levels due to the difference in their design, fan selection, fan operating point, and the extent of noise mitigation included (if noise mitigation is included at all). The suppliers usually provide an “A” weighted sound pressure level measured about three feet from the unit discharge, or claim an NC rating. These data provide little information in determining the resultant noise level in a cleanroom served by the units because the suppliers do not specify the testing configuration.
To determine the noise and vibration performance of the FFU’s, 9 units were tested, all of which were 2 feet x 4 feet. The noise measurements were conducted in a large office with the unit hung six feet above the floor, which was covered with sound absorbing material. The discharge sound power levels were calculated from the measured average sound intensity levels across the discharge of the FFU, or the measured average sound pressure levels.
Figure 1 shows the octave band sound power level spectra of FFU’s measured at 75 feet per minute. Figure 2 shows the sound data measured at 100 feet per minute. Figure 3 shows the octave band sound power level spectra of one representative FFU measured at different air velocities. Clearly, the sound power levels increase when the air velocity increases.
Also shown in Figures 1 and 2, and in Table 1, are the calculated maximum allowable sound power level spectra to meet NC-55 and NC-60 in a cleanroom. In the calculations it was assumed that (1) the filters cover 100% of the ceiling of a “ballroom” cleanroom, and (2) the cleanroom has sheet metal walls and a concrete floor. The figures show that most of the tested FFU’s will not meet NC-60 when used in the assumed condition. Usually a decrease in filter coverage will not significantly decrease the sound pressure levels in a cleanroom. This is because the room sound absorption, which is mostly provided by the cleanroom filters, is reduced at the same rate as the sound power input.
FAN-FILTER UNIT VIBRATION
Manufacturers use a range of methods for vibration isolation - from none to very soft stack-neoprene isolators. The authors’ experience suggests that so long as the fan/motor assembly is well balanced, vibration is not a significant issue. The dynamic forces from multiple fan sources are incoherent and thus represent a less intense vibration source than large, single units.
Vibrations were measured on 9 different units, with the FFU’s suspended on soft rubber cords. The vibration force of the FFU is calculated as the product of the mass of the unit and the average of the acceleration amplitudes measured at four corners. Figure 4 show the calculated vertical vibration force of several units.
Table 1. Calculated Maximum Allowable Sound Power Levels for a 2 ft x 4 ft FFU (dB re Watt)
Octave Band Freq. (Hz) 63 125 250 500 1000 2000 4000 8000
PWL to meet NC-55 in a Cleanroom 64 57 53 50 48 46 45 44
PWL to meet NC-60 in a Cleanroom 67 61 58 55 53 51 50 49
FIGURE 1. Sound power level spectra of fan-filter
units measured at 75 feet per minute.
FIGURE 2. Sound power level spectra of fan-filter
units measured at 100 feet per minute.
FIGURE 3. Sound power level spectra of a fan-filter
unit measured at different air flow velocities.
FIGURE 4. Vertical forces of fan-filter units
measured at 100 feet per minute.