Air Filtration at High Temperatures
Air filtration at elevated temperatures presents multiple challenges in filter performance and filter integrity testing. Many standard cleanroom practices and HEPA filter test procedures do not work, and some may result in hazardous operation of equipment.
Most current high efficiency air particulate (HEPA) and ultra low particulate air (ULPA) filters are designed and constructed to accommodate ambient and near ambient temperature conditions. They can achieve levels of removal of 0.3-micron and larger particles that exceed 99.95%, and, depending on the concentration of particles in the incoming air, produce air of Class 1 (ISO Class 3), Class 10 (ISO Class 4), or Class 100 (ISO Class 5) which, in English units, are particles of 0.5 micron and larger particles per cubic foot.
However, when these filters are used to perform the same filtration function at temperatures exceeding 180°C, many factors, including temperature ramp rate, method of challenge for integrity screening, and seals need to be modified to achieve the best and safest end result. Failure to recognize these differences when utilizing filter techniques at elevated temperatures may result in, at a minimum, substandard filter performance. At worst, high temperature operation may present physical danger to the operators and processes being performed in the equipment.
The primary application of HEPA and ULPA filters at elevated temperature equipment occurs in ovens designed for use in the medical device, pharmaceutical, and microelectronics and semiconductor manufacturing operations. These ovens may be performing sterilization or depyrogenation of instruments or glassware for use in the life sciences or be used for holding, curing, and/or annealing electronic components that must be kept clean. These processes can range in temperature from 100 to 400°C and require temperature ramping rates from steady state (i.e., parts introduced at the process temperature) to as much as 15+ C°/min. Hence, high temperature use of HEPA filters can present a number of challenges to the use of filters which we most commonly associate with ambient operation in a cleanroom.
The two main standards which have and are being applied to HEPA filtered ovens are U.S. Federal Standard 209E (FS209E), which is being superceded by the International Organization for Standardization (ISO) standard 14644-1, an international cleanroom standard. As has been detailed elsewhere1ü these standards mainly address cleanroom requirements for particulate classification and compliance. Other portions of ISO 14644 address various aspects of testing (14644-2, 14644-3), design/construction/startup (14644-4), operations (14644-5), and separative enclosures (Draft Standard 14644-7).
ýSO 14644-1 was finalized in 1999 and parallels much of FS209E, which first came out in 1963. FS209E contains a number of useful definitions, descriptions, and discussions of particulates, air sampling, and the statistical analysis required when measuring a large number of sample points. ISO 14644-1 provides similar information, but in an abbreviated and totally metric format. Neither of these standards specifically discusses ovens and how to apply the standards to the measurement of particulate concentration inside equipment at elevated temperatures. As is discussed in more detail elsewhere in this paper, the changing temperature in an oven and the large range of possible temperatures for HEPA filtered ovens (to >400C) forces the user of these standards to find ways to measure particulates under much more difficult conditions than would be experienced in an ambient temperature cleanroom.
The filter can generate particles from organic binders and adhesives as well as media shedding, and any organic filter challenge compounds used to test the filter can also produce condensable material and particulates. The oven and product load can also generate particles from the various materials of construction, abrasion, heating and shifting of components with temperature, with HEPA filtration present to remove particulates from these often unavoidable sources. The particulate sampling train and parNicle counter can also be adversely affected by these potential contaminants.
As stated in ISO 14644-1, the manufacturer and customer must agree on the testing methodologies used for the as-built oven. The user must then determine what their industry requirements are to meet the appropriate processing and product specifications, and then perform installed and operating testing to verify compliance. It would be in the user’s best interest to avoid sources of particulate wherever possible, with organic challenge agents such as dioctyl phthalate (DOP) or Emery 3004 (polyalphaolefin) being the most obvious potential problem for maintaining a clean and safe oven. The FDA does allow use of ambient particulate challenge in some cases for the pharmaceutical industry, which should move toward solving this area of concern. In the electronics industry, use of DOP has long since ceased2 due to contamination concerns. For electronic components and wafers, particle settling is a much more serious concern, so while meeting the various ISO 14644-1 or FS209E particulate classification standards is important for their cleanrooms and ovens, it is the problem of particulate contaminants causing surface defects to the ever smaller feature line sizes that is the key issue.
Most HEPA/ULPA filtration systems use a common glass fiber media type, and include organic binders which are used to maintain integrity during manufacture and use. These binder materials are outgassed and oxidized when the filters are run continuously at temperatures in excess of 180 to 200°C. Some new types of high temperature filters are available which have a different media and frame bonding that perform better than the previously available filter styles. Photo 1 shows a typical silicone-sealed HEPA filter used in a clean process oven (Photo 2).
One of the undesirable effects encountered in high temperature operation of HEPA filters is particle shedding from the filter media during rapid thermal transitions. This has been documented in papers presented to the pharmaceutical community as far back as 1988.3 Transition shedding of filters can be significantly reduced using four different approaches:
> The ramping rate can be reduced to the point where thermal stress on the filter structure does not elicit excessive shedding. This relationship is shown in Figure 1.
> Filter construction can be modified to minimize the thermal effects by making the pleat structure smaller.
> Various techniques, including that described in a 1991 patent4, can be utilized to maintain the filter at a constant temperature, even though the equipment is transitioned. (See Figure 2 for diagram and patent reference.)
> Others have proposed that the entire air stream be reduced in temperature before it passes through the filter, and then reheated to the process temperature afterwards (Figure 3).
Each of these methodologies has advantages and disadvantages (Table 1). Another aspect of elevated temperature use of HEPA filters is the problem associated with particulate measurement at elevated temperatures. Since most methodologies require the use of an active particle counter, which are sensitive to the incoming gas stream temperature, measurements generally rely on some means of reducing the gas stream temperature prior to particle counter entry. This can present problems associated with potential condensation due to sample cooling within the sample lines along with possible particle settling. Figure 4 shows a typical particle counting sampling train. Another solution, not mentioned in Table 1, is to use a composite HEPA filter to allow for even higher temperatures and ramp rates.5 Figure 5 shows typical data for such a filter. The pressure drop versus flow characteristics and limited size availability of this composite filter somewhat restricts its application, however.
The final problem associated with filters used at elevated temperatures involves the use in some cases of challenge materials in the medical device and pharmaceutical industries. Typically, challenges with organic compounds such as dioctyl phthalate (DOP) and Emery 3004 (polyalphaolefin) have been used to evaluate the integrity of filters before service or after periods of service. Challenge material is hydrocarbon based and is trapped in the filter after testing. This creates a major problem when heat is applied as the challenge material decomposes creating combustible smoke and vapor. The vapor will often condense on cooler portions of the equipment.
Use of organic filter challenge agents has long since been abandoned by the electronics industry in favor of ambient particle challenges and particle settling tests. Photo 3 shows a surface particle deposition measurement system.
The pharmaceutical industry, however, still uses organic challenge agents in some locations. Note that it appears that most HEPA filter manufacturers do not recommend use of these agents. Ambient particulate challenge provides a solid measure of a HEPA filtered oven’s performance. DOP is also a suspected carcinogen and while Emery 3004 may not be as toxic, it still has the same low exposure limit for personnel. The National Fire Protection Association (NFPA) has a standard, NFPA 86, which requires ovens containing flammable organic materials to meet Class A requirements, which may not be met by standard HEPA filtered ovens. The use of these organic challenge agents therefore may also present safety and liability issues that must be addressed by the oven user.
In conclusion, high temperature air filtration presents a number of challenges to the user of HEPA filtered industrial ovens. The user must determine maximum operating temperature and the desired temperature ramping rate and then determine if an oven and filter combination exists that will accommodate those needs.
1 Grier, P., “ISO 14000: Putting the Standards to Work,” A2C2, May 2002, pp. 9-12.
2 Donovan, R.P., “The Downside of Fibrous Filters,” CleanRooms, June 2001, p.10.
3 Melgaard, H. L., “Filter Shedding and Automatic Pressure Balance in Batch Depyrogenation Ovens,” Pharmaceutical Engineering, Nov/Dec 1988.
4 U.S. Patent No. 4,988,288, 1/29/91, Hans L. Melgaard, Despatch Industries, Inc., Material Heating Oven.
5 Melgaard, H.L., Kiser, B.L., “Redefining Process Time Limitations in Batch Depyrogenation Ovens,” Pharmaceutical Processing, Jan. 2001, pp. 68-70.