HIGH TEMPERATURE HEPA FILTERS FAQs
HEPA filters, when exposed to elevated temperatures, present multiple challenges for filter performance and filter integrity testing.
01/23/2013 // Riverdale, NJ, USA // Camfil Farr USA // Sean O'Reilly
HEPA filters, when exposed to elevated temperatures, present multiple challenges for filter performance and filter integrity testing. It is one of the few areas of air filtration where there are simply no black or white answers.
To summarize Camfil Farr’s experience on this subject, the following are some recommendations and answers related to high temperature filters.
What is the primary application of HEPA filters at elevated temperatures?
These filters are used in ovens and tunnels designed for use in the Life Science and Microelectronics industries. This equipment may be performing sterilization and depyrogenation of instruments or glassware (vials) in Life Science and die-bond curing or other semiconductor packaging processes. Applications can range in temperature from 212° to 752° F (100° to 400° C) and require ramp-rates (burn-in) from steady state to as much as 60° F + per minute (15°+ C/min.). These variations create tremendous stress and challenge to the filter’s construction and therefore filter performance integrity.
What are the options for conventional high temperature filters?
There are two common types of high temperature HEPA filters, in simple terms, silicone and ceramic sealed filters. For the silicone type, one is for moderately high temperature and is often used in the supply and exhaust (“cool” zone) of an oven in addition to the recirculated HEPA filters and for a variety of other applications. The distinguishing characteristic of this filter is the red high temperature silicone potting compound used to seal the metal-separated media pack to the metal filter frame. There are sometimes slight manufacturing differences, with different locations within Camfil Farr. European (EU) manufactured filters in this class are rated at 482° F (250° C), in the United States (US) they are rated at 500° F (260° C).
The other high temperature HEPA filter type has a white ceramic sealant, and is most commonly used in the “hot” zone of a tunnel and for other very high temperature applications. These filters incorporate a ceramic material to seal the metal-separated filter pack to the metal frame. Again, there are slight differences in temperature ratings due to some specific design criteria, EU is rated to 662° F (350° C) and the F series in the US is rated to 750° F (398° C).
Are there improved high temperature filters available?
There has been specific high temperature filters developed in conjunction with equipment manufacturers and end users. The Termikfil design features a monolithic inorganic, non-metallic frame. The unique design and assembly method allows all of the bonded materials to react to temperature changes in a similar manner. This reduces thermal stress and helps to ensure a product that is more resistant to multiple cycles and rapid temperature changes. Metal components, such as the grille, are designed to “float” and not interfere with other components as they expand and contract in response to temperature changes. This filter is rated at 662° F (350° C) and is delivered ‘burned in’ at 300° C (573° F) from our facility in France.
There is also a V-bank type HEPA filter that is rated to 446° F (230° C) continuous, with one hour peaks to 482° F (250° C). This filter is used where high process air flow is present at a temperature slightly lower than the traditional aluminum separator type ‘red silicone’ filter. Filter efficiency ranges from 99.95 to 99.995% in accordance with EN-1822 (IEST-RP-CC-001 Type E & K).
How rapidly can I heat up or cool down a high temperature filter?
This common question is second only to… ”Our filters have failed and I need replacements by tomorrow”. The official position of Camfil Farr is to default to the equipment provider. It might seem like a dodge, but experience tells us that no two ovens/tunnels are the same. The equipment provider has ‘validated’ how a specific filter has performed in their specific equipment, often re-labeling filters at higher temperatures than may have been recommended by the filter manufacturer. This is okay; filter manufacturers are a conservative bunch. If the equipment manufacturer is comfortable with increasing the ramp-up rate and pushing the temperature limit, that is between them and the end user. When pushed, Camfil Farr recommends (with no guarantees) that you not exceed temperature change rates greater than 100° F/hr. (1.67° F/min., 0.63° C/min.). The Termikfil, because of its construction, is rated at a temperature change rate of 33° F per minute (1° C/min.)
What is filter “burn-in?”
Filter burn-in is the initial tempering procedure following the installation of new filters into an oven. With the exception of the Termikfil, as this filter is ‘conditioned’ at 572° F (300° C). This means that all of the organic material used to bind the filter media, and any other residual organic material used in the materials, are still present on the filter. Additionally, if integrity testing was performed after installation, the test aerosol is also captured in the filter media. These materials must be removed by thermal oxidation before placing the oven into service. Final tempering process on-site generates fumes and odors that might contain harmful or irritant compounds. Combustion gases are mainly carbon dioxide and water vapor. We can have small quantities of carbon monoxide and other substances (MSDS available on request).
It is recommended to take steps, as much as possible to lower exposure to personnel, for example:
Avoid people present in the vicinity during the conditioning or burn-in process.
Run the exhaust system and ensure good ventilation of the room without recirculating the air.
How long does a high temperature HEPA filter last?
Filter life will depend upon the operating conditions. Because these filters are often operated under very clean conditions, the common HEPA filter recommendation to replace the filter when the initial pressure drop has doubled may not apply to a high temperature filter. Filters should be changed if they are no longer able to maintain the cleanliness of the air for the process environment. Exceeding the temperature limit, or heating the filter more rapidly than recommended may significantly shorten filter life. That said, Camfil Farr’s experience has shown that if the owner has a history of pass/fail on periodic certifications, patterns of ‘failure’ can be predicted based on the specific site operation conditions in the given tunnel or oven, a 3 to 5 year life mostly based on certification ‘failure’ is a reasonable assumption.
What does the FDA say in relation to leak testing?
The FDA states that among the HEPA filters that should be leak tested are those installed in ovens and tunnels that are commonly used to depyrogenate glass vials. It is important to note that where justified, alternate methods can be used to test HEPA filters in the hot zones of these tunnels and ovens.
Should we integrity-test high temperature filters?
This is a very difficult question, and this is where many people run into problems with certain high temperature filters. The purpose of a high temperature filter is to supply appropriately clean air to the downstream process. Unlike most clean room filters that operate in a “single pass” mode and have only one chance to remove particles, most process ovens operate in a “multi-pass” mode in order to avoid excessive heat loss. This means that the air entering the filter during steady state operating conditions is very clean. In these situations, assuming the process does not generate lots of particles, the presence of a small pin-hole or small area of penetration greater than average is of little consequence to the overall cleanliness of the process.
In addition to visual inspection, many operators elect to conduct integrity testing after filter installation and prior to filter tempering to ensure there are no large areas of damage from handling or installation of the filter media pack. Effective clamping can also be evaluated, as the absence of a seal on filter-to-hardware mating surfaces can be detected.
After burn-in, the emphasis shifts to particle testing of the environment inside the oven downstream of the filter during normal operating conditions to ensure that the desired cleanliness level is achieved. Some certifiers have built elaborate devices (an air or water cooled ‘jacket’ for example) in order to rapidly reduce temperature so discrete particle counters can function. It is important to apply the ideal gas equation to determine the actual count per cubic meter of hot air. A sample of 1 cfm would be approximately double at 650° F (343° C), so this needs to be included in your calculations. Particle losses can also occur due to the extended length of sample tubing required that is typically longer than recommend in guidance documentation. Those who later elect to conduct integrity testing subsequent to filter burn-in and operation may find that there are areas of penetration around the edges of the filter media pack greater than 0.01%, while hot particle testing showed acceptable cleanliness levels in the process environment. This finding does not necessarily indicate a problem with the high temperature filters. In these situations, a decision to keep or replace the filters based on facility policy and procedures needs to be made by the process owner and quality assurance department.
These responses pertain to air filters manufactured by Camfil Farr only. It should not be assumed that these answers necessarily hold true for products manufactured by other companies.
Commercial Air Filters
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