Preamble
Leaky HEPA filtration (translated with the term 'Bleed-Thru') is a pre-existing condition in Pharmaceutical cleanroom construction, particularly important at high cleanliness levels such as Class A (ceiling 100% filtration). . The problem of leakage filter cannot be solved by any method of dressing (cannot stop bleeding ☺☺). As a result of these small problems, which will arise during a period of operation, the HEPA filter actually breaks 'Bloody'. Therefore, HEPA leaks need to be checked before a problem occurs.
Leak filter - 'Bleed-Thru'- is defined: when leak test on the back of the filter, there is a leak that exceeds the leak parameter on the allowable standard.
For example, if the percent penetration (% penetration) over the entire filter surface is measured as 0.02%, the percent allowable leakage is 0.01%. HEPA filter has failed. End users must change the filter, or the supplier must return the bad filter.
There are several factors that affect HEPA filter leaks:
There are misconceptions about what causes leaks. Conventional notions for the filter manufacturer or filter media produced the changes that led to the 'Bleed-Thru'. Most of the errors are attributed to the filter media's fault. Common complaints about causes:
The new materials used are of course cheaper and thinner than the MIL-SPEC material. Filter media may have different thickness but will have the same characteristics, the same percentage of leakage is used for making HEPA filter. In this case, the filter media thickness will not affect the leakage characteristics. however, it has an effect on their drop pressure and dust capacity.
Facts About Leaked HEPA- ‘Bleed-Thru’
As stated above the real cause of the leak is in the HEPA filter. Understand the first causes: inappropriate filter characteristics, filter surface velocity and particle size during filter testing, thereby providing solutions.
This is the primary cause of most 'Bleed-Thru' leaky filter problems. The parameter “Specific Filter Surface Velocity” is defined by HEPA filter manufacturers. For HEPA filtration used in Class A application areas the cleanliness level is 0.45 m/s (90fpm) or 0.5m/s (100fpm). When practically applied (in-situ), the maximum velocity value should not exceed the speed specified by the filter manufacturer. Precisely because surface velocity affects leakage, the maximum applied velocities must be actually tested by the filter manufacturers and they must ensure that they are suitable for all influencing conditions as well as being Performance conditions affect as well as ensure filtration performance and leakage. Most of the HEPA filter specifications given will guide practical requirements as in the IEST (Institute of Enironmental Science and Technology) document. The filter class “TYPE C” requires a minimum total efficiency of 99.99% with 0.3micron dust particles and a leak test on the entire filter surface (scan test) with a maximum leak of 0.01%. ().
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In IEST to test the filter using a laskin nozzle to determine the maximum amount of leakage thereby determining the minimum total efficiency, this problem is supported by the average mass of the particles generated by the tank. spray laskin oil with an average particle diameter of 0.7 microns. This eliminates the problem of posterior osmosis and allows us to find only the leak (note that a perforation does not select the particle size).
If a thermal aerosol is used, the average particle mass is much smaller resulting in a Bleed-Thru possibility. so smaller test particles will pass through, so the filter will have a lower efficiency than when testing in construction with these small particles.
As described above, the filter surface velocity As described above, the filter surface velocity is determined from 0.45 to 1.5 m/s for Bio-Pharm application. However, the actual wind speed at the installation site is usually higher. Velocity is usually tested to be 0.6 to 0.7 m/s even in some cases up to 0.9 m/s. with velocity changes specified by the manufacturer will have a profound effect on filter performance. Example with the following HEPA filter: (media pack)
As is known, Type C filters are required to be tested with 0.3 micron DOP particles. In the case of Class A (fully filtered ceilings) areas, the application standard requires testing by a portable thermal granulator intended to produce suitable upstream concentrations. One problem with this machine is producing particle sizes with dense density or MPPS form. If a factory has properly tested the filter with 99.99% efficiency with 0.3micron particles, then with a thermal sprayer in this case Bleed-Thru will appear when the MPPS test range is always lower than 0.3micron.
Contact technical support and quote:
Nguyen Phuoc Huy (Mr.)
Cell phone: 0988.408.134
Email: thtech.huy@gmail.com
