Let us take a look at particles which may enter a cleanroom from outside air. If we assume air outside particle concentrations are about one million 0.5 micron and larger particles, it would not be an unreasonable assumption as levels are much higher in many areas of the country. In this case, the number of 0.1 micron particles will be about 35 times as many as for the 0.5 micron particles or about thirty-five million particles per cubic foot. Now let us assume we are using a 99.999% efficient HEPA filter (rated at 0.1 µpoint of least efficiency), we would filter out 34,999,650 particles but 350 one-tenth micron particles would remain. This is the maximum limit for a Class 100 cleanroom! Obviously, we need a number of additional methods to help address the required reductions of particle concentrations.
One way the methods is to use a small percentage of outside makeup air and mix it with recirculated air which gets progressively cleaner, a primary reason along with energy savings to use recirculated air. Another method is to use dual banks of HEPA filters, one set in the make-up handler and a final room set, which is a recommended general practice (using a dual bank also protects the final set from high levels of particle exposure, thus increasing their lifetime and reducing the buildup of pressure drops). At this point, we need to not only note, but also emphasize,that the use of any HEPA filters of less than 99.99% efficiency should never be considered. For even the crudest of cleanrooms, the initial cost savings, if any, will certainly be more than offset by certification costs and the poor results.
A 99.97% efficiency filter is tested with a gross leak measurement indicating that 0.03% of all upstream contamination may be passed through the filter. For a 99.99% filter, the test measures each small area so that no more than .01% of upstream contaminant may be passed in a individual leak. Since most of the filter does not have a leakage rate anywhere near the .01% limit, the result is that the gross leak is far less than 0.01%. Thus, a scanned 99.99% filter is far more than 3 times better than the gross leak tested 99.97% filter, generally on the order of from 1000 to 10,000 times better!.
Rule 1:It is just as important to understand what a HEPA filter cannot do as well as what it can do. No HEPA filter can reduce the amount of contamination introduced downstream of the filter. Repeat: No HEPA filter can reduce the amount of contamination introduced downstream of the filter. While this may seem inherently obvious, it is amazing how many times the excuse that the HEPA filters will take care of it is used!
HEPA filters can remove a portion of upstream contamination.
If the only function of a HEPA filtration system were to provide clean air to the cleanroom, we could pump the room full of clean air and then turn off the filtration system! In fact, ninety percent or better of the function of a well designed cleanroom HVAC system is to remove internally generated contamination and prevent it from adversely affecting the critical product or process. Conversely, delivering clean air to the cleanroom is only ten percent or less of the function. With this in mind, we need to ask, "Where does this internal contamination come from?"
Thus we cannot depend upon the filtration characteristics of the HEPA to remove the internally generated contamination. We already know that it will only remove a given percentage of upstream contamination. Thus, we must utilize HEPA filters as the valuable tools they are in the cleanroom, but at the same time keep in mind the constraints of their use.
- A person sitting or stopped generates about 100,000 particles per cubic ft.
- Sitting down or standing up generates about 2,500,000 particlescubic ft.
- Walking generates about 10,000,000 particles perc ubic ft.
- Horseplay generates about 30,000,000 particles per cubic ft.
- Grinding, sweeping, welding adds billions of particles per cubic ft.
- Two surfaces rubbing generate billions of particles per cubic ft.
- An open, non-airlocked door can add billions of particles per cubic ft.
- Process equipment adds particles
- Process materials add particles
- Maintenance activity adds particles
- Construction residue can generate massive particles throughout the life of the facility!
Rule #2:And that's the truth!
HEPA filters cannot remove contamination introduced downstream of the filter.