Change of HEPA Filters 3

[Fenixseravgon]

Is it true we muss change HEPA filter if there is a change in the pressure of the filter of 10%?
Which standar have the information about that?

Thanks a lot?

Sergio

 

[quark]

I never heard of such a rule nor practising it. Clean HEPA pressure drop is about 1" and can very well work upto 3" if the fan supports. The criterion for filter changing, particularly in pharma industry, is no. of air changes per hour. If no. of ACPH is less than permissible (this is again a topic of debate) limit, you should change it.

 

[tombmech]

I agree with quark's first statement - pressure drops of a few inches as HEPA's load up is not reason for alarm. Hopefully, the worst case is no more than a couple of inches, but certainly magnitudes higher than 10% pressure drop.

However, I'm confused about the mention of air changes. Air flow rates should not change with filter loadings. That is what is meant by "if the fan supports" - whether the fan and motor can maintain the same airflow despite higher pressure drops. In that respect, a 10% change in airflow may be a maximum limit to change the filter. (If even that much is acceptable.)

 

[quark]

The comment relates to the conventional systems where you don't have variable speed facility. If the designer keeps in mind the above 10% rule and if the fan static is designed accordingly, it may go to shut off condition with increase in system resistance.

If, for a constant speed system, the fan is designed at the expense of initial higher flowrates than required then the conditions can be maintained though the filters load up. The pharmacopeial requirement of no. of ACPH for class 10000 is mere 20 and for class 100000 it is 10. If the studies show the particle concentration below the class limits, air flowrate never matters.

Interestingly, HEPA efficiencies go up as they get loaded.

 

[tombmech]

OK, we are both in agreement that a 10% criterion for HEPA filter static pressure is extreme.

However, I think allowing airflow rates to change is peculiar to the application you cite. Most clean rooms and other applications that require HEPA filtration also have a critical dependence on air velocities and pressure differentials within the space. Airflow rates significantly affect those parameters. Ideally, the system is designed with components that allow flexibility. However, the loading of any filter is gauged by its pressure differential.

Perhaps your situation is one where you may judge HEPA loading by the affect on airflow rates (changes), or claim that "air flowrate never matters." That is not a luxury that some of us can afford.

 

[Fenixseravgon]

I´m evaluating a electrical oven with HEPA filters to depirogenizate ampoules ina pharma facilities.

 

[tombmech]

I am not very familiar with the drying oven with which you refer. I did a little research on some sterilizing and depyrogenizing drying tunnels, if that's what your talking about. It appears that some of them may depend on critical airflows and very small pressure drops to maintain integrity between chambers.

In that case, if they have recommended no more than a 10% pressure drop in their HEPA filters, then I would take them at their word.

However, much of what Quark and I have been telling you about HEPA filters is true. So, I would definitely put your 10% requirement in the category of a vendor's "gotcha" - one of those interesting bits of info that you wish you knew before someone bought it.

 

[quark]

Flow velocities, for depyrogenation ovens and tunnels should, generally, be laminar and in this case velocity becomes a judging characteristic. So, the average velocity should be 90fpm+/-10% and the localised velocities can be +/-20% of the average value(these values are as per IES contamination control recommended practices).

 

[tombmech]

Sorry to post on this again, but I did come across an interesting factoid that may be applicable. NSF 49 (BioSafety Cabinetry) states:

"When operating at the nominal set point velocities, without adjusting the fan speed control {or in lack of one}, a 50% increase in pressure drop across the filter shall not decrease the total air delivery by more than 10%."

Of course, any reference to a "filter" means "HEPA filter" in this context. So, my first post up there may be more appropriate: a 10% airflow drop with a HEPA filter is the maximum allowable in most circumstances. However, the pressure drop may be much higher.

I tend to think the vendor did not understand the difference, and just remembered the 10% figure.


P.S. Since the cross-sectional areas of an installed HEPA filter would not change, a % drop in air quantities would be seen as a % drop in velocity. So, quark's last post is correct, too.