Re: Air Filter Collection Efficiency -Reply

[Harold Chaney <HDC@nrc.gov>]

You wrote: 

> Does anyone have any information concerning the change in collection
> efficiency of an air filter as it becomes loaded with dust?  I suspect that
> the collection efficiency increases with dust loading, but to what extent?
> 

I look in two references I have handy:

- Nuclear Air Cleaning Handbook (ERDA-1976)
-  Industrial Ventilation (ACGIH, 19th Edition 1986)

Each of the above reference the fact that removal efficiencies for HEPA filters is a
function of airflow through the media.  Over pressurization is a function of filter
media resistance caused by various physical conditions (air density, wetting, and
debris buildup).   They also noted that efficiencies could be reduced by higher than
recommended airflows through the filters (tunneling).

Even though filtration efficiency actually increases early-on during debris buildup,
the stability of the filter media/system (ducts and filter units) is jeopardized by the
resulting high differential pressures acting on the system, especially downstream of
the filter unit.  At a certain point your system fails to perform it's function of
controlling airborne radioactivity, since there is too little air flow for collection of
the airborne radioactivity at the source.

H.  Dean Chaney, CHP -  USNRC, Region IV/WCFO
hdc@nrc.gov
ddchaney@castles.com

The views put forth above are my own and do not necessarily 
reflect the beliefs or policies of the USNRC or any other 
governmental entity.