[labnetwork] HEPA Filters

[John Shott]

Labnetwork:

We (Stanford Nanofabrication Facility) recently changed our 1985 HEPA 
filters for the first time since they were installed.  As a part of a 
facility upgrade that included some improved humidity control in the 
lithography area, we had to also add fire sprinklers above the HEPA 
filters.  Interestingly, as a part of the bid process, one of the 
general contractors commissioned a clean room survey of the existing 
condition ... expecting to find that our clean room would be terribly 
dirty due to our "antique" HEPA filters.  They were disappointed to 
learn that our 27-year-old HEPA filters were still sufficiently robust 
to pass Class 100 clean room certification standards.  As other have 
commented, however, we were likely paying higher energy costs to push 
air through these filters.

In terms of our pre-filtration ... which is am important part of this 
system .... we have the following three stages of pre-filtration.

1. All air coming into the building passed through banks of 2" deep 
Flanders MERV8 pleated filters.  Those are, effectively, high-end 
furnace filters and are rated, I believe, at an overall efficiency of 
35% (about 85% for particles in the 3-10 micron range).

2. Each of our AC units, which pre-treats all of the incoming make up 
air, has two sets of filters:

a. A 95% efficiency bag filter that is about 15" deep.  We used to 
change these on a timed basis, but now change them when they have a 
differential pressure of 0.75" WC.

b. A 99.97% HEPA filter!  These are changed every 2-5 years depending on 
pressure differential across them.  That insures that the "real" clean 
room ceiling HEPA filters don't see too many "big chunks" hitting them 
on a regular basis.

Finally, our campus facilities group monitors, pays for, and changes 
these three sets of pre-filters.

Let me know if you have any questions,

John




On 9/27/2012 7:03 AM, Iulian Codreanu wrote:
> Good Morning.
>
> Where are the prefilters some of you mentioned located? At the 
> discharge of the makeup air units and recirc units, right?
>
> Thanks for the feedback.
>
> Iulian