[labnetwork] final filter on makeup air units

[Tribble, Thomas]

Gentlepersons,

My thoughts:  The Harvard CNS setup is a very similar design, with recirculating fans delivering HEPA filtered air above the instrument bays and returning air through adjacent mechanical chases.  The supply air and recirculating air are separate systems.

The supply air intakes are located high on the building (8th floor - clean air) with air passing through MERV 8 pre-filters and MERV 15 final filters. Supply air volume varies with tools in use (gates open) and is balanced against exhaust to provide continuous 0.1 inch positive pressure relative to the area surrounding the cleanroom.  These "factoids" are important because fans are notoriously "finicky" with respect to pressure drop restrictions on the suction side of the unit. Too much restriction  and the fans "stall" or stop moving air.  Putting high restriction HEPA filters on the discharge side of your recirculating units is the right design.  Putting  makeup air intakes high in the building is a "best practice."  Running makeup air fans at less than full speed when volume demands of the equipment are less than 100% is good operating  practice (and reduces the pressure drop across the supply air filters).

The selection of efficiency for supply air pre and final filters depends on the fan curves for the fans you have installed, and the placement of the filter banks. There is a real possibility that trying to replace your MERV13's with HEPAs is not going to work.   The filter banks are usually placed on the suction (temperamental) side of the fan.  Chances are the engineers would have specified higher efficiency (higher pressure drop) final filters if the system design would have permitted.

Work with your facility engineers.  Get the fan curves for your fan (you need the air velocities for your system and pressures at those velocities), the pressure drop curves for the MERV 13, MERV15, and HEPA filters.  Adding in the pressure drop from the pre-filters, heating  and cooling coils, you should be able to calculate the maximum filtration that your fan can tolerate.  If you can get HEPA filters into your supply air, I predict your electric consumption and filter replacement frequencies are going to increase.  That may be ok if it solves a bigger problem.

Thomas A. Tribble PE

-----Original Message-----
From: labnetwork-bounces at mtl.mit.edu [mailto:labnetwork-bounces at mtl.mit.edu] On Behalf Of Iulian Codreanu
Sent: Tuesday, June 04, 2013 9:25 AM
To: Fab Network
Subject: [labnetwork] final filter on makeup air units

Dear Colleagues,

I am trying to convince some folks that having filters placed on the discharge side of a cleanroom makeup air unit (MAU) is not only a good
idea but a must.    I only have a MERV 8 and a MERV 14 on the intake
side and I am pushing for HEPA filtration on the discharge side; your input carries a lot of weight and I hope you can help.

My cleanroom is "two stories tall"; the makeup air discharges into the mechanical space (it houses the recirculating air units that push the air into the bay plenum boxes) that is above the the bay and chase cleanroom (no raised floor, sidewall air return).

Can you please share your MAU filter configuration; it would be helpful to list all the filters in the unit and where they are located.

Thank you very much.

Iulian

--
iulian Codreanu, Ph.D.
Director of Operations, UD Nanofab
149 Evans Hall
Newark, DE 19716
302-831-2784


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