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HEPA filter resistance and pressure dropHelpful Member! 

JAD0630 (Mechanical) (OP)
13 Jan 10 12:10
I have a laboratory that is currently bringing in approximately 1000 CFM of air. This air travels through 12" duct and then into a diffuser that is 48" x 24". CFM was calculated at the end of the 12" duct. I am wanting to install a HEPA filter just above the diffuser but do not want to comprimise too much CFM drop. What information do I need about the HEPA filter to calculate the CFM reduction? Would it simply be the resistance of the filter? Any information would be greatly appreciated.

Thanks,

Aaron
Helpful Member!  ChasBean1 (Mechanical)
13 Jan 10 13:19
Hi Aaron, more info about the system would be helpful, for example are there fan speed controls or VAV boxes that can compensate for pressure?

The luxury you have is it's only about a 125 feet per minute. Typical HEPA velocity limit is 500 fpm (some allow higher). With a clean filter, your added pressure drop will probably be about 0.3 in. w.c. For that duct alone, if the original drop along the run was 1" and now you have 1.3" total, the 1,000 cfm becomes about 880 cfm based on fan law estimation.

But be careful – the attached system matters a lot. The fan type and its curve matters a lot. If it's five equal ducts and equal flows, no fan speed controls or VAV boxes, putting a filter in one of the five ducts might cause you to greatly favor the other four and could reduce your flow to 200 cfm...
JAD0630 (Mechanical) (OP)
13 Jan 10 14:53
Thanks,

The air handling unit has no other ducts nor VAV boxes. Is there a way to calculate the CFM available without using the pressure drop. I do not have a pressure gauge within the system, nor would it be easy to install in our situation.  The resistance and pressure drop of the HEPA filter will be known shortly. Thank you for your assistance.

Aaron
atlas06 (Mechanical)
13 Jan 10 16:29
HEPA filters should not exceed 250 FPM.
Insalled in diffusers, we find that in laminar flow applications of Operating rooms in Hospitals.
HEPA filters can add up to 2.5" PD, your fan is likley not sized for this additional pressure drop. You could crack your duct system with that kind of additional PD.
In adition, you ought to add a pre-filter upstrea of HEPA filters to extend their lives (expensives these suckers).


Question: Why HEPA at the diffuser?
 
SAK9 (Mechanical)
13 Jan 10 17:34
The solution to your problem is fairly simple.If the pressure is not enough for a terminal HEPA filter,go for a FAn Filter Unit(FFU) from a company like Envirco(www.envirco.com).It has got anin built fan to overcome the HEPA resistance .Further the fan can be configured to speed up as the filter loads up.

But more questions need to be answered:

Why HEPA?



What level of cleanliness is required(ISO 8,7,6,5....)


What grade of HEPA?

What is the minimum air change rate required?

What level of pressirization is required to maintain cleanliness?
JAD0630 (Mechanical) (OP)
14 Jan 10 11:11
Gentlemen,

We are using HEPA filters because we are trying get 99.9%+ efficiency at .3ug/m^3. The price of ULPA filters is too expensive for this project.

To give you a little background of our system, we currently have a lab that is bringing in approximately 1000 CFM. Additionally, the lab falls into the ISO class 7 range. The minimum air change per hour is 60 but we will not meet that guideline for quite some time. We are more worried about particle counts. Thanks to everyone for your help.   
Zesti (Mechanical)
15 Jan 10 13:28

Don't you need the number of airchanges to actually obtain the desired ISO class?

Filtering the supply air is one thing but the air changes will dilute and remove the dust generated inside the room itself.

 
mauricestoker (Mechanical)
15 Jan 10 14:42
You might also want to consider any differential pressure/flow requirements. As the supply HEPA filter loads up, unless you have a VFD/dP sensor to overcome dirty filter condition, then supply would steadily degrade. Same would be true on the exhaust side.

I would recommend sizing everything on dirty filter condition, including a dP across pre- and HEPA, and dP across the lab perimeter.

If differential pressure is critical, I'd recommend also looking at an interlock between supply and exhaust. Should the system fail negative, postive or neutral.

If you're worried about capacity loss, then you may want to look at "deep", "extended", or "energy efficient" HEPA's. These are reducing static far below levels of just 10 years ago.

Last, keeo your maintenance people in mind. Filter  (and pre-) will need replacing, and you may want to think about whether you want swap out done inside the ISO area.


 

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