Air Baffel design needed to achive laminar flow 2

[MHBCE]

I have a fan powered HEPA filter that I am pumping air into a plenum at a rate of 300-650cfm. The plenum is 24 inches deep as is the 24x48 HEPA filter, but is 86 inches wide. I am trying to achive laminar airflow on the downstream side of the plenum using a difussion panel and baffels if needed. The air will be supplying a sterile air cabinet that will also act as a secondary plenum.

Anyone interested in helping me come up with a design?

Thanks,

Mike

 

[rb1957]

i'd suggest a set of vanes to smoothly transition between the two cross-sections.

650cfm is about 80ft/min in the smaller cross section (8 sq.ft at the fliter) = 1.5 ft/sec.

if you need laminar flow, what's the flow coming out of the filter like ... laminar ??

do you have lots of room to make the transition (from 8 sq.ft to near 15 sq.ft) ?

 

[MHBCE]

Thanks for the insight. Yep, out of the filter is reasonably laminar, BUT I only have a 4 inch plenum/transition area to distribute into.

Vanes is a good idea. I was thinking baffles.


What about possibly 2 diffusion panels with alternating holes to break the flow and pressurize? I guess if you make the holes small enough you overcome most of the lfpm face velocity, but would likely create to much of a pressure drop on the downstream side.

 

[rb1957]

you have 4" to double the area of the duct ?? at least the high speed (low area) flow rate is low (1.5ft/sec).

if you block the flow, you'll have to account for the losses in order to get the desired downstream flow rate (larger motor). i think too that this'll mess with producing a laminar flow.

 

[vortexman]

The first question is whether you really need laminar flow, or just plug flow. If you only need plug flow, in the sense that there aren't significant recirculations that could, for example, carry contaminants back upstream, you probably already have that coming out of your filter, as long as there aren't major expansions or turns downstream from it. If there are, flow-aligned channels (honeycomb) would probably take care of it. I would guess that the channels should be several channel diameters long. In a small section, I would suggest fabricating your own honeycomb by packing drinking straws. Some other kind of tubing, with larger diameter (maybe 1 to 3 inches), could perhaps be used in your case.

If you truly need laminar flow, you will want an initial screen, to generate turbulence with small eddies, so that they will damp out quickly. This needs to be followed by a section of honeycomb, several diameters (maybe 10) in length. Depeding how laminar you need it, you might need to have another bank. A bank of honeycomb backed upstream and downstream with screen could be pretty effective. The downstream screen should be finer, so that you end with small-scale eddies that damp very quickly.

If it's feasible to do the laminarization in a larger section followed by a very smooth contraction, the pressure drop will be lower, and the contraction actually helps to laminarize the flow, as long as it is very smooth.

vortexman

 

[KENAT]

Take a look at a wind tunnel if you can, they'll typically exhibit some of the ideas mentioned above.

KENAT,

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