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Offlinedarshan
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LF Hood Design Questions (now with pics!)
    #13322176 - 10/11/10 05:55 PM (13 years, 4 months ago)

Hi,

Tried search and didn't find a good answer.

Building a 2'x4' LF hood using a squirrel cage blower capable of 640CFM @ 1"WC (800CFM open air).  I wanted to know, is there any good reason not to put the blower in open air and have it push through the prefilter into the plenum, as opposed to building a box around it and having it pull through a prefilter?  Other than I'd have to remove the blower to change the prefilter, it seemed like I would be minimizing intake restriction on my blower, as well as removing an extra box and weight from the hood.

Thanks, Shroomers.


Edited by darshan (10/19/10 10:45 PM)


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Invisiblecurbstop
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Re: LF Hood Question [Re: darshan]
    #13322343 - 10/11/10 06:38 PM (13 years, 4 months ago)

IMHO,,,,,you are going to want to have the pre filter on the outside of your blower so that the air will be sucked threw the filter removing most dust before it gets to your blower.

this helps extend the longevity of the bearings in your blower :thumbup:

Also you dont have to build a box for the blower to go in you simply can attach your blower to the box and then duct tape your prefilter to the blower.

Ask RR to take a look at this thread he will show you a pic of what i'm talking about:thumbup:


--------------------
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Offlinedarshan
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Re: LF Hood Question [Re: curbstop]
    #13322746 - 10/11/10 08:16 PM (13 years, 4 months ago)

Fair enough.  I am pretty sure I've seen a picture of that hack.  Nothing against hacks.  Agar does the same thing, right?  Maybe I'm confusing the old hands.  Anyway, I think I'll go ahead and build the box anyhow, just cuz in some ways it seems a little tidier and I like building shit.  Also my blower is a twin cage, single motor affair, so the double taped ends would drive me a little crazy.

I guess it would be a moot point if I were already scrubbing the whole room with a 3M Allergen Ultra?  Hmmm....

Definitely a go on protecting the bearings, though.  Hadn't thought of that one.

Rad.

I'll post photos when it's done.

On a side note, any folks have experience with alternative materials like acrylic or such for the box?  I've made computer cases with acrylic that I seam-welded with acetone that turned out well.  That would be kind of rad for a hood.  What do you think?


Edited by darshan (10/11/10 10:14 PM)


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Offlinedarshan
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Re: LF Hood Question [Re: darshan]
    #13323355 - 10/11/10 10:03 PM (13 years, 4 months ago)

Also, I would like to ask for some opinions on the design.  I was going to use an approach similar to the FP flowhoods, where it's basically a box with the same X and Y of the filter, with a blower on top of the box blowing at 90 degrees to the face of the filter.  In such a setup, what is the optimal Y dimension (depth) of the box?  I would like to have as shallow a box as possible so as to leave me more workspace in front of the hood / prevent me from having to build an additional support structure BEHIND the work surface.

Thanks!!!


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Offlinetotal
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Re: LF Hood Question [Re: darshan]
    #13323418 - 10/11/10 10:20 PM (13 years, 4 months ago)

That blower is not going to be able to handle a 2'x4' filter...
Assuming you have a 2'x4' filter, that is a total of 8ft^2
You take that 8ft^2 and multiple it by the bare minimum of 100ft/min
And you get you will need 800cfm to achieve laminar flow...
Ok, so that said, you need a 800cfm at the filters static pressure, most filters are anywhere from .8-1" static pressure, plus a prefilter will add .2...so your looking at 1-1.2" static pressure...that 800cfm blower in free air will not achieve laminar flow

Im pretty sure my math is right here, you should check out THIS LINK It has alot of good info!
Maybe some of the older hands will come in and check my homework...

*edit*
Oh, and making your blower push air through a prefilter instead of pulling air through it via intake does not change the static pressure it creates.
The box on box FP one your seeing is a great design :smile: The top of that box is a filter and the blower is inside pulling air though the filter, into the blower, then out the hepa...filtering your blower might even make its life longer :shrug:


Edited by total (10/11/10 10:30 PM)


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Offlinedarshan
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Re: LF Hood Question [Re: total]
    #13323475 - 10/11/10 10:30 PM (13 years, 4 months ago)

Actually my filter is precisely 43 1/16 by 19 1/16 which works out to ~5.70^2 feet.  So at 100FPM = ~570CFM.  The filter is rated for 555CFM at .54" WC, well below the capacities of my blower.

Thanks for looking out, though.  And sorry for not being more exact in my initial posting.


Edited by darshan (10/11/10 10:39 PM)


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Offlinedarshan
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Re: LF Hood Question [Re: darshan]
    #13323530 - 10/11/10 10:44 PM (13 years, 4 months ago)

Like I said, I did use the "search" function...


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Offlinedarshan
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Re: LF Hood Question [Re: darshan]
    #13326760 - 10/12/10 05:01 PM (13 years, 4 months ago)

So it looks like the rule of thumb is plenum depth >= filter depth, which is 5 7/8" in my case.  I'm gonna be safe on this one and go 16" total box depth to give 10 1/8" space for pressurization, which shouldn't totally eat up my whole desk.

I'm going with a 3/4" particle-board with a formica finish on both sides, something that shouldn't fragment off or make its own dust.  I've already cut the pieces with a little wiggle room (since the actual filter doesn't come til Thursday), and I'll be sealing the exposed particle-board ends with glue or a resin of some kind.  Quite looking forward to how this turns out.

Another thought I had is that I don't want to just silicone the filter directly to the box.  For one, I might not like the box or I might come across some better material and want to rebuild it.  So my thought was that I would install some trim along the interior of the box, set about 6" back, and put some foam air-filter gasket around that.  I would be able to set the filter in against that, and it would have the same profile as the bezel of the aluminum filter frame.  I would build another bezel, also the same shape and size as the frame bezel, also with gasket material, and put that on front of the filter and compress it down with hardware of some kind.  Perhaps some swing-out bolts with butterfly nuts on the end.  This way I could remove the filter if need be.  Anyone done something like this?  Anyone done something different that also allowed for removal of the filter?

Thanks.


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Offlinedarshan
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Re: LF Hood Question [Re: darshan]
    #13360038 - 10/19/10 10:45 PM (13 years, 3 months ago)

Here's my hood.  Sorry for the crummy photos.





For the box around the blower, it's actually just a tube of sorts, with weather stripping on each side.  The weight of the box and the suction holds a tight airlock.



Unfortunately I made some errors.  The divider is less than ideal, as it introduces turbulence.  I effectively have two 2x2 hoods, which actually is fine, I just have to remind myself of it.  Furthermore, my blower has two outputs, each 3"x3", which makes for a high pressure relative to the output, and therefore is highly directional.  I should have made my plenum deeper to accommodate this, as the flow is not quite laminar.  It seems good enough for government work, as they say though, because I have done testing with open agar plates and am seeing no contaminants.

Totally worthwhile project and cost about $340, not counting the screws and silicone, which I had around.  Stoked!

Next one will be stainless though :wink:


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Re: LF Hood Question [Re: darshan]
    #13360073 - 10/19/10 10:57 PM (13 years, 3 months ago)

Sure is pretty! :smile:
:homerdrool:


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