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Hot damn you want to build a flowhood.
First of all, don't. Go buy a used lab hood. You can get one for $2k in working condition no problem, all day every day. Even cheaper if you get lucky.
Is that a lot of money? In what sense? For a pack of gum yes. For something that can make you $10k a month legitimate business, FUCK NO.
And I don't mean an FFU come on get the real thing made to do the real job. A real hood has an enclosed work space that works like a tube to keep the flow organized, like a pipe. And adjustable speed motors and often a magnehelic and nice table and drip protection etc etc...
Just get a real hood. You know, the thing made to do the thing you want to do, not the thing made to hang from a ceiling in a clean room...
Ok so they weigh 400lbs and are a huge pain to move. ▄▀▄▀▄▀▄▀▄▀▄▀▄▀▄▀▄▀▄▀▄▀▄▀▄▀▄▀▄▀▄▀▄▀▄▀▄▀▄▀▄▀▄▀▄▀▄▀▄▀▄▀▄▀▄▀▄▀▄▀▄▀▄▀▄▀▄▀▄▀▄▀Alright cool you didn't listen to me and want to build a hood anyway? Wow you are a real fucking piece of work, you know that!?
Don't buy a 12" thick pack, yes I know they are easier to find on the internet. They have half the static pressure at the same speed as a 6" pack.
Why? because they aren't twice as thick they have twice the surface area. The actual filter is the same piece of shit 1/4" thick continuous sheet of fiberglass paper, it's just folded twice as deep.
Here take a look ya pervert.
Just don't do it. Static pressure is your key to an even flow.
12" thick filter run around 0.3" static pressure at 90 fpm.
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6" thick (aka 5-7/8 or 5.875) 99.99% HEPA filters that we typically use in hoods run about 0.6" static pressure at 90 fpm. With the addition of a properly sized pre-filter adding 0.2" static pressure brings us to 0.8" pressure on the fan at 90 fpm. This is practically all you need to know about the static pressure alright.
Too fast of a hood makes MORE TURBULENCE. Anytime air hits anything it makes turbulence. The faster the air the worse the turbulence, i.e. the wider the swath of turbulent air. You want as little turbulence as physically possible. That's the point of the whole fucking thing.
Turbulence can ride backwards up your shit and fuck it up alright? We call that an eddy. Fuck Eddy we don't like that guy.
90 fpm is what you want. In a lab the hood must be 90 fpm +/-10 fpm across the entire surface measured in a 6" grid or they will literally throw it off an overpass.
So just take your filter size and multiply the sq ft of filter area x 90 fpm and you have the target CFM @ 0.8" static pressure, now go buy a fan that matches.
you can get a more accurate size by removing the frame with an easy calculation: Sqft - (Perimeter length * 0.75 / 144)= actual size so a 24x24 4 sq ft minus (96" perimeter x 0.75 / 144) = 3.5 sq ft actual size * 90fpm = 315 cfm@ 0.8" static pressure target.
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Speed controller? Not wise, these common blowers come in several electrical schemes and some of them will burn up on a speed controller.
They utilize a start capacitor winding that is physically tied to a centrifugal force switch, basically the capacitor runs until it gets to a certain speed and shuts off. When it stays on this can make a lot of problems.
Some of them can work but most Dayton blowers specifically say specifically NOT FOR SPEED CONTROL
Pro hoods have better blowers that use PSC or PWM speed control.
You can just get a blower slightly over sized and block the intake side with some duct tape. Easy to adjust and remove.
I don' recommend overshooting your target too much, there is only so much you can reduce it by blocking even 100%.
Yes the filter loads up over time and will need to be adjusted.
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You won't know shit about your hood unless you have a hot-wire (thermal) anemometer and a particle counter.
This is the one and only way to verify proper functionality.
You don't NEED to verify it, but if you want to that's what you have to use. You could be looking at $250 for these each used on a good deal or much more new so...
Fan anemometer is totally worthless for this use. Better off with a fresh Bic.
You may as well be interpreting random gorilla shrieks as numbers when taking such readings.
A half ass hood may work 95% of the time but be really hard to pin down the problems without the proper tools.
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Prefilter size: Prefilters are designed to run 300 fpm to get the target pressure. Heres a tiny pic of some common pressures.
So take that 315 cfm and / 300 and you want 1.05 sq ft of prefilter for that 24x24. If it is much oversized you will be adding less static pressure to your 0.6" hepa and have more air going through than you anticipated.
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Take a look at this glorious video.
Pay attention to the 35 and 2:45 second area but watch all of it to understand what's going on in your hood. Sorry about the lady.
I will add to this as I can but thought it may be helpful to someone somewhere. Feel free to ask any questions.
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