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MeltingPenguin
Pooh-Bah


Registered: 08/29/01
Posts: 2,138
Loc: new england
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Is an inline duct fan or for a flow hood?
#10306271 - 05/08/09 02:29 AM (14 years, 9 months ago) |
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12" In-line duct fan, It's got plenty of CFM (975), and It would fit nice on the filters 12" collar. I was just wondering if an in-line duct fan would be ok, or if a squirrel cage is totally necessary.
Because it's a lot cheaper then the squirrel cage blows I've been finding that are only half the CFM.
-------------------- Growing anything is good for the soul
Edited by MeltingPenguin (05/08/09 02:33 AM)
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sillycibe
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Registered: 03/10/09
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Re: Is an inline duct fan or for a flow hood? [Re: MeltingPenguin]
#10306356 - 05/08/09 02:53 AM (14 years, 9 months ago) |
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you're going to have to find the cfms that match the static density of your filter, you cant just pop any filter to any fan.
for five hundred and twenty five bucks, u can order a VERY nice hood from fungiperfecti pre assembled.
there are TONS of "flowhood" designs that will match your fans CFM's.
you'll notice, that every flowhood you see, even my ULPA, has a squirrel cage fan,
it's no coincidince, if then one you're looking at, that's a fraction of the cost worked, then everyone would be using them.
I apologize i couldn't be of more help. Maybe someone will come along and give you more hope on the matter, but i don't see your fan working well if at all.
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MeltingPenguin
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Registered: 08/29/01
Posts: 2,138
Loc: new england
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Re: Is an inline duct fan or for a flow hood? [Re: sillycibe]
#10306385 - 05/08/09 03:06 AM (14 years, 9 months ago) |
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Well I already have a 4" filter. It's 24"X48" enclosed in aluminum with a 12" hole on the back to attach a duct. It's rated 720 CFM flow at 0.50 inches. It's basically a hood without a motor. I'd trying to figure how I would attack a squirrel cage to it....
-------------------- Growing anything is good for the soul
Edited by MeltingPenguin (05/08/09 03:14 AM)
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sillycibe
Stranger

Registered: 03/10/09
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Last seen: 14 years, 8 months
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Re: Is an inline duct fan or for a flow hood? [Re: MeltingPenguin]
#10306412 - 05/08/09 03:16 AM (14 years, 9 months ago) |
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MeltingPenguin
Pooh-Bah


Registered: 08/29/01
Posts: 2,138
Loc: new england
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Re: Is an inline duct fan or for a flow hood? [Re: sillycibe]
#10307087 - 05/08/09 08:09 AM (14 years, 9 months ago) |
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SO i need to cut a hole in the top of the aluminum? Not sure if I know what tools i need to do that...
-------------------- Growing anything is good for the soul
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veda_sticks
Cultivator




Registered: 07/29/07
Posts: 14,191
Loc: UK
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Re: Is an inline duct fan or for a flow hood? [Re: MeltingPenguin]
#10307115 - 05/08/09 08:17 AM (14 years, 9 months ago) |
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Post a picture of what you have. Usualy you house your filter in a sealed box with a hole cut out to fit your fan. Ive only seen 2 types of fans used for flowhoods. squirrel cage and centrifugal, which are used for industrial applications.
The cheaper fan may work but how long i dont know.
the cfm is too high for your filter though, you want it close to what you need for your filter. I could be wrong, but if its too strong it might not produce laminar flow properly.
The CFM needed for the static pressure of your filter is what is needed to give you laminar flow with your particular filter - thats right isnt it??
Quote:
Every filter has a different static pressure at the working point. The working point is where the amount of the air flowing through the filter is sufficient to meet the requirement of the laminar flow.
Match a blower to the filter
Once you decided on the size of the HEPA filter, you have to match a suitable blower to this particular filter. This is a very important step, so make sure to study the directions thoroughly.
Every filter has a "resistance" when air blows through it at a certain speed, this resistance is called the "static pressure".
Press your hand against your mouth. Now try to blow through it. Dependant on how firm you press it against your mouth, you will have some difficulties blowing air out and you will feel some resistance, this is the static pressure.
Every filter has a different static pressure at the working point. The working point is where the amount of the air flowing through the filter is sufficient to meet the requirement of the laminar flow.
The static pressure is expressed in inch of water column in the English units, a typical value would be 1", the SI unit for pressure is Pa(Pascal).
1" water column is around 250 Pa. Each filter has a data sheet (consult the manufacturer if this is not the case with your filter) where the static pressure at the working point is entered. Before the air enters the blower it is usually pre-filtered by a furnace filter around 1"(2.5cm) thick placed in front of the blower to protect it and the HEPA filter from big particles like dust and hairs. It can be assumed that the static pressure for this prefilter at the working point is around 0.2"(50 Pa)
According to Stamets (Paul Stamets and J.S.Chilton: The Mushroom Cultivator p. 347 ff) the air speed of the air flowing from the filter surface should be (at least) 100 feet per minute(fpm).(around 30 meter per minute or 0.5 meter per second). Determining the correct blower for a filter consists of several steps:
1. Find out the area of your filter by multiplying the width and the hight in feet (for instance the smallest reasonably usable filter would be 2ft x 1ft) 2ft x 1ft = 2 ft2
2. Multiply the required air speed(the one Stamets specifies, 100 ft/min) with the area of your filter 100 ft/min x 2 ft2 = 200 ft3/min
So 200 ft3/min(= cfm = "cubic feet per minute") is the amount of air your blower must deliver at the sum of the STATIC PRESSURE of the HEPA filter + prefilter.
NOTE: 1 cfm= 1.7m3/h
So if you use the above filter with 1"(250Pa) static pressure and a furnace prefilter with a static pressure of 0.2"(50Pa) your blower must deliver 200 cfm(340m3/h) of air at a static pressure of 1.2"(300Pa).
Finding the correct blower
Each blower has a data sheet (consult the manufacturer if this is not the case with your blower) where the correlation between the volumetric flow and the static pressure is represented by a graph or table.
Here is such a set of curves for 4 blowers(numbered 1-4). NOTE: Each model of a blower has his own characteristic curve. This chart shows the curves for 4 different particular models of axial duct blowers. What you can clearly see is that the bigger the static pressure the less air the blower delivers, up to the maximum static pressure where the air output is zero.
Sometimes these data are presented in form of a table.
By now you know how much your blower must deliver and at which static pressure. In our example it was 340m3/h at 300Pa. So what we do now is to go in this chart and find the value of 300Pa(=1.2") at the vertical axis. Then you move to the right until you reach 340m3/h.(=200cfm) Now, ideally there should be a curve of a blower going exactly trough this point, but this is not always the case. So we have to choose a blower that best approximates our requirements. In most cases we should choose a stronger blower, if it's not too strong. How strong is too strong? I'd say the blower shouldn't deliver more that 20% more air than we calculated, so in our case the strongest acceptable blower would be around 400m3/h. In this example we have a blower that delivers 300m3/h(according to our calculation it should deliver 340m3/h though). This is a difference of mere 10%. The next stronger blower in this chart delivers 530m3/h at 300Pa, which is way too much.
In this case we go for the smaller(and cheaper) blower(Nr.4) instead of the much bigger one(Nr.1, which delivers 530m3/h at 300Pa), despite the fact that it's generally recommended to take a stronger one.
NOTE: This chart is only an example for a set of 4 particular blowers. The curves of the blowers available to you may look a bit different, but similar. Usually, if you search long enough, you will be able to find a blower that exactly matches your HEPA.
The design in this pictorial uses an in-line centrifugal fan, another good option (actually the one most commonly used for flow hoods) is a squirrel cage blower (also called shaded pole blower).
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MeltingPenguin
Pooh-Bah


Registered: 08/29/01
Posts: 2,138
Loc: new england
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Re: Is an inline duct fan or for a flow hood? [Re: veda_sticks]
#10324162 - 05/11/09 08:25 PM (14 years, 9 months ago) |
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The filter is housed in an aluminum box. There's two inches behind the filter, and a 12 inch hole set up to take a 12" vent in the back. I think it's meant to be a drop down filter, but i've used this size 4" thick filter before as a horizontal hood with great success.
I'm just wondering if anyone has any ideas for setting this up, or if there is such thing as a low profile squirrel cage that could fit inside the 2" space, or hang partially outside on the back without making the whole thing back heavy.
Or should i just take the filter out of the aluminum and build a wooden one?
-------------------- Growing anything is good for the soul
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RogerRabbit
Bans for Pleasure



Registered: 03/26/03
Posts: 42,214
Loc: Seattle
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Re: Is an inline duct fan or for a flow hood? [Re: MeltingPenguin]
#10324214 - 05/11/09 08:41 PM (14 years, 9 months ago) |
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Quote:
MeltingPenguin said: Well I already have a 4" filter. It's 24"X48" enclosed in aluminum with a 12" hole on the back to attach a duct. It's rated 720 CFM flow at 0.50 inches.
That's a filter, but you're not going to get laminar flow from it even if you found a way to hook a motor up. At that flow and that static pressure, there's no way it's going to give laminar flow. Look for a flow hood HEPA, which will be at least 5 1/2" thick. Most are 7" to 11" thick. You'll want to build a plenum behind the filter roughly as large as the area of the filter itself. This pressurized plenum is how you obtain laminar flow. You'll want to pressurize the plenum to .9" to 1.2" WC. If you ever put that much pressure behind the filter you describe, it would blow far too hard for mycology use. RR
-------------------- Download Let's Grow Mushrooms semper in excretia sumus solim profundum variat "I've never had a failed experiment. I've only discovered 10,000 methods which do not work." Thomas Edison
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anevsky
Dilettante



Registered: 07/30/08
Posts: 143
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Re: Is an inline duct fan or for a flow hood? [Re: RogerRabbit]
#10324290 - 05/11/09 09:03 PM (14 years, 9 months ago) |
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My filter is 6" in depth but with twice the static air pressure. I have never seen a 99.97% or 99.99% filter with static pressure less than .8". So, you should probably check into the stats of your filter.
A centrifugal "inline" fan is fine as long as it fits the filter. You will need to see the graph of resistance vs CFM to get everything right though. A typical prefilter will be .1" of static pressure. One nice thing about a centrifugal is that they are often quieter and more energy efficient - but not always.. YMMV..
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