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Unfolding Nature Shop: Unfolding Nature: Being in the Implicate Order

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Flow hood Filter Resistance = 1 inch / 250 Pa. Why?
    #24940368 -

Hi guys,

I have been searching a lot around about flow hood projects and filters, and I realize there are always 2 assumptions made beforehand that guide the design of a "suitable" flow hood. They are: the air speed leaving the filter should be around 100ft/min; the filter resistance should be around 1 inch or 250 Pa. I am not questioning the first assumption (it is in Stamets book), but I really could not find where on Earth the second assumption came from (it seems to have come from RR, as far as I could track down).

To be clear, I am not questioning the fact that the filter should offer some resistance to the airflow (it will always have some resistance anyways). I am not questioning either the fact that some amount of resistance might be good in the formation of a laminar flow. In fact, I can imagine that if the filter's resistance is too low, there might be some inhomogeneity in the air pressure inside the plenum, which would make the airflow right outside the filter uneven.

But why 1 inch of resistance? Why not 0,5 inch? As it so happens, I am having some trouble finding 1 inch resistance filters (at 100ft/min air speed). It seems that modern filters tend to have lower resistances, and that is making the project more difficult, which, if you think about it, doesnt make much sense. Why would this people be making filters (which are basically used only for building flow hoods after all) that are less suitable for flow hoods than the older ones?

Furthermore, if you have a higher resistance filter, you need a more powerful blower in order to get the same airflow. So if you can relax the 1 inch back filter pressure assumption, and maybe get a 0.5 inch one, you could get away with a much smaller blower. So the fact is that whether 1 inch or 0.5 inch is enough is quite a big deal, and I dont see any evidence to support the 1 inch case.

Extras: Filter Print Post Top
Re: Flow hood Filter Resistance = 1 inch / 250 Pa. Why? [Re: vsoares]
    #24940718 -

Because we want laminar flow.

Thicker filters usually have lower resistance. (Because there's more surface area of filter material in the same flat surface area at the front.)

Thicker ones are made for scrubbing air but not laminar flow(think hospital HVAC)

1 inch initial resistance usually ends up being about 2" by the end of the life of the hepa. You want a blower that can handle the increased resistance over time.

At 1" w.g. we get into filters that will develop laminar flow when the right blower is attached.

The "evidence" is that 0.5" filters don't necessarily produce laminar flow when tested with an smoke wand.


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Extras: Filter Print Post Top
Re: Flow hood Filter Resistance = 1 inch / 250 Pa. Why? [Re: bodhisatta]
    #24942654 -

bodhisatta said:
Because we want laminar flow.

Thicker filters usually have lower resistance. (Because there's more surface area of filter material in the same flat surface area at the front.)

Thicker ones are made for scrubbing air but not laminar flow(think hospital HVAC)

1 inch initial resistance usually ends up being about 2" by the end of the life of the hepa. You want a blower that can handle the increased resistance over time.

At 1" w.g. we get into filters that will develop laminar flow when the right blower is attached.

The "evidence" is that 0.5" filters don't necessarily produce laminar flow when tested with an smoke wand.


Hi Bod, thanks for answering, by the way, your easy AF tek videos are great!

Yeah I know thicker filter usually have lower resistance, but that doesnt explain why people are making lower resistance filters nowadays (even thinner ones), if we absolutely need a higher resistance filter in order to get laminar flow...

Has anyone ever built a hood with a lower resistance filter in order to check if there is laminar flow or not?

Extras: Filter Print Post Top
Re: Flow hood Filter Resistance = 1 inch / 250 Pa. Why? [Re: vsoares]
    #24942884 -

While researching the topic I read that you need a filter with an initial resistance between 0.6 - 1.2 "w.g.

I made mine with a 24x24x6 filter rated 0.68"w.g. Built the box with an 8" plenum.
Works great, nice even flow across the whole filter.

Extras: Filter Print Post Top
Re: Flow hood Filter Resistance = 1 inch / 250 Pa. Why? [Re: foragedfungus]
    #24952010 -

Thank you foragedfungus, that will make my life much easier!

Extras: Filter Print Post Top
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Unfolding Nature Shop: Unfolding Nature: Being in the Implicate Order

Mushrooms, Mycology and Psychedelics > Mushroom Cultivation
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