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Pod tries to dumb down flowhoods * 1
    #27144577 - 01/12/21 09:52 PM (6 days, 2 hours ago)

Pod dumbs down flowhoods Date started: 01/09/2021 **My 50th Post HOORAH**
I'm going to assume you at least have an understanding about what a flow hood is and how it functions in general, we will expand on that general knowledge.

Hepa filters 
|_ About HEPA and why we use it in our Lfh.
“Willis Whitfield, Claude Marsh and Gordon King discovered that the air emerging from newly developed Hepa filters did so at a uniform and predictable speed, and that the flow could carry away particles in its path,” says David Brande, lead consultant with Cleanroom Project Management, Inc. “That phenomenon became known as ‘laminar flow,’” but that term is a widely adopted misnomer.” That makes sense but why? In comparison to our Ulpa counterparts a hepa filter is cheaper, lasts longer, and has less air restriction while filtering out 0.3 microns or larger. Perfect for our application. Anything below 0.5 microns tend to act more like gases in which case will most likely blow past if it manages to make it through the fibers of the filter. Among all other options Hepa filters does the job and does it well.
|_ Choosing a filter. Understanding size and why it matters.
You can and should build your flow hood around the filter you intend to use. LxH should be totally dependent on the size of the flow hood you are building and what you can afford bigger is better but remember to budget in a fan as well. Wider filters last longer and catch more airborne particles in exchange the static pressure climbs.
|_ The nerdy stuff about hepa filters  Static pressure? Inches of water? Pascals?
Static pressure (Ps) is the resistance against air flow, we are dealing with measurements on a small scale it makes the most sense use inches of water (w.g or wc), opposed to PSI which is what you're probably familiar with when you think pressure measurement a typical measurement would look like this [1" w.g]. Inches of water can be converted to the metric system (SI for nerds) unit know as pascals (Pa) and a typical measurement would look like this [500 Pa]
For Reference: 1" w.g = 250 Pa

The goal of 100fpm
Paul Stamets states that the air flow leaving the filter screen should be 100fpm (feet per minute or 30 meter per minute).
|_ What are you talking about?
The velocity of outgoing air leaving the filter should be 70-100fpm. This is the optimal exiting air velocity range to keep particles airborne and settling away from the work area.

Blowers
|_ Why such specific blower motors? What gives?
Centrifugal blowers are built for resistance, they deliver steady streams of air of a higher pressure in comparison to their axial counterparts that deliver massive volumes of air at a low pressure. Since you're putting the airflow up against resistance a centrifugal blower makes the most sense.
|_ Why not an axial fan? If the numbers match up, it should work, right?
The swirling air produced by the fan would make it hard to hold a steady pressure in that area, static pressure is there to maintain even pressure across the back side of the filter, which helps to ensure laminar flow out the front, with turbulent air on the back side of the filter, you're counting on the resistance of the filter to buffer the oscillations.
- RR (Inlines vs. blowers - https://www.shroomery.org/forums/showflat.php/Number/17725558)
|_ The nerdy stuff about blowers *Giggles in geek*
Blowers push out air, the volumetric flow is measured with the general term CFM for our purposes, this is different then Ffm which is the rate at which the air moves.
(For idiots, Cfm for volume, Fpm for velocity)

Keeping up? You should have a general understanding of filters, fans and how laminar air flow is achieved using them.

Cfm Calculation ***If you so choose to convert the measurements be sure to check that it's all been converted properly.
|_ Note beforehand
You shouldn't have to calculate anything.  Your filter should come with a chart.  Simply match the desired flow to the pressure drop(static pressure) and then then select a blower which will deliver that flow at that static pressure.
- RR (Understanding static pressure - https://www.shroomery.org/forums/showflat.php/Number/20192547)
|_ Solve for face area The filters front face.
To find out the surface area for the face of the filter find the dimensions and use the formula Length x Width = Area of Face ² ft
|_ Solve for required Cfm It's easy
You can solve for CFM by multiplying the filter area by the the air flow velocity goal (100fpm, you knew this. Why'd you look? :frown:).
Now you should have your CFM number, congratulations!

Matching all our numbers up
|_ That's it for the numbers?
Not yet, blowers aren't safe from big particulates. Like your bedroom fan, dust, hair and debris can collect quickly in your blower. This is why a pre-filter comes into play. Pre-filters take debris from the air before said air enters the plenum. The hepa only has to filter out what sneaks past making the blower and filter last longer. We need to take pre-filters static pressure into account but since they're around .5" - 1" thick, their resistance isn't much. We can assume the static pressure of this filter which will be .2wg while the static pressure of the filter of your choice should come with a static pressure rating.
|_ Ok, what do i do with these static pressure numbers now?
Add the prefilter and hepa filter static pressure together and you should have your resistance.
|_ Making it all work together
Take your Cfm and your resistance and find a blower that can deliver that air through that resistance.

You're done but here's an example to work through on your end to solidify it in your head.
Jake found a HEPA filter for his flowhood, the dimensions are 24" x 12", with the resistance of 1"wg, in order to achieve laminar flow what Cfm would Jake need from his blower?
A) 350 @ 1.2"wg
B) 200 @ 0.8"wg
C) 200 @ 1.2"wg
D) 350 @ 1.8"wg
Answer: (Do the work, when you do the answer is under the citation at the bottom)


Lazy Citation aka Sources
SP Scienceware. Cfm calculation explained https://www.belart.com/corporate/Resources/Fume%20Hoods%20CFM%20Calculation%20Explained.pdf
So you're building a flowhood....easy flowhood math template (05/18/20), P9hu7 - https://www.shroomery.org/forums/showflat.php/Number/26679345
ULPA Filter vs. HEPA Filter, hepacart (March 2020) - https://www.hepacart.com/blog/ulpa-filter-vs.-hepa-filter-whats-the-difference-why-does-it-matter
5- Fundamentals of HVAC - Pressure Measurement, Author Unknown - https://www.you-tube.com/watch?v=YmKhE1d1Wws
David Richardson (2014). An introducton to static pressure https://www.huduser.org/Datasets/IL/IL08/in_fy2008.pdf
Anemostat (Year Unknown). laminar flow https://www.anemostat-hvac.com/literature/laminar_flow_app.pdf
“Laminar Flow Hood - Build a HEPA filter flowhood” FungiFun, 14 Nov. 2020, http://www.fungifun.org/English/Flowhood
Static pressure, what is it and why does it matter? Dominion Service Company, 12 April. 2014, https://www.dscr.com/blog/static-pressure-important/
Wikipedia. 2020. “Inches of Water.” https://en.wikipedia.org/wiki/Inch_of_water
Wikipedia. 2020. “Pascal (unit).” https://en.wikipedia.org/wiki/Pascal_(unit)
Wikipedia. 2020. “Actual cubic feet per minute.” https://en.wikipedia.org/wiki/Actual_cubic_feet_per_minute
Answer: (C)


--------------------

Pod dumbs down: Flowhoods


Edited by Podamous (01/12/21 10:22 PM)


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