|
dfwerydfhg
Stranger
Registered: 05/04/20
Posts: 194
|
|
Oh and in theory, there's no minimum velocity for laminar flow. Obviously at a certain point air will only be coming from some parts of the filter, which is not good for maintaining an even flow profile. But as long as every part of your filter is working evenly, it'll be laminar. But as I say above super low velocities are not desirable because air movement in the room, your movements, etc. will then have an easier time getting into your laminar zone and fucking things up.
|
sandman420
Saint PP



Registered: 06/17/04
Posts: 5,384
|
Re: Flowhood questions [Re: ouuwee]
#26837858 - 07/22/20 09:56 AM (3 years, 6 months ago) |
|
|
Quote:
ouuwee said: I saw your updated math template thread but i'm confused on what static pressure I should be using before adding the prefilter. Is it 0.8 wg or 0.45wg? Because if I start at 0.8wg and add ~0.4wg from the prefilter, I get 1.2wg and most motors I've looked at doesn't have the required CFM at 1.2wg
Also, can you tell me what furnace filters u use for prefilter?
its around 0.45....you need to calculate the SP at 100 fpm you had the right idea. Most filters have a rated SP not at 100 fpm and it gets all these people tripped up buying too much fan.
Edited by sandman420 (07/22/20 10:06 AM)
|
sandman420
Saint PP



Registered: 06/17/04
Posts: 5,384
|
Re: Flowhood questions [Re: Gan]
#26837861 - 07/22/20 09:57 AM (3 years, 6 months ago) |
|
|
Quote:
Gan said: I've always wondered... is the upper limit of the fpm flow capped because eventually at higher flow velocities there is no longer laminar flow? Or is it just because at high velocities it will blow shit around and make it a pain in the ass to work unless you nailed everything down?
Laminar air flow can only exist at air speeds of around 90-120 FPM, anything more is not physically possible to be laminar.
Edited by sandman420 (07/22/20 10:06 AM)
|
dfwerydfhg
Stranger
Registered: 05/04/20
Posts: 194
|
|
Out of interest I just ran the numbers- for room temp air in a 24" x 24" channel, at ~0.5 m/s (~100 FPM), the Reynolds number is actually firmly in the turbulent regime.
But what is actually happening, is that because the filter is pleated so deeply, the actual flow area of the air through the filter is some multiple of the channel area. So as it passes through and exits the filter, the air is still in the laminar regime. Then, as it exits it will begin to transition to turbulent flow. But this takes some time to develop (usually 10+ channel diameters), so for the distances involved in flow hoods it's irrelevant.
Edited by dfwerydfhg (07/22/20 10:22 AM)
|
RoscoeReturns
Crotchety chode man


Registered: 02/12/18
Posts: 1,738
Loc: State of Confusion
Last seen: 2 hours, 50 minutes
|
|
Another thing to consider is that the filter efficiency drops at increasing airflow. Even in HVAC hepa filter banks, where laminar flow was not a consideration, we routinely kept airflow at or below 150fpm. Anything above that, and we would see “leaks” during integrity testing. No leaks were present, and once velocities were reduced the filters passed testing.
|
sandman420
Saint PP



Registered: 06/17/04
Posts: 5,384
|
|
you my old homie roscoe?
|
ouuwee
Garden of Weeden



Registered: 03/22/20
Posts: 297
Last seen: 20 days, 4 hours
|
|
Quote:
sandman420 said:
Quote:
ouuwee said: I saw your updated math template thread but i'm confused on what static pressure I should be using before adding the prefilter. Is it 0.8 wg or 0.45wg? Because if I start at 0.8wg and add ~0.4wg from the prefilter, I get 1.2wg and most motors I've looked at doesn't have the required CFM at 1.2wg
Also, can you tell me what furnace filters u use for prefilter?
its around 0.45....you need to calculate the SP at 100 fpm you had the right idea. Most filters have a rated SP not at 100 fpm and it gets all these people tripped up buying too much fan.
Thank you for clearing up the confusion I had. I've looked at some filtrete prefilters and they supposedly have a 0.2 wg resistance. What MPR/MERV should I get? With that said, and rounding up to 0.8 wg, it seems like the dayton 1TDT8 matches well with the 24x24 HEPA filter. At 0.8wg, I get 520 CFM. I'll be getting a replacement 1TDT8 to save some money. Can someone else confirm that this blower will work before I go ahead and purchase it?

I was able to find someone locally selling brand new 3-pack filtrete furnace filters 16x25x1 for $10..... https://www.amazon.com/Filtrete-Allergen-Reduction-Delivers-Throughout/dp/B07GFSKDKM Would these work as prefilters? MERV rating is 13/Filtration level 2200 and has a pressure drop of 0.21
Edited by ouuwee (07/22/20 05:58 PM)
|
|
|
You cannot start new topics / You cannot reply to topics HTML is disabled / BBCode is enabled
Moderator: Shroomism, george castanza, RogerRabbit, veggie, mushboy, fahtster, LogicaL Chaos, 13shrooms, Stipe-n Cap, Pastywhyte, bodhisatta, Tormato, Land Trout, A.k.a 1,205 topic views. 32 members, 213 guests and 35 web crawlers are browsing this forum.
[ Show Images Only | Sort by Score | Print Topic ] |
|