Hey guys, Is there a way to calculate airflow loss through different angled bends? Basically, I'm making a light-proof vent that is a bunch of stacked "Z" shaped aluminum pieces with 1/8" spacers between them. I read that airflow loss is minimal, but I've also read otherwise and common sense tells me there will be some loss. Is there a way I can calculate this so I can adjust the vent sizes to compensate for the proper air flow? As of now I am making them bigger than I need to (as in, bigger than the 2x passive intakes) to compensate, but I would really like to know some exact measurements for peace of mind. P.S. Here is the basic concept of my vent (side view): That is for 100% light-tight (between veg and flower) and I will be using just the V shape for passive intakes on the veg chamber: Would love to nail down these measurements! Thanks guys!
How much space will be between the pieces of material? I.E. will there be 1 inch of separation between them? Also, how wide is each piece? How many pieces will you stack? Those answers should be enough information to calculate the square footage/inchage(spelling? lol) of your opening. Then we can move onto the loss of cfm. Edit - Wait, do the 1/8" spacers run all the way down in between the z-shape pieces? I assume not and if they don't, you need to know how wide of an opening for each piece is open to allow air flow. Pretty much, you need to first calculate how much free area you will have for air flow. A good website to help you understand this is below. If you want to search for more information on the matter, you are pretty much creating a vent louver. I just googled "how to calculate vent louver cfm". http://www.archlouvers.com/Louver_Free_Area.htm
1/8" spacers between the pieces, the pieces are about 12" long (may be more like 11" though, after the spacers are in there).. I mean I can calculate all that, but how would you calculate the cfm loss?
I was a little late editing my post with some more information, but to calculate the cfm loss, you need to know how much "starting" CFM you have. How much CFM is your fan(s) rated for?
200 CFM (5" fan) And yeah I was gonna say, the number of stacks is dependent on how much CFM the vents can allow through.. so I guess I am aiming for 100CFM (I am making two of these for between veg/flower)
And this is to play it safe, I may even end up putting my fan on low, which is around 135 CFM.. but better safe than sorry! No one ever complained about too much airflow!
I'd love to calculate the same thing for the "single" passive vent (V shaped) to allow 200CFM to pass through (each stack, same dimension.. 11" x 1/8").. I imagine there is a much lesser loss of CFM with the single V instead of the Z shape.
Ok. So to calculate the free space for "each" vane, they are 11" wide by 1/8" tall. 1 / 8 = 0.125 so 11 x 0.125 = 1.375 Each opening is 1.375 sq. inches of free space. Now, lets say you want to use (10) of them, 10 x 1.375 = 13.75 now with (10) of them, you'll have 13.75 sq. inches of free space (for air to flow through). Now, you have a 5" fan. A 5" circle has 19.63 sq. inches of free space (area). If you are shooting for the same free space as your 5" ducting/fan, you'll want to divide the space of the 5" circle by the free space of each vane so... 19.63 / 1.375 = 14.2763636 You'll need at least 14.3 openings to achieve the same free space as your 5" ducting/fan. lol, but no such thing as a .3 opening , so to start, you'll need 15 openings at least. Now this doesn't calculate the loss through the bends. Give me a little to research that one.
Are you wanting to have the same size intake as your fan? Are you using your fan to exhaust the air? Are you using a fan for air intake? If you are using just an exhaust fan and making this "louver" to be used as a passive intake, it's generally accepted to use a passive opening of 1.5 to 2 times the size of the exhaust. I personally don't have too much experience with this, but most people want a little negative pressure in the grow area and immediately think they can achieve this by making the passive intake smaller than the exhaust. Again, I'm not 100% on this, but I believe that will make the fan work harder. I'm just asking these questions so we can find out how big of an opening you need for your intake. (Is this an intake at all or is it a passive exhaust?)
Yup.. the 200CFM fan is for exhaust, and all passive openings are 2x the area of the 5" fan, so if it had no air flow impediment the passive intakes should total ~50 sq inches.. but that's where my problem is.. in the impediment of airflow through these vents.
I originally calculated it assuming a 60% air flow and would have needed about 20-21 "stacks" - but that was just sort of guesswork
Disclaimer, I'm used to calculating from the ground up seeing what my CFM requirements are and not calculating the inefficiency of it. But I'm sure we can do it backwards. From what I've been reading about ducting, I see that you want to add about 50% - 70% for each 90-degree bend. Also, on a side note, you want to add about 10% for every foot of straight ducting. Not sure about the efficiency differences between smooth metal ducting and the flexible ducting, but I'm sure the numbers are pretty different... lol (smooth ducting I'm sure win in that contest). So, if you went with the V-shape, you would only have to calculate (1) bend being the hard 90-degree bend in the middle. You wouldn't have to add the (2) 45-degree bends because the air would come in upward into the vent at the 45-degree angle, turn 90-degrees down, and then would just simply exit downward out of the vent. Without even doing any math, I think you are very close if not right on with the 50 sq inch of net free space in your vent to not only give you twice the amount if incoming airspace, but to also make up for the inefficiency of the louver by adding a few extra openings. Let's see... As I said earlier, a 5" circle (hole) has 19.63 sq. inches. Double that so you have (2) times the amount of incoming air space then exhaust and you have 39.26 sq. inches Now, from using my very basic formula of calculating the inefficiency of the 90-degree bend in the vent louver, I multiply the 39.26 sq. inches by 1.5 (By multiplying by 1.5, I'm adding 50% to make up for the inefficiency of the bend). You could also just take 50% of 39.26 which just happens to be 19.63 and add that to the 39.26, but below is quicker. 39.26 x 1.5 = 58.89 I calculate that if you make your vent louver with 58.89 sq. inches of net free space, you'll have enough intake space to act as double your exhaust space. This means that your 58.89 net free space louver would be the same as making (2) 5" holes. Now, your openings are 1.375 sq. inches in each stack. Now, dividing the required 58.89 sq. inches by 1.375 will tell you how many "stacks" (sounds better than my "vanes" lol) you need. 58.89 / 1.375 = 42.83 or just round up to 43. That's a lot lol. Now, each stack is about 1/8" (0.125") high. 43 of these would make your entire vent about 5.375" high (not including the thickness of the metal for each stack). Sound about what you were thinking? ----------- On a side note, I was at the same place wanting to do a vent louver a couple months ago. I looked into the Doran/Adorama Light Proof Vent Louvers. Then I started thinking about how to incorporate a filter of some sort into it... It just got to be too much... lol... Have you thought about just doing an intake filter like the ones Phresh makes? (**I'm not talking about their carbon filters, but instead their intake filters**). They make them in 4", 6", etc... You could just install a duct collar on the inside of your space, and fix one of these filters to it. They not only filter the incoming air, but they also block the light... Just a thought. They even give the max rated CFM of each filter. Model #'s (701255 - 701295) http://www.phreshfilter.com/about/size-selector.aspxM
All the bends are actually 90 degrees, so there are the two main 90 degree bends and the small lips, which I'm not really counting. My guesstimation brought me at about 8"x12"ish for those ones. I was going to make two 4"x12" ducts for between veg and flower. Gluing the spacers on now so the openings are 11.5" I'mma have to come back to this and try to input those calculations you did (on my phone now) thanks man!
And I'm way less concerned about the passive intake in the veg chamber, I might just do something else for that, it doesn't even have to be light tight like between veg and flower
Ah, I missed that part. True. Let me know what you come up with. Can't wait to see/hear how it turns out. Are you making it now? Are you painting the stacks flat black or something?
Yup, I got a big stack of aluminum drip caps from Home Depot and cut them to about a foot or so long. Used gorilla glue to make them into the N shape and am in the process of glueing the spacers on all of them. I'm using those free Home Depot paint stirrers cut into little 1/4" or so pieces, and those aluminum drip caps are 99 cents for almost 4 feet long pieces, so it's a fairly cheap project. Definitely gonna spray paint them all flat black and stacking them, then framing them somehow, not sure yet.. I have black cloth duct tape I will probably use that somehow (Like lining the sticky side of the tape with the black side, but not as big, so the black is still the only thing showing on the inside, if that makes sense..) and make a nice metal frame for it.
Trying to calculate the double 90 degree bends, but I feel like there is less loss on the second bend after the air impediment on the first bend. I could be wrong though. Any idea on how to calculate that? Judging by the original calculations, having it end up being almost 6" high, I think I will still end up going for the 2 vents to be 4" thick (11.5" x 0.125" openings), so the free space area for each vent would be roughly 45 square inches, making both of them 90 square inches. Assuming these vents lose 50% airflow, that would still give me a little more than enough. I'm just not sure 50% is anywhere near accurate or not for 2 90 degree bends. At least the good thing about these vents is that I could always add more layers if needed!
Cool. If you are so inclined, you might want to pick up an air flow velocity meter. I'll be getting one eventually. You can play around with how well your vent louver performs before and after placing it in your intake hole. I would measure the CFM with just the hole, and then with the louver. You would know how much % loss of CFM you get. You could even mock up some baffles that would go in your square vent louver hole and maybe do one with a 5" hole, then another with (2) 5" holes. Then measure each and see what CFM you get. This one is about $55 and it does the calculation of CFM. You just have to plug in the area of the duct/hole in sq. inches. http://www.amazon.com/dp/B009TQ6ILQ/ref=wl_it_dp_o_pC_nS_ttl?_encoding=UTF8&colid=3VBXOPTS7NQXM&coliid=IDWSV5QT5N5MO
Oh wow I feel like that is an invaluable tool for us DIYers! I thought about getting something like this but assumed it would be a few hundred bucks at least. Very cool thanks!!
I was thinking about this more - is there a way to calculate the maximum CFM one of these slots can achieve? I'm still a little hesitant on what size to make them. I know the passive intakes are based on twice the exhaust hole opening, I would much prefer to dial this in to allow 100 CFM through each layered vent (so two vents would equal 200CFM, the max my fan can go.. plus my fan will most likely be running at 135CFM) I would love to figure out how many "layers" I would need for 200CFM and 135CFM As of now I'm still going for about 3-4" x 11.5" - which would be roughly 25 layers each, but I want to go as small as possible due to space concerns. I'm wondering if I can get away with 3" x 11.5" (3 of those metal shims are about 1mm, so I'm not sure if they need to be taken into account or not, I think it's better to figure out the max CFM allowed through one individual slot and just multiply that by the number of layers... if that is even how it would work, not sure if adding more layers would change this or not. I think, anyway, it's better to go smaller as I can always add layers easier than taking them away. I suppose if I get no solid answers I will go for 3", and the passive intake in the back 5-6" x 11.5" (only one 90 degree bend on that one, so there would be less loss I believe) Really have no idea about how to calculate this haha.