Aw crap! I was reading the damn thing backwards. I go now to hang my head in shame and embarrassment 
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Understanding air flow
- Thread starter hoosierdaddy
- Start date
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- Tags
- air flow
Right on, Haggis!
:friends:
We can use what we now know about fan configuration to help us in many situations.
Take for an example; our box has a 250cfm fan at the exit. We are drawing our intake air from a duct that leads to outside air...and we have no way to increase the size of the duct, nor can we add an additional duct. We are stuck with what we have for intake.
Now, the intake we are stuck with is just slightly under the size it needs to be to satisfy the air flow. We know this because when we shut the door, we can hear the fan get louder, and it sucks in on the door a bit.
Adding an additional fan in parallel is not going to help the situation at all. We already know the fans cannot increase the static pressure in parallel, and since we cannot add additional air intake, they cannot produce any additional air flow.
BUT...adding an additional fan in series (stacked) will indeed increase the static pressure, and will help to increase the cfm as a result.
The secret of joy in work is contained in one word - excellence. To know how to do something well is to enjoy it.
-Pearl S. Buck
Take for an example; our box has a 250cfm fan at the exit. We are drawing our intake air from a duct that leads to outside air...and we have no way to increase the size of the duct, nor can we add an additional duct. We are stuck with what we have for intake.
Now, the intake we are stuck with is just slightly under the size it needs to be to satisfy the air flow. We know this because when we shut the door, we can hear the fan get louder, and it sucks in on the door a bit.
Adding an additional fan in parallel is not going to help the situation at all. We already know the fans cannot increase the static pressure in parallel, and since we cannot add additional air intake, they cannot produce any additional air flow.
BUT...adding an additional fan in series (stacked) will indeed increase the static pressure, and will help to increase the cfm as a result.
The secret of joy in work is contained in one word - excellence. To know how to do something well is to enjoy it.
-Pearl S. Buck
Last edited:
HempHut
Active member
Static pressure, that's the concept I've had in mind, but not the term for it.
That was the premise on which I was basing the concept of the "helper" fan inbetween the two chambers. I was working with PC fans, and as we know they are poor when they hit resistance, so that's why I thought about an interim fan to ease the load on the exhaust fan. In essence, "stacking" them but with one at each end of the chamber instead of right next in line with eachother.
Excellent thread with lots of good input. Thanks guys.
That was the premise on which I was basing the concept of the "helper" fan inbetween the two chambers. I was working with PC fans, and as we know they are poor when they hit resistance, so that's why I thought about an interim fan to ease the load on the exhaust fan. In essence, "stacking" them but with one at each end of the chamber instead of right next in line with eachother.
Excellent thread with lots of good input. Thanks guys.
HH, no matter where you put the additional fan, be it at the entry hole, or middle hole, it will be essentially the same as staking it directly on top of the one at the exit hole.
Only when we increase the intake flow to accommodate the fan power, will we increase efficiency. A fan alone in any configuration will not help to increase air flow. It must be accompanied by the proper amount of air.
Only time stacking a fan will help is with an already poorly designed system.
And you can only get to the top cfm rating of the original fan. You cannot exceed the cfm rating without increasing the air flow intake/exhaust.
Only when we increase the intake flow to accommodate the fan power, will we increase efficiency. A fan alone in any configuration will not help to increase air flow. It must be accompanied by the proper amount of air.
Only time stacking a fan will help is with an already poorly designed system.
And you can only get to the top cfm rating of the original fan. You cannot exceed the cfm rating without increasing the air flow intake/exhaust.
BackToTheLab
Member
Very useful, thanks all
Dr Watt
Who What
Another aspect to airflow is tendency of air to flow in channels. The air really sticks to the walls in my 2.5'x2'x6' cab - the temperature on the wall is 22c but 27c over middle of bud canopy. I must point out that the cooltube inlet is quite close to the wall due to being in small box.
I've tried to point passive inlets away from walls but it looks like it'll need an active circulation fan. Anyway the situation highlights how careful you have to be with measuring heat.
Another aspect to airflow, I'll venture, is that it has limit on ability to cool when excessive watts are in a small volume - I've found increasing the volume of cabinet had a dramatic effect on lowering temps and is another variable that should be considered when forecasting temps (over light watts and fan cfm alone) such as with this formula CFM= (1.78xWatt)/DiffC or CFM = (3.2 x Watt)/Diff F
I've tried to point passive inlets away from walls but it looks like it'll need an active circulation fan. Anyway the situation highlights how careful you have to be with measuring heat.
Another aspect to airflow, I'll venture, is that it has limit on ability to cool when excessive watts are in a small volume - I've found increasing the volume of cabinet had a dramatic effect on lowering temps and is another variable that should be considered when forecasting temps (over light watts and fan cfm alone) such as with this formula CFM= (1.78xWatt)/DiffC or CFM = (3.2 x Watt)/Diff F
johnstreet said:Another aspect to airflow is tendency of air to flow in channels. The air really sticks to the walls in my 2.5'x2'x6' cab - the temperature on the wall is 22c but 27c over middle of bud canopy. I must point out that the cooltube inlet is quite close to the wall due to being in small box.
I've tried to point passive inlets away from walls but it looks like it'll need an active circulation fan. Anyway the situation highlights how careful you have to be with measuring heat.
Another aspect to airflow, I'll venture, is that it has limit on ability to cool when excessive watts are in a small volume - I've found increasing the volume of cabinet had a dramatic effect on lowering temps and is another variable that should be considered when forecasting temps (over light watts and fan cfm alone) such as with this formula CFM= (1.78xWatt)/DiffC or CFM = (3.2 x Watt)/Diff F
A logical and reasoned approach. Well said!
Namaste, mess
C
Cozy Amnesia
With all this talk about setting up two fans in series or parallel...check out these fans (go to page 12): http://www.hvacquick.com/catalog_fi...alog.pdf?PHPSESSID=ogjfk91msic78d32hehk6ge9r5
Anybody ever used these fans before? I just ordered the TD-125 for less than $100 shipped.
Anybody ever used these fans before? I just ordered the TD-125 for less than $100 shipped.
B
badugi
Excellent thread.
BlueberryTerry
Member
awesome post!!
Hoosier, dude, I really can't thank you enough for this post. I was absolutely stumped before now and really just running on guesswork a lot of the time. My only complaint is that I didn't see this earlier.
I do have a question if you don't mind. If I run my 150mm cooltube in the correct orientation for good light spread in my unfinished cab, the open intake end of the cooltube is only 50mm from the wall.
I was thinking this wouldn't be enough but after reading your thread and thinking outside the box, I got some string and roughly measured the inner circumference of the cooltube. Lets call it 45cm. Would I be correct in saying that the erm, "size" of this section of the airflow is equal to a hole 5cm x 45cm?
I do have a question if you don't mind. If I run my 150mm cooltube in the correct orientation for good light spread in my unfinished cab, the open intake end of the cooltube is only 50mm from the wall.
I was thinking this wouldn't be enough but after reading your thread and thinking outside the box, I got some string and roughly measured the inner circumference of the cooltube. Lets call it 45cm. Would I be correct in saying that the erm, "size" of this section of the airflow is equal to a hole 5cm x 45cm?
Thanks for the kind words, folks. Glad I could help.
ScrubNinja, yes you are on the right track.
Your 15 cm tube needs 176.68sq cm of air flow, and your gap provides ~47cm x 5cm = ~235sq cm.
ScrubNinja, yes you are on the right track.
Your 15 cm tube needs 176.68sq cm of air flow, and your gap provides ~47cm x 5cm = ~235sq cm.
Incredible! You would never think it just looking at it. I had even started a thread here because I was so worried about it. But that is awesome, keep spreading the knowledge.
A very helpful thread for me, as I'm currently building a tiny little NGB-style box - up until now, I've been going on what I've picked up from other builds when it comes to inlet size.
I'm trying to build a cab.. and ran into a little math problem. My cab or I should say micro cab is 4.8 sq ft. I was looking into getting 50 cfm with a 3" duct: http://marketplace.hgtv.com/Product.aspx?Lid=4841-FV-05VS1
I was thinking about putting 8 1" inlets? I feel like I didnt do the math correctly.. yeaa any help would be greatly appreciated. hopefully i can get every set for my gro-op.. hehe
I was thinking about putting 8 1" inlets? I feel like I didnt do the math correctly.. yeaa any help would be greatly appreciated. hopefully i can get every set for my gro-op.. hehe
