What size fan for an ac box?

[darkhollo]

I built a fully enclosed AC box... tried some smaller fans, but the only thing that makes it work like a charm is a 6" vortex on a fan speed controller.

I shouldn't have fscked around for so long.. kept trying to avoid the $$$.. but in the end. the vortex is what worked for me.

-dh

 

[Guest 18340]

hoosierdaddy, i completely understand your explanation. It makes perfect sense.
Im wondering now about the intakes punched around the sides of the unit. Will the fact that it'll be sucking hot garage air and pushing that hot air thru the back coils, will that affect the ac ability to cool?
I would love you use your deisgn as you described in the second part, instead of an open back to the box, an open top thats sealed to an equal size opening in the ceiling board. More than likely i wouldnt need a fan. But i'm now worried about the hot air thats already in my garage. Any thoughts?

 

[hoosierdaddy]

My cab is in my garage. No heat controls at all, and it has been in the 90's.
The air that goes through the condenser coils goes through the veg chamber first, but it is ambient garage air. The unit functions fine.

Your unit running like that will help to pull in fresh air charge into the garage whenever the unit runs. Be sure you have a way for fresh air to enter the garage, so as not to have a low Co2 enviro. I think you will find your garage with cooler ambient temps than if the ac were not running, since you will be sucking the hot air out and replacing with new air all the time.

Also, your application would be a perfect place to use Dow Supertuff R insulation board as your ducting material (put it together with Gorilla duct tape). It can get round 190 inside and will be cool to the touch on the outside.

 

[Guest 18340]

Yup, definately gonna use insulation board. My lowes has that blue foam like stuff. I'll be building all this within the week and will post pics and results soon. Peace, Evl.

 

[OgreSeeker]

I don't want to beat the horse to a pulp...but...

You know I say yes you need a fan, but in fact you do not IF you have the right size duct going into the attic.
Take my sketch for an example...

What I have is a piece of duct that has an inside area equal to the total area of the back of the unit. My duct is running straight out the back, and would need no fan at all to allow it to work properly. IF you were to turn this rectangle ducting up 90 deg. and into the attic, and especially since you are only going a foot or so, you would need no fan at all for it to function as designed.
So to bypass buying a new fan, all you would have to worry about is a ~1'x2' hole cut in the ceiling board, instead of a 6" or 8" hole.

This is the whole point I have been trying to make here. And you have a perfect example of a situation that would work just like I am saying it will.

*Imagine taking the top off of my ~1foot ductwork and placing it on the back so my flow is now to the top. What I would have is a slight bit of restriction from what it is now by adding the 90 deg turn. And then you would have the restriction caused by the friction of the duct side walls for the additional 1-2 ft it takes to get to the ceiling. The turn and the side wall friction is what creates the "head" pressure of the duct line.

I can even choke the size of the rectangle duct down a bit if I needed to, because as I stated earlier, the real restriction on the exhaust side is the housing enclosing the fan blades inside the unit, which on my unit is a ~10" (78.5 sq in) opening. What I have now is the back of my unit being 17" x 11",
which gives me a total duct area of 187 inches!
I need a min of 78.5 inches to know I am not restricting the air flow, and I have 187 inches. I am golden for a few turns and some footage at the size of that duct.

I don't know how to explain this any better.

Earlier you said that you have a firm grasp of the dynamics but you haven't mentioned enough about pumping loss / flow loss. This plays a huge roll when someone is designing/building an ac box and your underestimating it's impact.
Do you realize how large your duct would have to be in order to "not restrict" air flow in 20 feet of duct without an added fan? Now in your case of running it only 1' then you would be ok but I'm not aware of to many people that are using such short lengths of duct...usually it's more like 10' to 20'.
Pumping loss NEEDS to be factored in.


I'm still not getting why you would cool your ac with cooled room air (have the intake inside the room)...that's just crazy inefficient! Your sucking your cooled air right outta the room!!!!!!! WHY???

 

[hoosierdaddy]

Isolating an AC unit is a three part deal.
1) the intake, which must get adequate fresh air from somewhere.
2) exhaust, which must have adequate air flow to evacuate the hot air properly.
3) the cool side of things that recirculates it's own air.
These three separations must be made, and with no air flow restrictions, to get by without a fan.

Nobody has said that the cold side air is being used to cool the condenser coils. The ambient air used to cool the condenser coils is coming from his garage...the cool side evaporator coils are inside his grow area that is within that garage.

Lets assume you have a fan pulling on the end of the exhaust duct. The AC unit is off yet your fan is still pulling air through the condenser coils...
OK..now inside the unit the fan (that is not running now because the unit is off) is normally enclosed by the casing plastic. The diameter of this enclosure (let's assume a 10 inch dia. opening) should be the restricting orifice (smallest hole in the air path), and if it is your fan can only pull so much air at a certain velocity. This is determined by the size of that orifice (10") and the power of the fan itself.

When the fan is sucking on this air path, the ducting can create a bit more resistance that has to be added to the resistance the 10" orifice places on the fan. For a smooth surface duct, like aluminum or galvanized steel, the friction factor is very low for ducting in this size range. So little that a straight run can go a very long ways before it starts putting much of a factor on the added resistance.

Turns and twists can start to add up the friction, as can the different types of ducts, especially flexible ducting. But when we are talking about ducting paths of less than 20ft, the pressure and friction are very negligible.
What makes the most dramatic change of resistance is the size of the paths.

If you do the math, a 10" dia circular duct is about equal in size to a 5.5"x17" rectangular duct. If I keep my exhaust ducting at a size of at least 5.5" x 17" then I will not be adding any restriction due to the duct. None that will be of any consequence...

When you choke it down from there, say to a 6" duct, then you have dramatically changed the resistance in the flow. Your fan simply cannot pull what it could when the smallest dia opening was 10", so you either have to open the ductwork back up to at least 10" dia circle or a 5.5 x 17 rectangle, OR increase the power of the fan to get the same amount of air through the coils.

We need to keep in mind that the AC unit was designed to perform properly with the power of the fan it has pushing through the size of the fan housing it has. When you add a fan you are assisting the existing fan, and only need enough CFM to bring the unit back to original design flow.
The only way to figure this out ahead of time is to know all the things we know above, plus what the exact CFM of the existing internal AC fan is.

 

[Guest 18340]

OgreSeeker, my intakes (which are punched into the sides of my unit) are gonna be on the outside of my growroom. My growroom happens to be inside my garage, so the intakes wil be sucking in air thats in my garage, not the actual growroom. Kinda confusing without pics but i cant get my computer to let me download pics.
If you look at hoosierdaddys' diagram, those slots that are punched around the unit will be on the outsdie of my growroom. The very front of the unit will be in the growrrom. Just like if the unit was hanging out a window. My concern was with the hot air being blown outta the back of the unit, so i'm gonna build a box( just like hoosierdaddys') and plumb the back of the box into the attic.
I wish i could use my sketch pad
I agrre with you though in that it would be stupid to use put those intakes in the growroom.

 

[OgreSeeker]

evlme2,

I understand exactly what you mean. No need to draw a sketch LOL.
The intake on your ac will be drawing air from the garage...only the face of your ac will be in the room.
It's just that the above drawing is a little misleading and I was trying to understand why it was designed like that.
By looking at that drawing, I would assume that the intake should be inside the room. Now if there were something in that sketch to suggest a partition between the front/face of the ac and the intake then it would make plenty of sense (like what your doing in your situation).
Just trying to clarify.

 

[hoosierdaddy]

Ogre...dewd...
You really shouldn't have any confusion at all on this issue. Hell, you made the very same observation in the AC thread you have in your sig line, and I explained the whole thing, in specific detail, including providing you with a drawing to help with the explanation.
Am I missing something? I fail to understand why we spend all this time on the same issue? Are we good and roasted?

For the sake of any confusion that may lurk in this thread, I will offer up the sketch of how my AC unit is in my cab. You can clearly see that there are three divisions of air flow.
1) Ambient air coming in from the veg chamber (used to cool the hot condenser coils)
2) Hot air exhaust that pushes the hot air out
3) Evaporator coil side that recirculates air within it's own space.

 

[OgreSeeker]

Actually yes HD. I've been on Oxycodone for about 4 days now cause of my F'ing bad back. So I'm pretty tore down!!
Here's what I'm asking...
Say someone wants to build an ac box. They do a search and find this pic...

If they build it like the one in your sketch then they need to go threw a wall, right (so as to suck air from a sourse outside of the growroom)? You couldn't just sit this in your grow room and expect it to run efficiently, correct?


Am I right so far...



Well that's what I'm getting at. Many people need an ac box cause they don't have a window or wall to put it through so they have to go with a setup like this...

(the first link shows this ac installed threw the wall but the ventilation on the box is constructed basically the same as an enclosed ac that sits within the room - the intake vents are inside of the box and cool air is drawn threw the box and ac)

http://www.icmag.com/ic/showthread.php?t=87627&highlight=ac+box

or this

http://www.icmag.com/ic/showthread.php?t=57828&highlight=ac+box


If you have an empty area outside of your grow room to draw air from (such as a garage) and can cut a large hole in the wall then your setup is perfect.
I understand how you and evl are construcing your ac boxes and how they will work but the reason I initially said he would need a fan is because I assumed he was building a standard enclosed ac box that would just sit in his room.

Would you agree that this needs a fan?

Good.

I hope this helps you understand me understand you understood? LOL I had to say that. Ah shit...anyway

Everything is pretty clear to me. You know what...
We should create a thread on the different types of ac boxes, how they are used, why etc and document the construction and how they differ (fully enclosed ac box, 1/2 enclosed w/ outer intake etc).
You already have several useful sketches and lots of information...it would be a great addition to ICmag IMO.
What program did you use to make those sketches?

 

[hoosierdaddy]

Another poster took the liberty to use one of my sketches for his explanation of things, and did not provide any link to the original post it came from. As a result, my drawing was taken out of context and created the original confusion you had about the separation of air flows.

That is why I mentioned him doing it right off the bat, and suggested in the future he provide a link when using others material. I could see the potential of the thing being misunderstood by simply seeing that sketch.

To be honest, I would have had enough sketches in this thread by now a person could build a box no problem, but the stinkin system seems to be on the fritz.

 

[OgreSeeker]

No shit man...I've got some pics that I've been wanting to upload. I lost my PGP password years ago to 5 dvds full of my pics. I JUST FOUND MY PASSPHRASE so I'm excited about posting some of my grows but the dogdamn uploader isn't working!!

 

[CovertCultivatr]

Stanley/lasko blower!

 

[green_tea]

to add a quick question to the thread, is it OK to use the expanding foam stuff they sell to seal up any holes etc in the actual AC unit

edit: AC unit as in sealing how the ac unit itself seals the one side, since the ac i have isn't 100% perfect seal.

and some of the things its touching are the cold side radiator tubing, and some possibly touching the compressor motor? possibly touching the plastic cap on the top (i doubt that gets hot to begin with)

 

[green_tea]

to add a quick question to the thread, is it OK to use the expanding foam stuff they sell to seal up any holes etc in the actual AC unit

edit: AC unit as in sealing how the ac unit itself seals the one side, since the ac i have isn't 100% perfect seal.

and some of the things its touching are the cold side radiator tubing, and some possibly touching the compressor motor? possibly touching the plastic cap on the top (i doubt that gets hot to begin with)

any thoughts on this? ^^

 

[Guest]

Those window shakers are suspect. I've seen those things shut down on hot muggy days when there was a gently breeze reducing the fans ability to push air across the coil. I would not add any restriction to the exhaust or intake. The condenser fan is not strong enough to handle the pressure, that would mean it was oversized initially and therefore less efficient. If you have to do it your way, definity add a fan to the exhaust. If your garage is sealed tight, open a window for the intake, it is a two way street. I never heard you mention a condensate(drain) line so it is probably an evaporative model. You sure you want to pump all the hot moist air into your attic?

Spray foam? I'm not touching that. Why? because I don't know anything about spray foam.

I would definatley torture test this setup. Restrict the exhaust air, the compressor will eventually shut down or blow the breaker. Remove the restriction. When the A/c overloads and doesn't reset itself will your lights turn off or just fry the girls?

I'm afraid to ask about the electrical. Fire Extinguishers are not optional go get several. And no I'm not just being a DICK.

Grow long and prosper.

 

[Guest]

Would you agree that this needs a fan?

QUOTE]


Omg, I'm going to take that photo into work and tell them I put A/C in my grandma's house but it wouldn't fit in the window. Too funny.

Does it work? What size flex and what size fan? Did you seal the stinky interior compartment from the outside air compartment.


Captain Obvious

 

[green_tea]

no i know

I did a run today, found out that the holes i had thought the water leaked from well dont do that... so now I have to work on getting a tray to fit the entire area, or just putting pond liner there and adding a hose to drain to a separate area.


fuck though, the wood there is wet as fuck, and i have to xacto my way though all the silicone sealant so i can just remove the AC unit from the box...

Found the differential though of the intake air temp to the exhaust air temp of the AC box/unit.

intake air was ~80F, exhaust topped at ~120F.
(so 40F differential)

 

[Bubble Puppy]

Thanks for explaining all this you guys ,I couldn't understand why my unit keeps cycling off.The added duct is the problem. Great Thread ,thanks again!!