Opinions needed: Where to place fan in hood?

[Syris]

I am upgrading my lighting from VHO to (2) 175w MH on my 75g. Should be seeing my MH today
I am halfway thru building my new hood and I want to put 2-4" fans on the side of the hood blowing in (back of hood will be 90% open). Where should these fans go height wise? The hood will be 11.5" tall.
Should the fans go low on the hood so to cool the surface of the water or high on the hood to cool the MH bulbs?
Which is most efficient to control from heating the tank?

TIA

 

[Garry thomas]

Syris , i run 4x 6inch fans in my 11ft hood. I'v got mine up high to cool the halides. It tends to cut down on evap. since putting my 400's on the tank actually runs for longer without the fans coming on, but i think that is down to the fact that the hood is now semi open , as before the hood was sealed. :roll: Also i run some fans over the sump as well, they all help.

 

[King Jason]

Syris , i run 4x 6inch fans in my 11ft hood. I'v got mine up high to cool the halides. It tends to cut down on evap. since putting my 400's on the tank actually runs for longer without the fans coming on, but i think that is down to the fact that the hood is now semi open , as before the hood was sealed. :roll: Also i run some fans over the sump as well, they all help.

How many gallons of topoff do you go through each day?

 

[MarkO1]

Since heat rises, I'd put them near the top.
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[King Jason]

You could also put vents on the top of the hood so the heat can escape through the top of the tank.

 

[Syris]

Jason,

I have an DIY auto topoff system that pumps from a 5 gal bucket which I fill every 4 days, so that would be 1.25gal/day for evap?

I already put some "vents" in the top of hood to help cooling.

Do most people cool their bulbs (i'm new to MH) or there water evap for heat control?

I would like to avoid a chiller.

 

[King Jason]

Jason,

I have an DIY auto topoff system that pumps from a 5 gal bucket which I fill every 4 days, so that would be 1.25gal/day for evap?

I already put some "vents" in the top of hood to help cooling.

Do most people cool their bulbs (i'm new to MH) or there water evap for heat control?

I would like to avoid a chiller.

Two reasons to cool the bulbs:
1. Keeps hood temp down, causes evaporation, cools tank.
2. Cool bulbs last longer (from my experience)

 

[danmhippo]

Whatever fan you got won't be able to make ripples on the water surface, especially the way the fan are angled. If your fan is to suck air out of the canopy, I would mount them high. If your fan is to push air into the canopy, I would mount them lower. Blowing cool air directly at the MH bulb may cause the light spectrum shifts.

If you want to avoid chiller, then water evap is the only way to go. You can increase water evap with fans blowing sump surface.

 

[Syris]

I did some more research and I found what danmhippo said. Not to have fans blowing directly on MH bulbs because cooling of the bulb will shift the color spectrum of it.
Found this info in http://reefkeeping.com/issues/2002-06/jg/

...do not blow cool air directly over the lamps, as bulb cooling will result in inefficient evaporation inside the arc tube and abnormal spectral variations of the bulb will result. It is important to remember that these bulbs are designed to operate at a pre-determined temperature.

Based on this and others opinions I plan to put the fans lower on the hood as I want them to be blowing in. I will also add a clip on fan to my sump to help heat dissipation.

Thx.

 

[sammystingray]

Evaporation sucks, but it's needed. The more water to air contact, the more evaporation, and the more cooling. Excellent idea to take advantage of the surface area in a sump, and cool it there as well. I run VHO, so don't have major heat issues, but they do try to raise it about 8 degrees, which is 90 degrees and too much. I agree with the statement, of course, that heat rises, and my fans are unattractively over the lights blowing up. Blowing across a heat source and then to the water would be light pointing a hair dryer at the water. Since the lights are actually MUCH hotter than the water, the air blowing by the bulbs heat might actually HEAT the water if the air is heated enough by the lights heat. I believe the best setup is to remove the heat in whatever fashion possible to not have the bulbs heat reach the water, and fresh room air should be what is drawn in to flow across the waters surface.......just my thoughts....