how should i place my fans??

[marinefish]

i am adding two 4 inch ice cap fans into my 15inch high cannopy (the cannopy has a complete open back side) with the 250 watt metal halide. The dimension of the hood is 20 inches (left to right), 13.5 inches (Front to back), and 15 Inches high (top to bottom). My reef tank is a 20 gallon and 17 inches high.
i am planning to have one fan pulling fresh ambient air into the hood and the other fan pulling the hot air out of the hood. the fans will be mounted directly on the opposite end of the hood (one on the right and one on the left side of the cannopy). my question is, should i mount the two fans closer to the metal halide (closer to the top of the cannopy) or closer to the water surface (closer to the bottom of the cannopy? Which will dissipate the most heat and prevent any future heat issues on my 20 gallon tank?
Also my second question is i was told that having any kind of fans pulling salty air out of the cannopy can really cut the life of the fan dramatically. I am confused on how i should orient the two of my 4inch ice cap fans. Should i have both fans pulling air inward or should i have one fan pulling air outward and one pulling inward?????

 

[dreeves]

If the back of the canopy is completely open...then what is or would be the purpose of the fans? Fans would need to channel the air either over a given surface or through an established route to cool an area. The flow of the fan needs to be routed for it to be effective.
If the canopy has a lense or something else between the lights and the water...you may be able to put the fans blowing right above the bulbs and out the back.
Otherwise...hmmm.

 

[broomer5]

If I were doing this - ideally I would place two fans in the ceiling of the canopy - both blowing out and up.
Heat rises.
Open back would allow room temperature air to be pulled in - warmer hot air would be exhausted out the top.
2nd choice would be one fan on both ends of the canopy. Placed high on the sides - both blowing out as well.
Air coming in the back - moving warm hot air up and away from the tankwater surface - is my choice always.

 

[sebae0]

1 fan on right and 1 fan on the left, 1 blowing in and the other blowing out seemed to work best for me. i still had a heat problem with only 1 on top pulling heat out, also i have my fans kinda blowing right at the halides. hth