Physics Question
 

I'm building a canopy for my tank and am installing two Ice Cap 4" Canopy Fans. Which is more efficient for cooling: drawing the warm air out of the canopy or blowing air on to the surface of the water?
I'm thinking drawing air out would be, but I'm not sure. Suggestions?

I vote drawing air out.

I vote push just to be difficult, but seriously if you are looking at cooling the water then I would think blowing cool air directly onto the waters surface might be more efficient as opposed to sucking air from random air leaks in the canopy.

blowing fresh air over the water is by far more efficient. The whole point of fans is evaporative cooling. fans pointed at the water will evaporate more water, therefore it will cool better. is this a closed canopy? Is there any reason you can't have one fan drawing fresh air in and directing it at the water, and have another fan drawing hot air out?

The Reef Aquarium Vol 3 recommends blowing air in because sucking air out will likely cause a lot of corrosion on the fan.

I can tell you for sure after I look over a doc at work (brain trust back National hired a Engineer and one of the things they had him studying was having a muffin fan blowing in, or the fan blowing out). Might as well get some of your tax money back.

Personally from looking at the fans I have around the tank/sump, probably better blowing in, rather than having the moist salt air being drawn past the fan. Hey, might be the difference of a fan lasting 5 years rather than 3.

+5 pushing air in, better cooling and fan does not corrode as fast

blowing air across the surface of the water, so pushing I would say

:deadhorse:

(yeah, I went there AGAIN whatigot)

It is a closed canopy. I thought about installing them this way originally, but was unsure if it was the right way to go.

Two fans, one each end. 1 pushing air in the other blowing out. my current canopy is set up this way and both previous custom canopies I have done have also been set up this way, cools the lamps and keeps the heat away from the surface of the water. If you don't have the fans yet I do have a spare 4" variable speed ice cap fan, for you I make to be good deal ya.

:doh: The fans just came in today. When am i gonna learn to call you first? I sometimes think you have enough equipment to open your own store.

That would be funny if it weren't true

I'm pretty sure I have a couple of ice cap fans sitting around here somewhere not doing anything too :lol:

Oh fer Chrissake!

Don't feel too badly it would probably take me a week to find them. I've had them for 3 or 4 yrs and never used them

It all depends on what your trying to do. But if your just trying to remove canopy heat from the lights, Pull. But fan will get corroded. The Push Pull config will work great on a fairly sealed canopy. That is, if the canopy doesnt have enough air leaks to match the fans CFM. But the total air exchanged that way is 1 fans CFM rating. If both fans are in the same direction, ie Blowing out, the CFM is the addition of the 2 fans, more air volume.. If wanting to cool the water itself, through evaporation, then blowing on the water obviously is best.

This is how i have mine setup as well. I have had no problems my canopy is about a year old. I have aqua medic fans and they have stayed quiet since the install and i have seen no signs of them rusting or salt creep.

Keith

It's about flow... if you push air in or pull air out... how's the air being replaced. The design with 2 matchedfan pushing and pulling is the best idea I think...

I have a closed canopy over a covered 75g with two 400w halides, and two bulk 4" fans from Princess Auto.... One pushes air in, the other draws air out at the other end... I have NO heat issues, my tank has only ever peaked at 83 degrees, when our air conditioner was not on for a few hours... during the day the temp is about 79 on average through the year.

If you can keep the heat away from the water... you wont have to cool the water.

For a techincal point of view
 

What would be best really depends on what you are trying to cool.

There is a lot of things to take into consideration here. CFM of the fans, how well the canopy seals and the air volume of the canopy itself. But here is a few ways to look at it.

Assuming reasonable air sealing and the "usual" amount of air space.

First option -- 2 pushing fans. 2x the airflow, and some air escapes through leaks. The air pressure would be slightly higher in the canopy, and thus you would have a higher heat transfer to the air. The fans blowing across the water or not, don't matter all that much if the volume of air is high. it is a safe assumption that the exiting air would be equal in temp to the water, and thus no more cooling could occur.
Interesting Enough, if the air volume is very high and the CFM low, you would acutally increase the heat transfer from the lamps to the water by the high air denisty and circulation.
This option also works well because you can put an air filter in front of the fans and reduce dust buildup, which insulates very well.

Second option -- 1 push , 1 pull. 1x the airflow, and equalized air pressure with outside air. Not, really any great bonus to this setup except for more control over the entry and exit points of the air (IE, no dust or dirt buildup around air leaks).

Third option. -- 2 pulling, 2x the airflow. The option works better if you are just trying to cool the water. The air pressure decreases, which would normally lower the heat transfer from the lights to the water (think a wannabe thermos). However, this option also increases evaportation of the water (things boil and evaporate easier in low peressure environments). So, you would cause your light to run a bit hotter, and transfer less of that to the water, and evaporate more water out. But only if the canopy seals well enough and has a small enough air space to acutally lower the pressure enough. (which I have not run the numbers on).

Of course, with the fans pulling, you do wear them out quickly. It's not so much an issue for me, as I have a perpetual supply of computer fans and 12v power supplies. But for anyone paying for fan, that might be an issue.

If you want to get real technical, it's the evaporation that accounts for most the the tank cooling effect, rather than the acutal temperature difference of the water. Very much so with the 0% humidity of a kamloops summer.

Now, If I could just find a cheap supply of 30cm 12vdc fans I would be in heaven. Finally a quiet fan :biggrin:

Unless this is a canopy for a 10g tank, i can't see these fans doing anything significant enough to actually drop air pressure to the point that evaporation increases and heat transfer decreases.

Thanks everybody for the advice. The canopy is to fit a 72x30 tank. I'm going with the draw at one end and blow at the other. Thanks, again.