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Thread: Lights, Heat, Fans and Venting

  1. #26
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    Quote Originally Posted by Psychojam View Post
    I'm not sure the diameter of the exhaust hole would matter. The important factor is the volume of air moving, not the flow rate. If you move the same volume of air through a large hole and a small hole over a given period of time, the air moving through the small hole has to be moving faster.
    The consideration here is not the speed at which the air flows but that pressure (forced movement) is not lost. Volume of air moving in will be relative to the number and speed of the fans. To keep it moving and push all the way down the lights, you don't want the air dumping out too early through too big of a hole. Thus having the hole on the opposite side. Size and location are only to keep the pressure and flow direction controlled.

    MythBusters. Take a spitwad and place it in a toilet paper or paper towel roll. Blow as hard as you can and see how far the spitwad goes. Next, take a spit wad and put it in a straw. Blow with just as much lung capacity and force and see how far it goes. In both cases, I doubt your lungs would change much size (volume) or speed (flow rate) for how much air you blew out. But, controlled through the size of the hole you are blowing the spitwad would travel farther in a smaller diameter tube. You can futher test by using the straw cut in half lenghtwise (i.e. not a straw anymore but a long C shaped thing ) and compare to give an example of an open back hood.

    Quote Originally Posted by Psychojam View Post
    I didn't think of the eddy current issue, but since the inside of a canopy is a relatively small volume, I wonder what impact this issue would have anyway.
    Eddies pooling heat pockets will form anywhere there is an obstruction since heat is being created by the lights that would also be creating the obstructions. Canopy size and volume is all relative since the size of the fans is relative and small as well. What is out of relative proportion to normal thought is the amount of heat crammed in to such as small space due to the lights. Little kids play ovens cook with lights less powerful that what we stick above our precious fish tanks. So in consideration, effeciency for an enclosed hood should be considered unless going with a chiller and forgetting about cooling at the source rather than just dealing with the cause. I have several relatively simple fluid dynamics simulations that can be done to show this but I think you all will have enough fun with spitwads for now.
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  2. #27
    Grand Master Reefer CarmieJo's Avatar
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    Papa, what a great analogy.

    Now class, I don't want to find any spitwads on the ceiling!
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  3. #28
    Assistant Moderator Skurvey Dog's Avatar
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    Lol! So many great minds in here, the brain vial is gonna explode! It is a shame that we are not filthy rich, solved the energy crisis and world hunger. We had a Picto Challenge, maybe we could of had this heating issue researched and put everyone's theory in motion and see which way produces the best results as they are all good ideas!


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    Assistant Moderator rayme07's Avatar
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    Lol! So many great minds in here, the brain vial is gonna explode! It is a shame that we are not filthy rich, solved the energy crisis and world hunger. We had a Picto Challenge, maybe we could of had this heating issue researched in virtual reality
    LOL In time lori in time well all have thousand gallon tanks and solved the crisises in time LOL and thanks I do have a great mind just dont think of useing it all the time
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  5. #30
    Insightful Reefer R. Deschain's Avatar
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    I see your point Papa...thanks. Nice post!

    I know that most hoods have air blowing in one end and out the other. I've just never considered the details/nuances of it all.

    The size of the hole thing still bugs me though. I'm having flashbacks of Physics class in college, (I was a microbiology major) and I'm trying to remember the Bernoulli principle. Again, I'm thinking about the sum total number of air molecules that flow out. Regardless of the speed at which they move, or the pressure, its moving from in to out. The spitwad may not pop out of the TP roll, but it will move out. For example, wouldn't it be ideal if the hood over your tank was open at both ends, thus making the biggest holes possible?

    Is it that you need to maintain a certain pressure to keep the air flowing in the right direction?

    Please clarify if you can so I can understand. Forgive me if I'm beating a dead horse, or if I'm being dense/obtuse. These are just the thoughts banging around in my wee brain.
    Last edited by R. Deschain; 08-28-2008 at 12:56 AM.


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  6. #31
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    Quote Originally Posted by Psychojam View Post
    I see your point Papa...thanks. Nice post!
    Is it that you need to maintain a certain pressure to keep the air flowing in the right direction?
    Right. And though I took college physics in high school, I am using less brainy experience rather than theory. I have long forgot the names to most of the principles and theories I learned almost 16 years ago.

    In my occupation in IT I deal with heat loads in server rooms, big fish tanks. The heat sources are the servers in racks lined up in rows. And if I was to use the same logic applied for a single rack or the server design itself, the concept is the same. You want to maximize the fans efforts by properly channeling the cool air flow in. A server blows cool air in the front and hot air out the back. If you take the lid off then the cool air comes in and only goes so far before it looses pressure. Then towards the back hot air just sits and only uses normal convection principles rather than forced air movement. In those cases, components farthest from the fan stay hot pooled in their own heat. Same thing goes for lights in the fish tank. Now the 1.5 is an estimate of size to start with since we are pushing cool air in that will heat and expand to some amount before leaving. The optimal option may likely be to open the entire side off the tank hood on the other end, but that would not look pretty. So at that point, it becomes a matter of balancing form and function. 1.5 would be a starting point. Oh, and I am thinking about fans that are most likely 4 inch square as the ones I bought for my 90 gal that is approx 48 inches long. That would have 32 sq inches open coming in and 48 sq inches for venting. That is a big hole so form has to be considered in balance with function.
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  7. #32
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    Having one in and out out moves cooler air into the hot chamber and the other pulls it out creating a cross breeze of sorts.








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