mmslue79
In our tanks we use powerheads to move saltwater in all directions. We want a somewhat random water movement, or at least we want the water to move from down deep to the top, and vice versa. Having the water circulate is the goal.
Air movement is quite a different story from my viewpoint.
If you want to get RID of the warm/hot air, meaning get it up and away from the water surface - mount the fans in the top of the canopy blowing air OUT, and allow the cooler ambient room temperature air to be naturally sucked in at the bottom of the hood.
Having fans mounted this way will impart a negative static pressure inside the canopy. The actual air pressure will be negative near the ceiling of the hood ( near the fans blades ) and will be a postive pressure on the discharge ( on the outside top "roof" ).
This differential pressure in part, is what makes the air flow from inside to outside. It's the difference between the pressure inside the canopy relative to the pressure in the room.
The static pressure, combined along with the "flowing" air pressure is called "total" pressure.
Total pressure minus the static is flowing pressure.
Cooler air will naturally be drawn in down near the water surface by this negative pressure within the canopy, and the net result is hot air moving up and out.
Every exhaust hood I've every worked on at work operates this way.
When you want to get rid of something, be it fumes or heat - exhaust the air - don't blow it into the hood.
The lamps are going to get hot no matter how much air you blow across them ( within practical reasons of course ), and the goal should be to keep the tankwater from taking on heat from the air column above it - not to move hot air down towards the tank water surface.
Blow hot air out
Suck cooler air in
Evaporation of the tankwater is what helps to remove heat of the water. It will take place no matter what we do. Forcing warm/hot air down will only increase the evaporation rate.
This is my opinion on fan placement.
Four 4" fans in the top ceiling may be overkill, but certainly would move a lot of air up and out, and not blow it in your face as you walk by the end of the tank.
If you want more information - let me know.
Q = AV Where: Q = Quantity of flow in cubic feet per minute.
A = Cross sectional area of duct in square feet.
V = Average velocity in feet per minute.
