Trying to cool a new highland terrarium

[Shimi]

I'm trying to build a new highland terrarium, using air blowing through cooled water to lower the temperature inside the terrarium.
The terrarium is built of styrofoam, with fluorescent lights above it.
The water cooling approach is based on projects such as https://www.cpukforum.com/forum/index.php?/topic/28146-my-cooling-unit/ and https://www.terraforums.com/forums/...33385-introduction-highland-grow-chamber.html.
I have a small fridge, inside which I placed a water-reservoir and pump. The water (actually a mixture of 2/3 water and 1/3 anti-freeze) is fed through a copper coil which is placed inside the fridge's freezer, after which the cold water exits the fridge and flows through a radiator located inside the terrarium. A fan blows air through the radiator. The water then flows back to the fridge and will (supposedly) cool down again. I copied some photos of the setup below.

My problem so far: the air blowing through the radiator isn't cool at all, which probably means the water isn't being cooled.
I noticed that when the pump isn't working, the copper coil is very quickly frozen and I need to stop the fridge from working so that the water inside it melts and can flow freely.
However, once there is water flow, the copper coil is not cold to the touch and as mentioned above, the air after passing through the radiator isn't cold at all.

Perhaps I need to replace the water+anti-freeze mixture with 100% anti-freeze, and lower the temperature to the lowest possible? Am I missing something else? Any other suggestion on what should be done to make this work properly?
Any help would be appreciated.

 

[Av8tor1]

Shimi, I got your message..

a problem i see is the copper tubing. The liquid in it is likely in laminar flow and the only heat exchange taking place is a small percentage of your total flow through the tube (the boundary layer against the tube). If you notice,your radiator has multiple tubes that are probably flatten to increase surface contact area. Also it causes the flow to be turbulent . Plain copper tubing also has very little surface area on it's exterior to dissipate heat.

laminar flow is good for airplane wings in flight but not so good for heat exchanges.

I would change out the copper tubing with another radiator to begin with. If that doesn't increase efficiency enough then you will need to look at thermal capacitance. In other words, use the cooler to cool a reservoir of water and then use the water to cool the radiator. That way you are banking the cooling until needed and increasing efficiency and capacitance . (Consider how car engines are cooled, you have a radiator that stores a significant amount of liquid that is allowed to flow through the engine when needed, giving it thermal capacitance.)

hope this helps,

https://sciencestruck.com/laminar-vs-turbulent-flow

https://www.britannica.com/science/boundary-layer

 

[adnedarn]

Yep, I agree with Av8tor1. What came to my head when I saw the photos was that there wasn't enough contact time on the cold side. Is that 3/8" or 1/2" copper? I think my approach to fixing it would be to reduce that to 1/4" and divided to 2 longer copper lines then recombine to leave the freezer. That would allow more water to cool (without leaving the center warm to mix with the cooled water). Think about water lines and how the smaller ones are the first to freeze when it's cold out. The two 1/4" lines would probably add restriction to flow to slow it down too, further increasing contact time.

OH! Also do not use more antifreeze than suggested for best freezing protection. Antifreeze actually sucks on its own at temperature transfer. When I was an auto tech we had someone put 100% antifreeze in and it would overheat super fast. Was very interesting to diagnose...

 

[Shimi]

Thank you both for the detailed response and the suggestions.

I'll start as Av8tor1 suggested by replacing the copper coil with a radiator (perhaps a larger one of 240mm and not 120mm?) and see if that helps. If it doesn't and if I do go in the direction of a larger, cool reservoir of water, I'll probably need to make more/larger changes to the setup (i.e. another pump).
Will update later on, hopefully with good news

Shimi

 

[adnedarn]

If going with the radiator, I would definitely add a fan- doesn't have to spin fast just enough to make sure the air within the radiator is changed out as it warms from the warm water flowing through it so it can continue to cool the fluid. It's basically the same concept I suggested but in a smaller more compact form where warm air could hide even if the air in the cooler is kept cold.
Good luck.

 

[Shimi]

You mean add a fan to the radiator inside the fridge?

If going with the radiator, I would definitely add a fan- doesn't have to spin fast just enough to make sure the air within the radiator is changed out as it warms from the warm water flowing through it so it can continue to cool the fluid. It's basically the same concept I suggested but in a smaller more compact form where warm air could hide even if the air in the cooler is kept cold.
Good luck.

 

[adnedarn]

Yes. Warm water will be flowing through that (since the cold has been extracted on the other side) so since it's got all the fins contained in a tight place you want to dissapate that heat that will be there into the cooler allowing the water to be cool again.
It'll probably work without that, but it will surely work better.

Sent from my SM-G960U using Tapatalk

 

[Shimi]

An update after some modifications: I added a radiator inside the fridge and submerged it in water; I also replaced the fan connected to the radiator inside the terrarium with a stronger fan by Noctua, per another tip I got (airflow is indeed much stronger).
Due to the additional radiator inside the fridge, the water supposedly spends more time inside the fridge where it can cool down again.

When the cooling system (pump+fan) is turned off for some time, the parts inside the fridge are cold to the touch (e.g. cables, water reservoir, etc.).
When I turn the cooling system on, cool air does indeed flow through the radiator inside the terrarium for a few minutes; however, relatively quickly water isn't being cooled (anymore/enough) and the components inside the fridge aren't cold to the touch anymore. Bottom line is that after some short time, it doesn't cool anything.
I guess that the cooling inside the fridge isn't enough to counter the heating outside it.

Any thoughts?

 

[Av8tor1]

Aquarium chiller based systems are the only DIY option I've seen that worked well short of using a chest freezer with a Plexiglas top.