Home Engineering Challenge #1!

[Ender]

Cooling A Terrarium! (yes, really)

My friend like to grow exotic plants - venus flytraps, tropical carnivores, etc. They are pretty cool. He recently moved from PA to TX, taking all his things with him. However, this has caused a bit of a problem. The plants, being tropical, need precise conditions to live in. In the more temperate climate of Pennsylvania a relatively simple application of mylar and lamps kept the temperature and humidity at the levels needed. In Texas, the higher daytime temperatures and night swings mean that the his setup can't maintain the conditions needed. He tried a setup that let him blow in cool air from a minifridge, with dismal results. So he asked for my help in setting up a cooling unit for this thing. Problem is that the engineering handbooks I have on hand don't have all the right equations, so my recommendations are only rough estimates. So I turn to those of you with the know-how, guts, or experience in jury-rigging and DIY projects with the first SDN Home Engineering Challenge.

First a few specs from him

I grow plants in a terrarium, and for them to be healthy I need a night time drop (optimally) to around 65 or so degrees Fahrenheit.

In my previous house, the ambient temperatures were always around 75 F (23 C) and I kept my terrarium down in the basement where the temperature would get to about 65 (18 C) at night with the fluorescent lights off.

I recently moved and am now in a house at around 80 degrees with no basement, and am about to go mad trying to figure out how to achieve a temperature drop to keep my plants happy. I recently tried a method I read somewhere about pumping air from a minifridge to the terrarium, but my results have been dismal.

Here's a diagram of my current setup.



Few basics, it's a 4x2.5x1.5 tank, 55 gallon. Acrylic sheets are between the lights and the terrarium (lights are about half a foot above). I have a humidity pump on either side keeping the humidity from 80-100%.

Swamp coolers are out because of the local humidity and the fact that evaporation cooling can't be relied on.

Now my recommendation to him was to build something like this



Using Type M 1/4" flexible copper tubing, with compression nuts and flaring to make joints where needed. Now as I see it, it needs some kind of 2 speed water pump so he can get the drop he needs at night. The issue is calculating the speed of the pump so he knows what to need, and the mass of water he will need for his heat sink in the fridge.

Now each plant is in an individual pot, and water build up from condensation is a concern as it can cause root rot. 3 of the 4 walls are covered by mylar, so that will impact heat transfer. So one also needs to take into account that - in 80-100% humidity how much condensation will there be, how will the flux from the light affect this, and what will he need to get the temp ranges he desires. Thoughts?

[Jaepheth]

Why not just build a double walled terrarium so that the interior is insulated enough that the temperature swings won't affect the plant as much?

Maybe have a thermostat inside that controls a 100 watt bulb and a mister/fan.

[Ender]

Why not just build a double walled terrarium so that the interior is insulated enough that the temperature swings won't affect the plant as much?

Maybe have a thermostat inside that controls a 100 watt bulb and a mister/fan.

Heat buildup; he still needs the drop at night.

[Akhlut]

For the humidity condensation problem: why not get an arbitrarily sized bucket of salt into the terrarium? It's cheap, it's plentiful, and if the salt gets overly saturated, it can be substituted out for yet more salt.

As for cooling, what about having the fan blow over the salt? That should pull the excess moisture from the air and should still cool down the plants a bit.

That's about all I have.

[Death from the Sea]

holy crap, they keep it at 80 degrees in their house? damn, dude turn on the AC!!!!

as for his problem, how about one of those fans that functions as a small cooling unit? something like this

[Vehrec]

Swamp cooler? Get a big old block of ice, put a fan next to it and circulate air over the ice. Keep the ice in a tray to make disposing of the water easier.

[Themightytom]

I've seen humidty and temperature control systems at the pet store for reptile tanks, They are probably more direct and efficient then using a minifridge or cobbling together an electrical deathtrap.

[Illuminatus Primus]

This sounds shady. I hope your friend is really growing venus flytraps.

[Lagmonster]

Not to put too fine a point on it, but the Venus isn't *really* tropical. I managed to grow them in peat moss pots on my kitchen windowsill in summer and winter them, well-watered, under in a plastic cone on my mantle, and they did fine. And I live in Canada.

[Darth Wong]

It sounds like your friend simply jury-rigged a refridgerator to turn it into a crude indoor air conditioner, and he can't use a swamp cooler because it makes that crappy humid air. So why not simply buy a professionally made indoor air conditioner, like this one? Then he can redirect the hot air exhaust hose out the window (and this particular unit has a built-in dehumidifier as well, which mists the water out through the hot air exhaust hose). Keep the whole unit in a small room with a closed door with the terrarium, or put them together in a tent.

The big problem with his scheme is that it's like using a cigarette lighter to do the job of a blowtorch. A mini-fridge is typically very weak; the one I had in university drew less than 100 watts running at full power, which puts rather severe limits on its cooling ability. The one I linked to draws around 8 amps, so it is obviously far more powerful. Refridgerators rely on the use of a small, confined, insulated volume of air, and I doubt that terrarium has anywhere near the isolation of the interior of a fridge.

In short, not enough environmental isolation and not enough BTUs. And it doesn't help that he is trying to draw air out of the fridge without also providing a return air channel for cycling through the fridge. As air pressure drops inside the fridge from his little hose/fan scheme trying to draw the air out without returning anything, the amount of air blown through the system will drop accordingly. He'd be better off just taking the door off the mini-fridge and making a small tent around the fridge and the terrarium with the back of the fridge outside the tent.

[Ender]

This sounds shady. I hope your friend is really growing venus flytraps.

I've stayed in his home and seen them. They are pretty nifty.

Not to put too fine a point on it, but the Venus isn't *really* tropical. I managed to grow them in peat moss pots on my kitchen windowsill in summer and winter them, well-watered, under in a plastic cone on my mantle, and they did fine. And I live in Canada.

He has more then just the Venus flytraps. I don't remember all of what he has, but some are kinda exotic.

Mike, thank you, I will pass that along, though I expect it will be too expensive..

[Ender]

His plantlog


ANd Mike, just to clarify

It sounds like his idea is mostly to setup a tent type thing that would be its own internal room, which would be cooled by the air conditioner to get the temperatures I was looking for, is that about right?

That is an accurate summation, correct?

[Darth Wong]

His plantlog


ANd Mike, just to clarify

It sounds like his idea is mostly to setup a tent type thing that would be its own internal room, which would be cooled by the air conditioner to get the temperatures I was looking for, is that about right?

That is an accurate summation, correct?

Pretty much. If you have a weak air conditioner, then the only solution is to reduce the volume of air that it's attempting to cool. His previous solution was critically flawed because of the lack of a return air channel. This would still be definitely inferior to a commercially engineered solution, of course. And I can't make any guarantees that it would be adequate, or that the fridge's operating lifespan won't be greatly reduced (it was meant to cool a much smaller volume of air after all, so its compressor will be running for much higher duty cycle than intended). But it would at least solve the most obvious problem with his previous solution.

[Ted C]

Would it be feasible to pipe all the output of a window air conditioner into the terrarium? That aught to be able to cool it pretty quick, and he might be able to get a used one cheap. Some even have timers. My main worries would be over-cooling and humidity control.

[Darth Wong]

Would it be feasible to pipe all the output of a window air conditioner into the terrarium? That aught to be able to cool it pretty quick, and he might be able to get a used one cheap. Some even have timers. My main worries would be over-cooling and humidity control.

It's the same concept, albeit with much more power than the mini-fridge but less control than an engineered indoor A/C unit simply cooling the entire room.

He could build a frame for it out of two by fours, make a tent in front of it, and then run the window A/C unit, with the back simply venting free inside the room. But as you noted, he would have to be careful that it doesn't produce wild temperature swings. If he's going to use a window A/C unit, he might as well find an actual window and then just try to cool down the whole room, if he can afford the resulting utility bills. From the sound of it, he's trying to do things on the cheap.