Re: DIY chillers

> From: "Jan Fidrmuc" <J.Fidrmuc at kub_nl>
> > From: JDAVIS at bio_tamu.edu
> > Subject: DIY Chillers
> [cut]
> > This water circulated through the 
> > frige and returned to the tank about 5 or 6 degrees cooler that the 
> > water in the sump of the wet/dry.  It wasn;t much cooling...but it was a 
> > little.  Now, if I would have used a 
> > freezer....hmmmmmmmmmmm.........
> Not that I have ever build a DIY chiller -- but perhaps you could 
> improve your fridge system by puting a large plastic container into 
> the fridge instead of a length of pipes. Depending on the size of the 
> fridge, some 25-50 litres could be more effective. That way, the 
> volume of water being in the fridge at any time would be greater that 
> with pipes. 
	Remember that the heat transfer will depend on the _area_ of
surface exposed to the refrigerator temperature, not the volume of
liquid in the refrigerator.  You really want a large surface area,
thin walls with good thermal conductivity and turbulent flow in the 
pipes, but you may have to settle for less.  If it is easy to get
the effect you want with sub-optimal design, why bother with the
fancy stuff?  .....finned tubes, forced air flow in the 'fridge....  :)

>  ..from someone else..
> John,
>   I would make a couple of suggestions which might improve the
> efficiency of your cooler.  
>   1)  Add a large container of water to the frige.  Bend a series of
> coils in your tubing and immerse the coil in the water.  Running the
> water through this coil will increase the heat transfer immensely by
> increasing the transfer surface area, and by using a water/plastic/water
> interface Vs air/plastic/water.  

	With this suggestion, the limiting factor may well be the
heat transfer between the refrigerator air and the container of water.
The water container would need a large surface area to make up for the
lack of flow in the container.  There would be convective flow in there,
but this would be weak.  The situation is worse than that in the previous
suggestion (two heat transfer surfaces in series, and near-stagnant
fluid in the middle)

>   2  Use metal if possible for the coil.  Plastic's heat transfer
> ability is poor.  

	Yes.  This would make a huge difference.

> Aluminum would be ideal.  Copper is questionable, but
> the pH and hardness of the water in your case may negate the problem. 
> As I recall, the higher the pH (over 7.0), the more difficult it is for
> the copper to stay in solution.  I believe the copper precipitates out,
> but I'm not sure if the precipitate would be available to plants.  If
> so, forget copper.  Any chemists out there with info on this? 

	I'm a bit hesitant to use any metals in my system.  I suspect that
aluminium is about as innocuous as you will get, because of the oxide
layer on the surface, but even there I would be concerned about exchange
of ions between the water and the oxide layer.  The trace elements
might end up being adsorbed to some extent.  This may reach equilibrium
quite quickly, and not be a problem.  I wouldn't use copper at aquarium
pH's - well maybe for pH 8.5, but not lower than that.

Paul Sears        Ottawa, Canada

Finger ap626 at freenet_carleton.ca for PGP public key.