[TriLUG] OT: thermodynamics of A/C question

matt at noway2.thruhere.net matt at noway2.thruhere.net
Wed Jun 27 11:05:12 EDT 2012


I was discussing this thread with a coworker this morning and I mentioned
periodically spraying the condenser unit with water to reduce the
pressure.  My coworker said that he has done that on occasion and he could
hear the difference in the motor as it caused the load to back off
substantially.  He then said that he used to have a window unit that the
condensate collected into a basin and that the fan was designed to go
through the water and mist the coils helping to cool them.

These next few days are going to be exceptionally hot (101-103F).  It may
not be a bad idea to spray your condenser occasionally to cool it off. 
This would reduce the load and allow it to better reject heat so that it
can in turn pull more heat out of the house.

We were also talking about why more systems aren't water cooled.  We could
see two problems with water/evaporative cooling.  One would be the
requirement of a makeup water source and the maintenance required.  How
many people even was their condenser coils as it is?  The second, and
really bigger problem, is that of water chemistry.  Corrosives,
alkalinity, and bio compounds would all be problematic.  You could treat
the water but after so many cycles a blow down (and chemical disposal)
would be required, followed by the addition of fresh chemicals.  For
medium to large systems, the efficiency would more than make up for the
costs, but it isn't so practical on small systems.

In terms of air cooling efficiency with regards to wet bulb, I agree that
this would be interesting data and I would like to look into it at some
point.  I assumed (correctly, or incorrectly) that it had to do with the
temperature approach in that a water cooled system can achieve a lower
point (wet bulb) than an air cooled (dry bulb).


> I wonder why A/Cs aren't water cooled. You might say "what's
> the difference? You're dumping heat from water at 160deg
> into air, which came from freon at 160deg, rather than
> directly dumping it from freon at 160deg into air."
>
> The compressor is expensive. Once it goes, the whole A/C is
> gone. It works at high temperature and high pressure,
> shortening its life (or making it more expensive). Thermal
> efficiency and backpressure of the A/C depend on dumping the
> heat as fast as possible. Air isn't a great coolant compared
> to water. So dump the heat to water. Even motorcycle engines
> are water cooled nowadays, mainly to increase engine life
> AFAICT. Water cooling uses a low tech pump operating on the
> cold side of the water and a car type radiator with regular
> anti-freeze and a big fan (presumably the same size at the
> one in current A/C units). If the water and freon flow
> counter current (opposite directions), then the freon
> emerging from the freon/water exchanger and entering the
> compressor, will be at air temperature.
>
> The difference with water cooling: the compressor operates
> at lower temperature and pressure.
>




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