[TriLUG] OT: thermodynamics of A/C question

Thomas Gardner tmg at pobox.com
Wed Jul 4 08:09:03 EDT 2012


On 7/4/12, Jeremy Portzer <jeremyp at pobox.com> wrote:
> [...]
>
> But Joe, the condenser unit already has a "radiator" to cool the
> compressed freon: - heat sink, fins to transfer the heat to the air, and
> a large fan, just like in your car.  What's the benefit of an extra
> cycle of water in addition to that?   You'd need a heat exchanger
> between the freon loop and the water loop - for what purpose?  The fins
> and fan do an adequate job transferring the heat directly from the freon
> to the air without the additional complexity of a water loop.
> Ultimately the heat has to get into the air no matter how many
> intervening cycles you add - unless you are talking geothermal systems
> or something else fundamentally different.

I think the idea here is to remove the heat from the freon more
quickly, and at the same time give the heat more time to dissipate
into the air.  If done right, the water in the secondary system would
be able to take on more heat more quickly than the air could have in
a more traditional air cooled unit.  While you're transferring that
heat to the water, the radiator would also be removing it from the
water and transferring it to the air at about the same rate as the
old completely air-cooled system would have.  However, the water in
the secondary system would act as heat storage, again, able to take
on more heat in less time than the air in the unmodified system.
Once the compressor shuts off, now you've got all this warmed up
water that has taken on all that heat from the A/C unit which you can
continue to pump through a radiator and blow air over to cool off to
prepare for the next time your unit cycles back on.

I don't think I'm explaining it well, but it does seem to me that
the compressor would run less, and the fan (which is now cooling
the water in the secondary system instead of the A/C coils directly)
would run longer.  The fan uses less energy than the compressor, so
I'm thinking the overall energy used should be reduced.

Of course, is it worth it?  That's the big question.  I have to think
it's unlikely to be, or manufacturers would be making such systems.
Also, if you're going to go that far, why not go the whole way and
put in a trench or well or something and do the whole geothermal thang?

Furthermore, I'm starting to really appreciate the effectiveness of
really good insulation:  I have the thermo in my shop programmed to
cool the place down in the wee small hours (as much as it can for a
half hour).  If I'm working out there, when I start to work up a sweat
(more from the labor than the heat, usually), I'll bump the thermo
down enough to run (current temperature minus a degree or two), and
whenever it finishes, I'll let it go back to its normal daytime preset
(85).  It stays cool enough to keep working out there into the evening
without coming on again.  I'm glad I did not go with geothermal out
there (I was tempted).  The builder did such a good job of insulating
that place that I don't think geothermal would have ever paid off.

I guess what I'm saying is, I doubt there's a lot of future in most
of this in the residential market.  As newer homes become better
insulated, the bang for the buck in doing this sort of thing becomes
smaller and smaller.

> [...]
> To get back to the spirit of the thread however, another easy to take
> action you can do to improve your condenser's efficiency is to simply
> clean the fins, so that they can transfer heat between the freon and air
> more efficiently.  See for example this link:
> http://blog.srmi.biz/energy-saving-tips/residential-air-conditioning-aircon-ac/cleaning-condenser-coils/

Indeed, probably the biggest bang for the buck.  Mostly because the
bucks are so small.

I'm wondering about using compressed air for this.  It sounds risky,
but I've cleaned off wicker furniture pretty effectively without
damaging it, and I'm thinking wicker is pretty delicate stuff.
Anyone have any stories about the safety (to the A/C radiator, I mean)
of using a blowgun for that?  It sure seems like it would be easier
than what this article tells you to do (mainly because you shouldn't
have to take anything apart), but I'd rather learn from someone else's
folly than use my own A/C unit as a guinea pig.  :-)

L8r,
tg.



More information about the TriLUG mailing list