[TriLUG] OT: thermodynamics of A/C question

[Matt Flyer]

On 06/23/2012 10:01 AM, Joseph Mack NA3T wrote:
> While it may be difficult to determine from the graphs, usage and
> drops in energy efficiency really do correlate heavily to wet bulb
> from our experience, to where it along with dew point have become
> accurate predictors of performance.
>
> this is for evaporative water cooling, or regular freon type A/C? (I'm
> sure you must have said this, but I've forgotten sorry.)
This is for regular type A/C.  We use a combination of machines that are
mostly R134a, but have a few left over R22, and even an R11 machine or
two that are tools of last resort.  The condenser side does make use of
evaporative cooling via cooling towers for efficiency.  Some of these
are building size structures while others are boxes about the size of a
couple of refrigerators.  The (hot) refrigerant tubes are bathed in
water which is then cooled rather than blowing air directly over the
tubes like a normal unit does, both otherwise it is the same.

The big electricity driver is being how low you can get the refrigerant
temperature on the hot side as this causes the pressure to go up which
is what the compressor needs to pump against, which takes horsepower. 
>From this perspective, if you can place the condenser in the shade you
will save money.  Just be careful to not restrict the air flow.
> I'd forgotten about this. I remember back in the 70's seeing
> solar/energy_efficient houses with large (3" ?) diameter columns in
> the house, full of water, to store heat/cold (depending on the season).
That is a rather neat idea.  I've seen discussions on how some places
are actually making ice at night, storing it, and then blowing air over
it during the day.  I've also seen that in places like Brazil, they
design the buildings so that cold air flows over the building steel at
night, cooling it down, and then chilling the warmer air the flows over
it during the day.

Another tactic, though I am not sure how well it would work in a
residential environment, at least without a capital expenditure, would
be to use variable speeds on the machines.  As power is a cubic function
of the speed, it is often times better to run two machines, slower than
one machine.