[TriLUG] OT: thermodynamics of A/C question

[Joseph Mack NA3T]

On Tue, 26 Jun 2012, Aaron Joyner wrote:

> Misting the HVAC unit seems initially very compelling, but 
> I'm not sure how effective it would be in the very humid 
> climate of the south east.

540cal/g is the same no matter what the humidity is. The 
water/mist will be hitting coils at 160deg. Even if the air 
is saturated at 100deg, at 160deg the RH of the same air is 
low.

At high humidity, you will likely need a longer path for the 
water to flow over the coils, so it has plenty of chance to 
evaporate, rather than dripping to the concrete at the 
bottom. Presumably you're going to need some water dripping 
to the bottom, or the coils will become covered in boiler 
scale. So you'll need excess water.

> I'd love to see some data on effectiveness.  Looking 
> around on the web for a few commercial installers seems to 
> confirm my suspicions, they cite great performance in dry 
> climates such as NV, AZ, Southern CA, etc.

that's valid data. I wonder what the missing piece of info 
is to reconcile a constant heat of vaporisation and 
deployment only in dry places.

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.

> On the whole, I've enjoyed this thread more than any OT 
> thread in memory.  :)

I was heartened to find that some of the people who enjoy 
messing with linux are off messing with A/C units. Some on 
the mechanical side and some on the programming side. It 
seems that people who enjoy messing with something are 
prepared to mess with anything. I expect then if you ask any 
technical question on this list that someone would be 
messing with it too.


> On Tue, Jun 26, 2012 at 1:30 PM, Craig Cook <cncook001 at yahoo.com> wrote:
>> On the thought of misting your outside unit, this is one 
>> option I am following:
>>
>> http://opensprinkler.com

neat project

Joe

-- 
Joseph Mack NA3T EME(B,D), FM05lw North Carolina
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