[TriLUG] OT: thermodynamics of A/C question

[Joseph Mack NA3T]

On Wed, 4 Jul 2012, Jeremy Portzer 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?

My initial posting was to ask

"if you're moving a fixed number of BTU/day, is it better to 
move them when the air is cooler, eg early morning 5am, 
rather than running the A/C full blast mid afternoon, when 
the outside air is hottest"

The assumption was that the A/C is a Carnot machine and in 
the early morning the condenser would be operating at a 
lower temperature. From the Carnot cycle efficiency formula, 
I expected I'd get 50% more efficiency.

No-one had an answer to this, and none of us know if an A/C 
operates at Carnot cycle efficiency (I suspect it doesn't), 
but we've had fun discussing other aspects of A/C as a 
result.

However if it's more energy efficient to have your condenser 
at a lower temperature, then you want your condenser 
operating against the lowest temperature heat sink. Air 
isn't great for transferring heat. If you had the condenser 
in a countercurrent heat exchanger with water, then the 
condenser would be operating at air temp, 80-100degF rather 
than at 160deg.

> And of course we've already discussed the cooling tower 
> systems that commercial A/C's use and the pros/cons of 
> those:  they aren't cost-effective for small-scale 
> systems.

evaporative cooling has been discussed. A couple of people 
have talked about sprinkler systems to cool the condenser in 
home A/Cs, so some people think it can be scaled down. 
No-one has shown that evaporative cooling can't be scaled 
down. It might be true, but I haven't seen any numbers 
showing it. Whenever someone says "it can't be done" and has 
no numbers or proof, I assume they just don't want to do 
bother doing it and are making excuses.

> 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/ 
> .

they tell you that the fins are dirty even if they look 
clean. Strike 1. They don't have numbers for effectiveness 
($, efficiency) of their suggestion. Strike 2. I expect this 
is an infomercial flogging their books.

Joe

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