[TriLUG] OT: thermodynamics of A/C question

Neil L. Little nllittle at embarqmail.com
Wed Jul 4 09:50:20 EDT 2012


I have an LG window mount A/C unit that is designed to have the fan 
fling condensate water onto the fins of the cooling coils to increase 
cooling and evaporate the water. It even says so in the user manual.

73,

Neil Little, WA4AZL


Joseph Mack NA3T wrote:
> 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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