re: frugal ac tips
5 aug 2002
>>>>if nick's house was an automobile...
>>but it isn't.
>you picked the wrong section of my post to quote for your response.
should i have picked the section in which you implied that
heatflow depended on grant money, or the one in which you
implied that 12 watts is a large percentage of 1000 watts? :-)
>>here's a quote from kreider and rabl's book heating and cooling of
>>buildings (mcgraw hill, 1994):
>> the performance of air-cooled condensers can be improved if the
>> air-side surface is kept wet with purified water. evaporation from
>> the condenser will enhance performance markedly because the driving
>> potential for a coil cooled by evaporation is the wet-bulb, not the
>> dry-bulb, temperature. since the wet-bulb temperature is 15 to 25 f
>> (8 to 14 c) below the dry-bulb temperature, evaporative condensers
>> will operate at temperatures substantially lower than those of
>> air-cooled equipment and somewhat lower than those of water cooled
>> condensers employing a cooling tower. low condenser temperatures
>> result in lower compressor power needs and longer compressor life.
please note the last words above.
>...and i think you'll note that i said evaporative systems do have
>their place. i never said that evaporative systems wouldn't work...
if you say so.
>if you go back to my initial post it was saying that spraying water on
>coils wasn't a good idea because of the buildup of calcium and crud.
>> the key consideration in the design of evaporative condensers is
>> the water composition. if minerals are not controlled, they will
>> accumulate on the condenser sufrace and foul it...
>well, i guess you are saying you agree with me by quoting this.
i've never disagreed.
>>goswami's 1993 asme paper...
>> the performance of such systems can be improved by employing
>> indirect evaporative cooling such that no moisture is added
>> to the supply air stream. this can be achieved by evaporative
>> cooling of the outside air before it passes over the condenser
>> coil, resulting in a larger overall temperature difference
>> across the heat exchanger and thus greater cooling effect.
>and cooling the air around the entire house in this manner can improve
>performance of the ac and reduce the delta t across the structure.
this seems like an impractical non-sequitur. with an average 8 mph
july windspeed in phila, cooling the air around an entire house "in
this manner" would require a huge swamp cooler box or an impressive
artificial jungle with a serious water supply. how would you cool
the roof? you might enclose your house in a large dome with a few
openings at the top and the bottom, but that would create another
problem: solar heat.
>> the above problem can be eliminated if an alternative system, eg a
>> media pad evaporative cooler is used...
>in case the point hasn't been driven home enough, this is saying that
>spraying water on the coils is not the best idea.
no, it's saying media pads are better. trickling rainwater on the coils
seems like the best idea. simpler than swamp cooling them, with better
performance, since water vs air contacts the coils.
>>on a typical day, they measured 2.9-3.0 kw of compressor power without
>>and 2.4 kw with evaporative cooling, with an eer increase from 9 to 11,
>>ie a 22% system performance increase.
>i can certainly agree that in a non-humid climate such a box will
>reduce the temperature of the coil to near the wet bulb temperature,
mechanical engineering professor d. y. goswami, phd, p.e. did this
in florida, with support from the florida energy extension service.
he is the director of the solar energy and energy conversion lab at
the gainesville campus of the university of florida. he's written 10
papers since 1998 on liquid desiccant hybrid solar air conditioning.
>what i find missing from all this is the reduced lifespan of soft
>steel sheet metal, aluminum fins (notably susceptible to corrosion)
the aluminum greenhouse kits i mentioned last a long time.
>as always, there are trade-offs. can the cost of energy saved over
>the reduced lifespan of the unit be more than not only the cost of the
>water, but the cost of constructing the swamp cooler box and the
>financial loss from the reduced life of the ac unit?
of course. then again, kreider and rabl say "low condenser temperatures
result in lower compressor power needs and longer compressor life."
>i think this equation will vary over different climatic areas. in new
>orleans where humidity is high, performance will suffer to the point
>of ineffectiveness, making the cost of construction a near total waste
>of time and money.
it might not work on the moon either.
>some people might not like recycled plastic 55 gallon drums sitting
>underneath their windows, or wiring up a bilge pump. some people
>might not want to encourage mosquitos, frogs, and snakes by
>constructing a swamp cooler on the ground around their ac.
it might attract life-threatening alligators in florida.
>while the idea is interesting, i'm not about to rush right out and buy
>a bilge pump.
my $9.99 11 watt 41287-ivga "mini-submersible pump" from harbor freight
tools is plugged into the into the same power strip as the window ac...