On 5 Jun 2004 09:58:51 -0400, wrote:

Chris J... wrote:

The reason I want to install an Evap cooler is to reduce my electric
bill because using my present AC can cost up to $200 a month.

I'm surprised you spend so much at 7,000 feet in Northern AZ.

That was my total electric bill, so I'm guessing $140 for the AC, but
that was an unusually hot month. I also could not open windows as we
were in the ash fallout area of a major fire, and my screens would not
have stopped all the ash blowing around (that hung around for weeks
after the fire). A more normal usage for June or July would be $80 for
the AC.

NREL says
July is the warmest month in Flagstaff (el 7005', with an average air
pressure of 11.4 vs 14.7 psi at sea level), with a 66.3 F 24-hour average
daily temp and 50.5 and 81.9 average min and max and humidity ratio

Correct, Flagstaff is at the same elevation, and it's only 80 miles
away, but it's a little warmer here due to fewer trees and also
airflow up from the nearby low deserts. My warmest month is usually
June. (Monsoons start in July, and often have a massive cooling
effect)

However, the evap cooler wouldn't be much use in July or August, due
to very high humidity.

w = 0.0081 and a 97 F 30-year record high. Prescott (el 5023') has 57.9
and 73.1 and 88.1 average temps and w = 0.0091 in July. Phoenix has 81.0,
93.5, and 105.9, with w = 0.0105 and a 30-year record high of 118.0.

Prescott is only 15 miles away, and I'm on average ten degrees cooler,
but wind direction affects that a lot. BTW, your data is old; Phoenix
had an official high of 123 over a decade ago. I was there.

Opening windows at night might do a lot,

That's one big reason I want an evep cooler and air filters; It's
often 30 degrees cooler at night, but there are lots of tiny flies in
this area that can get through window screens and they are attracted
to coming inside, both day and night, which often precludes open
windows during the summer. Pollen and Dust are also an issue, though
not as serious. So, I'd use the cooler for non-evap air inflow quite
often, basically as an alternative to opening windows.

or reducing indoor electrical
energy use or the amount of sun that shines in windows,

Sun in the windows is a big issue; I've got lots of them with a
Western exposure. I have thermal windows in most places, and vertical
blinds that I close, but some windows aren't practical to cover, due
to being 20' off the floor.

or increasing
the thermal mass of the house. At 10 cents/kWh, $200/mo is 67 kWh/day,
which could move 200 kWh or 682K Btu/day of heat, with a COP of 3.
$70 is about 10K Btu/h.

Thermal mass seems to be fine; The home has a very low heat loss or
gain over time when the AC or heater is off. I've got a great deal of
brick, granite, marble and tile, which helps.

With no indoor electrical use or sun into windows, you might cool
a 5K Btu/F house thermal mass with a 200 Btu/h-F house-to-outdoor
thermal conductance (RCday = 25 hours) to T (F) at dawn and warm it
to 75 F at dusk, where 75 = 81.9+(T-81.9)e^(-12/25), so T = 70.7.

If 70.7 = 50.5+(75-50.5)e^(-12/RCnight), RCnight = 62.7 hours, so
R = 62.7/5K = 0.01253, eg a mass airfilm resistance of 1/5K h-F/Btu
in series with a 1/cfm fan resistance, with a tiny 81 cfm fan or
open windows at night. With an 8' height difference and an average
23 F temp diff, 81 = 16.6Asqrt(8'x23F) (using one empirical chimney
formula) requires an open window area A = 0.36 ft^2, eg a 3" slot
in each 2' window.

My heater/AC is in a utility room on the lowest floor of my home, and
ten feet from a wall that divides the living area from a dirt and rock
floored crawlspace (the air in there is cool and dry; no moisture in
the crawlspace at all.). The crawlspace has twelve-foot ceilings in
that area, and I plan on installing the evap cooler in the crawlspace.

If the ambient vapor pressure Pa = 29.921/(1+0.62198/0.0081)11.4/14.7
= 0.2983" Hg, 100(Pw-0.2983)/(Tw-66.3-460) = -1 (using Bowen's 1926
formula) at the average wet bulb temp Tw (R). If Pw = e^(17.863-9621/Tw)
(using a Clausius-Clapeyron approximation), Tw = 9621/(22.47-ln(556.1-Tw)).
Plugging in Tw = 510 R (50 F) on the right makes Tw = 516 on the left,
Plugging in Tw = 516 R (56 F) on the right makes Tw = 512 on the left,
then 514.8, 513.1, 514.2, and 513.5 R, ie 53.5 F, or less, if the cooler
runs more at night.

You might install it on top of a water reservoir (eg a plastic 55 gallon
drum) and cool the house indirectly with an indoor fan-coil unit that adds
no humidity to the house air,

I wouldn't mind some extra humidity; the air is extremely dry right
now.

I'm very interested by the fan-coil concept you mention, though. I've
never heard of it.

or evaporate some water inside the house to
make it cooler still. A $150 2'x2' MagicAire SHW2347 duct heat exchanger
or a $35 used auto radiator and fan might have a water-air conductance of
00 Btu/h-F. With 53.5 F water and an indoor heat gain of 10K Btu/h, it
might keep the house at temp T, where (T-53.5)800 = 10K, so T = 66 F.

You might improve the cooler controls by adding a room temp thermostat
and a Honeywell RH sensor that turns off the fan-coil when the room is
comfortable and a differential thermostat that turns off the cooler when
the water inside the cooler is warmer than the reservoir water. You might
also improve the water-air ratio inside the cooler: too little airflow,
and the RH inside the cooler rises to 100%, with no evaporation and no
cooling; too much airflow, and the water temp inside the cooler is the
same as the outdoor air temp, with no cooling. Weather conditions change,
but this air-water ratio doesn't, in most swamp coolers.

The thermostat is a great idea, I'll definitely do that. I'm going to
have to study all the math you cited, though, as it's a bit beyond me.
However, thanks very much for posting it; I'll definitely look into
it.

For more heat storage, you might sprinkle a 4' or 8' cube or cylindrical
rock gabion with water at night, under an updraft fan.

I need something automated, or I'll never remember to do it. Also, I
can't see the rock Gabion you describe being that effective in a 4000
Sq. Ft. house?