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re: tricky cooling problem
19 oct 1998
mike hardy  wrote:
 
>nick-help! is there a method of finding out or calculating [the
>temperature of the earth 3 to 4 feet down in morocco] apart from
>me going there and digging down:)

you could send me there to dig a hole and report back. i've always
wanted to go to morocco and try out my french on the unsuspecting
natives ("aieee! death to colonialists!") i'd like to see tattooine
too, or at least learn how to spell it.

the ground is probably close to the average annual air temperature,
a cool source in series with something like a us r-value of 10. but
you may quickly heat up the ground, if you try to use it for cooling
a building...

otoh, morocco is dry, so those exotic nights would doubtless be cooler
than days, and you might somehow save some nightime coolth for the day.
and some form of evaporative cooling may work well, eg those traditional
buildings you see pictures of in scientific american with wind scoop
towers and fountains or pools of water under the ground surface.

you might fill the theater with plants and pools and fountains, eg a
waterfall wall. perhaps one of the sci.engr.heat-vent-ac psychrometric
gurus can perform some elegant mathematical analysis and tell us the
optimal ventilation rate to maintain the lowest interior temperature
or the most comfortable combination of temperature and humidity.

there must be one: say it's 100 f outdoors, with a humidity ratio of
0.005 pounds of water per pound of dry air, which corresponds to a
dew point of about 40 f; with no ventilation, the theater air temp
will be 100 f, with 100% humidity; with an infinite ventilation rate,
theater air is outdoor air, at 100 f, with a humidity ratio of 0.005;
at some inbetween rate, theater air should be cooler and damper than
outdoor air. what is that rate?

it seems to me this isn't the same as a swamp cooler, btw, since the
internal humidity generation rate does not directly depend on the
ventilation rate.

i guess morocco also has lots of sun, the better to concentrate licl...
"unglazed collector/regenerator performance for a solar assisted open
cycle absorption cooling system" by m. n. a. hawlader, k. s. novak,
and b. d. wood of the center for energy system research, college of
engineering and applied sciences, arizona state university, tempe, az
85287-5806 usa, in solar energy, vol. 50, pp 59-73, 1993 describes
"an ordinary black shingled roof... used as a collector/regenerator
for the evaporation of water to obtain a strong solution of [lithium
chloride] absorbent... experimental results [using a 36'x 36' roof]
show a regeneration efficiency varying between 38 and 67%, and the
corresponding cooling capacities ranged from 31 to 72 kw (8.8 to
20 tons)", ie about 1 ton per 100 square feet of roof area :-)

in the house "water [the refrigerant] is sprayed into an evaporator,
evacuated to about 5 mmhg of pressure, where it immediately flashes
into vapor... cold water, pumped from the bottom of the evaporator,
flows through a fan coil... that blows cool air into the conditioned
space. the absorber acts as a vapor compressor and condenser for the
system. water vapor from the evaporator flows over the absorber where
it is absorbed by the concentrated absorbent. the continuous absorption
of water vapor maintains a low pressure in the system and permits
flashing of water in the evaporator... the product of the absorption
process, a weak absorbent solution, collects at the bottom of the
absorber to be pumped [up over the roof] for concentration."

"the dilute licl solution was delivered to the collector surface
through a spray header spanning the top of the roof and made from
50.8 mm (2 in) diameter cpvc pipe fitted with 35 evenly spaced brass
nozzles. the concentrated solution collected at the bottom... in a
pvc rain gutter, and returned via gravity feed to a 1608 l (425 gallon)
fiberglass tank... in the event of a rain, fluid flowing off the
collector could be manually diverted to a 946 l (250 gallon) wash tank
or to a roof drain. during the initial phase of the rain, residual salt
would be washed from the roof and collected in the wash tank to be
stored for later regeneration. after sufficient rainfall, the rainwater
is diverted to the roof drain."

nick




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