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re: pressurizing a misting system...?
23 jun 2004
indoor misting with a vent fan for outdoor air works like a swamp cooler,
when 4500(wi-wo)>to-ti. the left side is the amount of latent cooling per
cfm of vent air. the right is the amount of sensible heating per cfm of
vent air. as they become equal, swamp cooling and misting no longer work.

the ashrae comfort zone has an efficient corner at ti = 78.5 f and
wi = 0.0126 (about 60% rh), which makes to < 135.2-4500wo. nrel says
phoenix has an 88.2 f average temp in june, with an average daily min
and max of 72.9 and 103.5 and average humidity ratio wo = 0.0056 pounds
of water per pound of dry air in june, so misting should help until
the outdoor temp rises to to = 135.2-4500(0.0056) = 110 f.

evaporating a pound of water takes about 1000 btu, and air weighs about
0.075 lb/ft^3, so a house with no internal heat gain or unshaded windows
and a 200 btu/h-f conductance that's 78.5 f indoors with wi = 0.0126 when
it's 88.2 f and wo = 0.0056 outdoors needs about 1000x60c0.075(0.0126-0.0056)
= 31.5c = (88.2-78.5)(200+c) btu/h of cooling, including cooling c cfm of
air from 88.2 to 78.5, so c = 89 cfm (not much), with 4.5c(0.0126-0.0056)
= 2.8 pounds or 0.34 gallons of water per hour, which might come from a $5
0.5 gph mister mister nozzle and a solenoid valve scrounged from an old
washing machine in series with a 78.5 f room temp thermostat and a $5 
humidistat that turns on a $12 window box fan when the rh rises to 60%. 

herbach and rademan (800) 848-8001 sell a nice
brass $4.95 navy surplus humidistat, item number tm89hvc5203, with a
20-80% range, a 3-6% differential, and a 7.5a 125v switch that can be
wired to open or close on humidity rise.

pw = 0.996 "hg at 78.5 f and 100% rh, and pi = 0.594 "hg with wi = 0.0126.
ashrae says a square foot of pool evaporates 100(pw-pi) = 40 btu/h, so we
can evaporate 2.8 pounds of water per hour with 2800/40 = 70 ft^2 of wet 
surface, counting droplets in air. masonry floors and walls or an indoor
cylindrical rock gabion with a fountain pump (in a toilet tank?) with high
thermal conductance and mass might allow more efficient cooling with cooler
night air and keep the mass of the house cool with the fan off during
the heat of the day.


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for an all-day workshop on new solar house heating and natural cooling
strategies ("hvac nonsense") on july 9 in portland, or--see page 25 of

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