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re: pressurizing a misting system...?
22 jun 2004
wayne   wrote:

>>>...we made a much larger investment in a commercial misting 
>>>system, professionally installed.  the pump operates at more than 10 
>>>times the pressure of the old system, pumping through copper lines to 
>>>bronze misting heads with orifices fine enough to produce clouds of
>>>vapor rather than "rain".
>> i think you mean "smaller droplets." if it really produced water
>> vapor, it wouldn't cool at all.
>that's all vapor is...smaller droplets.

common matter comes in solid, liquid, and vapor phases. vapor contains
no liquid--no droplets and no latent cooling power. you can't see vapor.
clouds are not vapor. high pressure systems make smaller droplets that
evaporate faster because they have a higher surface to volume ratio and
a higher vapor pressure, so we don't get as much liquid on the floor...

>>>...this system cools the patio at 15 degrees lower than the
>>>surrounding temperature, ofen more.  there is no "wetting" of people
>>>or surfaces, and no mineral deposits left on surfaces.

where do the minerals go?

>> do you believe that a gallon per hour evaporating from "clouds of
>> vapor" produces more coolth than a gallon per hour evaporating from
>> a concrete patio? where do your minerals go?
>absolutely!  water evaporating from a patio floor is clearly not going to 
>have a significant cooling effect 5-6 feet above the surface.

"clearly," eh? :-) i disagree, altho some of the floor water may be
wasted, if misting isn't continuous, even for shaded floors. 

>while the net effects of each gallon may be equal technically,
>the perceived effect is radically different between the two.

water on the floor makes cool air that is less easily blown away in
a wind. and radiant temperature is more important to comfort than
air temp. misting people (making wet clothing) cools even better. 
low-pressure systems are simpler and cheaper, altho it's nice to
see all the water that falls on a floor evaporate, with no runoff.

>do you have a personal problem with mist?

no. seems like a good idea, altho less wasteful if done indoors, with
a vent fan to provide outdoor air. this works like a swamp cooler,
when 4500(wi-wo) > to-ti. the left-hand 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 chart has an efficient corner at ti = 78.5 f and
wi = 0.0126 pounds of water per pound of dry air, which makes the
equation above to < 135.2-4500wo. nrel says phoenix has an average
wo = 0.0056 in june, so indoor misting should help until the outdoor
temp rises to 135.2-4500(0.0056) = 110 f, on an average day.

for optimal ventilation, a house with a 200 btu/h-f conductance that's
78.5 f indoors with wi = 0.0126 when it's 100 f and wo = 0.0056 outdoors
needs about 1000x60c0.075(0.0126-0.0056) = (100-78.5)(200+c) btu/h of
cooling, including cooling c cfm of outdoor air from 100 to to 78.5 f.
so c = 430 cfm, with 60c0.075(0.0126-0.0056) = 13.5 pounds or 1.6 gallons
per hour (or "1.1 tons" :-) of water... from 4 $5 0.5 gph mister mister
nozzles 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%. 

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 13.5 pounds of water per hour with 13.5/40 = 336 ft^2 of
interior surface, not counting droplets in air. masonry floors and walls
or indoor cylindrical rock gabions with fountain pumps have high thermal
conductance and mass that can allow more efficient cooling at night and
keep the house cool with the fan off during the heat of the day.


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system, etc to use? ask us.

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

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