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re: evaporative cooler question
15 jan 2005
rich  wrote:

>...i am trying to see if you can think.

amusing. try some premises, rich. do we agree that:

1. most people find 80 f at 60% rh (w=0.012) comfortable. 
2. a c cfm airstream has an effective thermal conductance of about c btu/h-f. 
3. it takes about 1000 btu to evaporate a pound of water.

cooling more outdoor air than we need to wastes water and energy. what's
the minimum outdoor airflow c in cfm and water evaporation rate p lb/h
needed to cool a house with a 128 btu/h-f conductance to 80 f at w=0.012
if the outdoor air is 91.1 f with w=0.0066? air weighs about 0.075 lb/ft^3.

>you are somehow going to magically evaporate 2.62 lb/h of water.  as far as
>i know that can only be achieved with the largest of coolers using extremely
>hot and dry air.

but your "5000 cfm" cooler might evaporate 5 gallons per hour.

>i want to know what apparatus will do such a thing in an environment such
>as you describe.

we haven't talked about that. more premises:

4a. an a ft^2 pool of water loses about 100a(pw-pa) btu/h by evaporation,
where pw is the vapor pressure ("hg) at the water temp and pa is the vapor
pressure of the air around the pool (ashrae), or if you prefer,

4b. the ratio of heat loss by evaporation to heat gain by convection r =
100(pw-pa)/(tw-ta), in t (f) degrees, regardless of windspeed (bowen, 1926).
(r = -1 defines the wet bulb temp.)

5. a ft^2 in a v mph wind has about a(2+v/2) btu/h of airfilm conductance.

>the portable unit you picked can't evaporate more than about two gallons
>a day using 10%, 100 degree air.

would you have any evidence for this article of faith?

>that portable one probably has about 500 square foot.

try 4a.

>how are you going to make it evaporate 7 gallons using air that already has a
>higher water content and temperature approaching 80 degrees.

it helps if the cooler is near a window with incoming air. more premises: 

6. pa = 29.921/(1+0.62198/w) "hg and

7. pw = e^(17.863-9621/(tw+460)) "hg (a clausius-clapeyron approximation.)

>a well designed pad for a whole house cooler  has 3000 to 4500 square foot
>of surface area for evaporation.

but that's way too much air and water, with poor controls.


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