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re: evaporative cooler question
25 jan 2005
abby normal wrote:
>...if you got a rock bin or storing ice we have some
>valid thermal capacitance. typical house, no.
concrete stores about 25 btu/f-ft^3, so a 1200 ft^2 x 4" thick slab
has c = 25x1200x4/12 = 10000 btu/f of valid thermal capacitance, or
more, if it's in contact with soil with no insulation beneath.
the house needs 24h(91-80)128 = 34k btu/day of cooling, no?
the slab can provide that with a 34k/c = 3.4 f temperature swing,
plus a smaller amount that comes from the slab airfilm resistance:
when it's 106 f outdoors, the house needs (106-80)128 = 3328 btu/h of
cooling. a dry slab with a u1.1 radiation conductance under slow-moving
air from a ceiling fan with a u1.5 convection conductance can provide
3328 btu/h of cooling with a 3328/1200/(u1.1+u1.5) = 1.1 f slab-air
temperature difference. to keep the house 80 f all day, the slab has to
start the day at 80-1.1-3.4 = 75.5 f.
>> and turn on your 25k cfm fan :-) on an average day with a sinewave
>> temp curve, it's only 76 for an hour or so.
>
>how many hours at 77,78, and 79 :)
more. it's a sine wave. with some indoor evaporation at night, the fan
can be a lot smaller.
>i lost my entire library recently to a flood and i cannot check all the
>formulae you are quoting,
sorry to hear that. it's mostly just "ohm's law for heatflow." line 50
below is an approximation for the ("hg) vapor pressure of water at temp
ta (f). it's close to the numbers you'd find in table 3 in chapter 6 of
your ashrae hof, if you still had one.
50 ps=exp(17.863-9621/(ta+460))'outdoor vapor pressure at 100% rh
>> andersen says an average family of 4 evaporate 2 gallons per day,
>> including all that. that's minimal (and helpful) compared to the
>> cooling water flow. no need to predict. the total makes wc = 0.012
>> by design and control. this corresponds to the 54% exhaust fan
>> humidistat setting at 80 f.
>
>you are assuming 0.012. you are not proving.
i'm not sure what you are getting at, abby. we evaporate water inside
a house, with an exhaust fan that turns on when the indoor rh reaches
54%. the outdoor air is much drier. what's to prove? would we need to
prove that a house with an ac and an 80 f thermostat setting is 80 f
indoors? we might wonder if the ac has sufficient capacity...
>i think in arid regions with evap cooling indoor rh will be lower.
why do you think that? perhaps you imagine that an indoor wet surface
will just stay wet, without evaporating any water. i used a simple ashrae
swimming pool formula. a ft^2 of wet surface at temp tw in air with pa
"hg loses 100a(pw-pa) btu/h by evaporation, where pw is the saturation vapor
pressure at tw. it's a little surprising that this formula doesn't include
the air temp, but it's well-known and often used. an airtight house with
an indoor humidity source will eventually have 100% rh indoor air...
>the indoor condition will have to be very close to the same wet bulb
>as the ambient.
brrr. why can't it be warmer than 65 f? especially if the water only
turns on when the room temp reaches 80 f, using a thermostat?
>> >show the pychrometric process...
>> no thanks. feel free to do so yourself. what do you think you will find?
>
>you will find that the room temp will rise above 80.
why would you say that? conservation of energy (energy in = energy out,
over a day) says otherwise.
>adding sensible and latent heat to air, is sort of like horizontal and
>vertical vector components on a right angled triangle. the problem is
>the angle of the resultant is fixed with evaporative cooling.
i don't understand that. perhaps it has more to do with a box on the roof
than this indoor scheme.
>i think you will have problems coming up with 0.0012 unless it is for
>internal latent gains such as from occupants, or the fact that your
>design emulates a sauna. don't forget sensible gains from occupants
>as well.
why would you say that, exactly? we can always add more details, but
that won't change the performance much, imo.
>you keep talking about super insulated homes, massive thermal storage.
>my argument is evaporative cooling is high airflow per unit cooling.
that's true, traditionally.
>you fictitious numbers do not change this.
in what ways are they fictitious? ok, it's a tiny house. we could scale it
up and add lots of air leaks and remove the insulation and thermal mass :-)
>you are merely shaving the load.
i'd say we are discussing more than that.
>> >a boy scout trick to cool a bottled beverage is to wrap the bottle
>> >in a wet newspaper. evaporating water from a newspaper cools the fluid
>> >inside the bottle but i suppose it grabs some heat from ambient air as
>> >well.
>>
>> it has to, by energy conservation. where else would the heat come from?
>
>most of the heat comes from the fluid inside the bottle. the
>evaporating water draws heat from the remaining liquid water which
>cools off and draws most of its heat from the bottle.
of course. i didn't see that.
>> >your wetted slab will experience a similar effect. heat from the
>> >ground is going to evaporate water.
>
>> not much, if the ground is dry or the slab has insulation beneath.
>
>now of course the slab is insulated.
or on dry soil.
>...you keep changing the house parameters,
sometimes, as potential improvements arise.
>but evaporative cooling is still high airflow per unit cooling.
by tradition.
>i guess its going to be the wetted ceiling slab next...
nonono. cool floor, warm low-e ceiling.
>> >...you can add ceiling fans...
>
>> exactly. they allow a larger mass temp swing and more precise room
>> temp control and setbacks during unoccupied times. radiation helps too.
>
>> >the most practical way to do this is to run this hot air through an
>> >evaporative cooler first.
>>
>> i disagree.
>
>let's see, 106 degree outdoor air is going to enter a room.
not a good idea, imo.
>is it better not to control and remove the sensible heat from this room air
>first before it creates discomfort or to walk around on a wet floor?
the floor should never be "wet," and the house should never admit
106 outdoor air. i wonder if fu-tung cheng's fancy concrete floors
http://www.chengdesign.com are porous to water or water vapor.
>treating the air first before it can add its heat to the space is like
>getting a vaccine to prevent a disease. trying to deal with it later,
>is like going to the hospital to be healed.
is indoor evaporation at night like mother theresa?
nick
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