Re: Cooling attic (and home) by spraying water on roof (shingles)- good/bad?

["Phil Scott" <philscott@xxxxxxxxxxxxx>]

"Some Guy" <Some@xxxxxxx> wrote in message
news:42BF0BE3.2C314AE5@xxxxxxxxxx
> Jim Baber wrote:
>
> > >Besides the glaring fact of WASTING WATER?
> > >
> > Bill has a point here, but if you were to use misters
>
> Doesn't a mister waste water by atomizing it and then the
wind will
> blow it away?

     The latent heat of evaporation for water is somehing like
970 btu's per lb... one gallon of water is 8 lbs...so its
about 7800 btu's of heat required or the equivalent amount of
*sensible heat removed from the air to evaporate a gallon of
water.  Thats equivalent to about 3/4 of a ton of
airconditioning  (average home takes about 3 tons).

So you would have to evaporate about 4 gallons of water an
hour to cool the average home in a very dry climate... say 10
hours a day.. thats 40 gallons a day,..or 1200 gallons a
month.    The average home uses about 500 gallons of water a
day the bill is say 25 dollars for that 15,000 gallons a
month.   So the water used in 100% evap cooling sells for
around 3 dollars a month. or lets say 10 dollars max.

Cooling the house with refrigerated HVAC will cost 200 to 300
dollars a month or more.



The cost of a gallon of city water is less than a cent...so
its a very good deal cost wise.. and ecologically much better
than running a one HP motor (approx HP required to deliver 3/4
ton of cooling if you count the fans).   That power is
generated by burning foscil fuels in most cases.

Evaporative cooling would be limitlessly popular if it did not
add an equal amount of humidity in the form of 'steam' to the
air it was cooling to *sensibly lower termperatures.     There
is sensible heat, measured with a thermometer, and Latent heat
thats water vapor in the air..it takes about 100 times as much
heat to create steam as it does to raise the same amount of
water 1 degree F.

So latent heat is a big deal in humid climates... in dry
climates you can use evap cooling to take advantage of the
situation.


 The human body feels both this humidity and the sensible
temperature as one...so evaporative cooling only works in very
dry climates where an acceptable rise in humidity fits well
with human comfort requirements.

It will become a lot more popular in commercial buildings in
the south west shortly..as fuel prices go up.  Right now its
straight refrigerated air.


>
> I would think the ideal way to cool something with water
would be to
> completely wet the surface of something you want to cool
with a low
> volume of flowing water.  That would avoid staining from
minerals in
> the water.  The heat you're taking away is the difference
between the
> inflow and outflow temp multiplied by the volume flow rate.
Some
> evaporation will also happen.
>
> I know that water can cool best if (all of) it evaporates,
but that
> will leave mineral stains, and also it probably won't get
you down to
> the low temps you can reach by continuous flowing water.
>
> My tap water comes from lake Huron and is probably around 50
degrees
> (f).  I pay 3.86 cents (CDN) per cubic foot (3.165  USD).  I
think
> that's about 0.516 cents (CDN) per US gallon (or 0.423 cents
USD).
>
> I pay 5 cents (CDN) per kWh for the first 750 kWh (per
month) and 5.8
> cents per kWh after that.  That's about 4.1 and 4.8 cents
USD
> respectively.
>
> > At 10.8 gal. a day for 67 days that's not much water
>
> That's 5.6 cents (CDN) per day, or $3.74 (CDN) for 67 days
($3.06 USD)
>
> > > The various minerals causing deposits, the likelihood of
algae
> > > other mold/fungus growth and even it's weight are all
factors
> > > to consider.
>
> Deposits, yes - but perhaps negligable if the water is not
allowed to
> evaporate.  Mold/fungus - I'm thinking no because this would
only be
> used on mostly sunny days for maybe 1/2 hour to maybe 2
hours of the
> day.  When the water is turned off, the roof will dry up and
I
> wouldn't think that would give any mold or fungus any
ability to grow
> given that the shingle temp would probably climb back to
well over 100
> degrees.

Thats about right.


 A thin layer of flowing water on the roof is equivalent to a
> gentle spring or fall rain shower and nowhere near the
weight of the
> snow loads we see during the winter.
>
> PS:  Given gasoline costs at $2 to $3 per gallon (or $1 CDN
per liter)
> what are the costs to generate electricity (on a kWh basis)
using a
> gasoline powered generator?
>
> PPS:  Are there gasoline or propane-powered AC units, and
are they
> more economical to run vs electric?
>
> PPS:  What are the pro's and con's of immersing your outside
AC
> condensor coil in your swimming pool (and therefor doing
away with the
> cooling fan) ???  Heat your pool and remove heat from the
coils much
> more efficiently?


That would be what is called a 'ground water sourced heat pump
system' those work very very well.  And are popular.    For
some reason you dont see them piped to pool water very often.
But its workable....to a limit.   Depending on the size of
your pool, in the summer the system would warm the pool...but
you wouldnt want it to go over 85 degrees or so...then it
would have to switch back to air cooled or a ground loop.

In the winter you would be cooling the pool while heating the
house... the pool temp would then drop to 40F or so and start
absorbing heat from the surrounding ground in most climates...
and if the pools was large enough and had a pool cover .. it
would make a good heat source for winter heating...but of
course then too cold to swim in..

so for that reason pipes burried in the ground are common.
The engineering and install can screwed up easily and often
is..then its a mess.  If done right its a very good deal.


On the attic temps though the poster who mentioned temp at the
top of the insulation had the best response.. myself in your
case I would not wet the roof.  I would ventilate the attic
with a fan.



Phil Scott
Mech Engr HVAC contractor since 1829.