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re: well when should i take my pool cover off??!!
7 oct 1996
ken tait  wrote:
 
>>a pool blanket reduces heat loss by   a) reducing evaporation.
 
god (big g) replied via god (np), assuming he wasn't making this up:

>...here's one version of a formula for the ratio of heat loss by convection
>to the heat loss by evaporation, for a pond, independent of windspeed, from
>p 664 of duffie and beckman's 1991 _solar engineering of thermal processes_:
 
>qc   0.001(tp-ta)     where tp and ta are pond and ambient temps (f) and
>-- = ------------           pwp and pa are vapor pressures at the pond
>qe     pwp - pa             surface and ambient vapor pressure in inches
>                            of mercury, 29.9" hg being 1 atmosphere. 

if tp > ta, it looks like we have heat loss by convection and evaporation,
since pa has to be less than pwp, since the pool surface air is saturated
with water, and warmer air can hold more water than colder air, right?

when should we take the pool cover off on a day in october in phila, with an
average temp of 56.4 f, daily min, 46.4, daily max, 66.3, record min, 35,
record max, 96 f, average humidity ratio, 0.007, and average 1020 btu/ft^2/d
of sun that falls on a horizontal surface, if the pool temp were 80 f?

on an average day, we might remove the cover and gain 30 btu/ft^2/hr of
solar heat, at noon, and lose (80-66.3)1ft^2/r1 = 13.7 btu/hour by conduction.
pwp(80f)=exp(17.863-9621/(80+460)) = 1.079" hg, or 1.033 from page 6.4 of the
1993 ashrae hof, and pa=29.921/(1+0.62198/0.007) = 0.333" hg, so we'd lose
1000(1.079-0.333)/(80-66.3) = 54.45 times more heat by evaporation than by
convection, ie another 746 btu/hr-ft^2, losing a total of 760 btu/hr-ft^2
and gaining 30 btu/hr-ft^2, maybe, for a total loss of 730 btu/hr. we might
evaporate 3/4 lb of water per hour per square foot of pool this way. we might 
be able to evaporate 500 gallons of water, if we did this over an 8 hour
average october day with a 24'x36' pool. so it looks like this strategy won't
work very well in phila, on an average october day.

how close would the air temp have to be to the pool temp, to make it worth
taking the "solar pool cover" off? looks like the breakeven point is when 
30 = (80-ta)(1+1000(1.079-0.333)) = 747 (80-ta) = 59760 - 747 ta, so 
ta = (59760-30)/747 = 79.96 f. so it might make sense to remove the cover
to let more sun shine into the pool, if the air is no more than 0.04 f
cooler than the pool, and there is no wind.

if tp = ta, there will be no convection loss, right? and if the
ambient air is not 100% humid, there will be evaporation heat loss.  
 
does this equation apply when the air is warmer than the water, ie ta > tp?
there's an old reference: bowen, i. s. physical review, 27b, 779 (1926)
"the ratio of heat loss by conduction and by evaporation from any
water surface." the title does not mention condensation, or talk about
heat gain at all, or suggest water can lose heat by evaporation and
gain by convection at the same time, but hey, why not?

>what else? clausius-clapeyron: ln vp = 17.863-9621/(t+460),
>and humidity ratio w = 0.62198 pw/(p-pw). and qe is roughly 
>(tp-ta)(2+v/2) btu/hr-ft^2 with a windspeed of v mph. and a
>clear sky can suck heaps o' radiative heat out of a pool.

baruch givoni has measured roofs that are 10 c cooler than surrounding air,
on clear nights. if the wind blows, it warms up the roof. seems like this
would happen on a clear day, too, as another form of heat loss.
why is the sky blue? why are "solar pool covers" blue? 

>>a pool blanket reduces heat gain by   c) shading the pool.
 
>it might "shade" the pool by reflecting and absorbing 10% or 20% of the sun
>that falls on the cover... some of that eventually ends up in the pool...
>so we might take the pool cover off when the sun is shining if we believed
>that the increased gain (30 btu/hr-ft^2, ie 10% of the peak solar input?)
>would be more than the 1 btu/hr-ft^2-f increased loss by r1 conduction and 
>evaporation (which might be much more than that.) 

a perfectly transparent pool cover would not "shade the pool," nor would
a perfectly conductive one, i guess. "solar pool covers" are close to both.

>>examples:  assume pool is at 80f.
 
>with what air temp (80 f?) and humidity ratio (0.0133, like philadelphia,
>in july?), and how much sun, and clouds and wind? 
 
humidity ratio (w) is the ratio of water weight/dry air weight. perhaps
nrel characterizes humidity by monthly average humidity ratios for various
places in their _solar radiation data manual for buildings_ since w depends
less on air temperature than relative humidity (rh). perhaps this says
there's a certain amount of water in the atmosphere, in a particular city,
and if the air temp goes up, rh goes down, and vice-versa, unless
condensation or evaporation occur, eg to or from swimming pools.

>>	its sunny, warm and no wind	-off
 
if the air is warmer than the pool, and the air is moving, the pool gains
heat by convection, and loses heat by evaporation, right? if the air is warm
enough and humid enough, the pool gains heat by condensation, right? how warm,
and how humid? would we ever want the pool warmer when that happens? maybe.

>>>is there some quirky relationship between air temp and water heat
>>>transfer that will enable me to know when its beneficial to take the
>>>cover off at other times?
 
sure. i wonder what that relationship is. 

nick

  the main point is i want you to have fun in doing what you are trying to
  do for yourself or india or whatever. there is in fact no agenda that is
  or has to be laid out by me, and you follow it. you are self motivated by
  your fun and curiosity. let me tell you a story.

  when i joined tifr, we worked (or played) in a place called holiday camp.
  we were entitled to 90 days vacation in an year. so we were called 
  vacation officers in holiday camp. we were not assigned any jobs. no
  agenda. we used to frequent the coffee tables, then magazine sections,
  then lunch, gossip, then coffee, and by 3pm, it was time to go. after six
  months of such fun, the fun became boring. we started wondering: why the
  heck we were being paid at all? from then on the story was entirely
  different. work became play and play became work.

  please note: it is not my project you are doing. it is your project,
  whatever that you are doing for the fun of doing it. you do not have to
  prove or disprove my ideas. you generate your own and be self referred.
  this is what i would like to see happen.
						sastry

  guruji feels that it will be imporant to teach organisational skills,
  team spirit and bonding, since the schools are teaching other standard
  educational aspects such as reading, writing and arithmetic. we all hear
  of how we indians don't do well in groups, right? there is the well-known
  story of the indian crabs. also, someone was saying a few days ago that
  "one indian equals three japanese, but three indians equal one japanese".
  not that we want to compete and beat the others, but the japanese do seem
  to offer a valuable lesson to us here, in making the group (japan inc)
  an important entity. guruji has been very impressed by the japanese school
  next door, where the kids spend a few hours everyday in the sun, doing
  various group exercises such as forming patterns, then reorganising 
  themsleves etc. (the kind we see in parades). guruji feels it will be
  good for us work on fun themes that involve group work and hence teach
  the value of this. 

  some ideas we were discussing here were: have a set of circles each of
  some given colour, organised in some initial pattern (say a set of
  parallel lines). then "morph" this into some other pattern, by moving the
  circles appropriately. this is a symbolic representation of the japanese
  children's daily exercise. each group (or child) is  assigned a certain
  colour, and by moving their circles appropriately, they see how a pattern
  froms by their individual moves. or maybe, they are given building blocks
  to build a house--one child puts them together, another paints, another
  takes care of the environment around the house etc. maybe we can start off
  creating group puzzles and activities like this that are fun for the kids,
  then maybe make them software packages that the children work with. 

  i think this can also help us enhance our group activity: i think many of
  us really dream of contributing to the work of svt (or other such groups)
  and have largely similar goals, but we do seem to become unorganised and
  unfocussed at times, right? maybe projects such as this will help us split
  the work appropriately through the internet, and jointly brainstorm etc.

                                                   aravind

  thanks aravind, this message itself tries to incorporate the group spirit
  we are talking about. together, we can fly much higher than any one of us
  individually can (as long as we sit in each other's laps :-) have fun!

						   sastry



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