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Fast & Cool or Slow & Hot

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Posted by Ian McDonald on October 22, 2004, 12:56 am
 
I've been lurking here for a long time and now I'm ready to ask questions
that will help me build a solar water heater for my swimming pool. Over the
years I've experimentally informally with black hoses etc but I'm ready to
move to something a bit more efficient.
Question:
As a general rule, (if it exists), which of these is true?
(a) It's better for water to flow slowly through a heater, so that it goes
into the pool very hot (but not much hot water goes in)
(b) Flow quickly, so more water goes through, but at a lower temperature
(c) makes no difference, because raising a small amount of water a high
temperature is the same as raising a large amount of water by  a small temp.
Instinct tells me (c), but I'm very willing to have my ignorance corrected.

Ian.



Posted by Duane C. Johnson on October 22, 2004, 2:04 am
 
Hi Ian;



 > to ask questions that will help me build a solar water
 > heater for my swimming pool. Over the years I've
 > experimentally informally with black hoses etc but I'm
 > ready to move to something a bit more efficient.

 > temp. Instinct tells me (c), but I'm very willing to have my ignorance
 > corrected.

No, Since the water is being heated with a solar collector the
the higher the collector temperature the lower the efficiency.


There are two exceptions.
1. The pumping losses for the higher flow out weigh the gain
    due to the increase in efficiency. One could this electric heat.
2. This is a special case where the ambient air temperature
    is greater than the pool water. In this case the solar collector,
    in addition to a solar heat source, is also an air heat source.

    The optimum flow in this case is dependent on a number of
    factors and I don't know how to evaluate them.


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Posted by nicksanspam on October 22, 2004, 9:55 am
 

With more and more powerful pumps, that can always be made to happen,
unless the plumbing explodes :-)


Faster (more mass per second per square foot of collector) is also more
efficient in that case, since that makes the water in the collector
cooler, so it collects more heat from warmer air.

Nick


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