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Thermosyphoning freeze protection

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Posted by Nick Pine on July 11, 2003, 8:53 am
Andrew Swingler suggests thermosyphoning as one form of freeze protection
for evacuated tube collectors above a heat storage tank. This might be the
only form of freeze protection needed for freeze-tolerant heat-pipe tubes
below a pressurized header with some insulation on the header and its supply
and return pipes, and a pipe with minimal elbows and a circulation pump that
doesn't block flow when stopped.

Cool water weighs about 62.67 - 0.003T lb/ft^3, with T in degrees F. The
density difference caused by the temperature difference between up and down
pipes causes a pressure difference proportional to the height of the water
column, making water flow through the resistance of the pipe loop. With 16'
of height and a dT temperature difference, dP = 0.048dT lb/ft^2.

Bill Shurcliff says a pipe with radius r and length L in feet and pressure
diff dP has laminar flow Q = Pir^4dP/(8MuL) ft^3/s. Viscosity Mu is about
6x10^-7 lb-s/ft^2 for 32 F water. With, say, 32' of 1/2" pipe, 16' up and
down, Q = Pi(0.25/12)^4dP/(8x6x10^-7x32') = 0.004dP ft^3/s or 43dT lb/h,
which moves 43dT^2 Btu/h. If the heat pipe collector header is 6"x6"x8' long,
with 16 ft^2 of surface and R4 insulation and a 16/4 = 4 Btu/h-F conductance
from 60 F water temp to outdoor air at, say, -20 F, (60+20)4 = 320 Btu/h
makes dT = sqrt(320/43) = 2.7 F, if I did that right :-) Water goes up at
60 F and returns at 57.3 F, well above 32 F. At -40, with about 400 Btu/h
of heat loss, the water might return at 60-sqrt(400/43) = 57.0 F.  


Posted by Fred B. McGalliard on July 11, 2003, 2:10 pm
Nick. Isn't this a description of a convection driven water chiller?


Posted by News on July 11, 2003, 4:39 pm

In short it takes heat from the stored water in the tank below and emits
this to the night sky "every" night.  Not very efficient at all.

If you have a roof mounted water solar panel one ant-freeze method is to
have two stage frost control.  A pipe stat senses the water in the panel is
on 0C - the pumps runs, however it does not extract heat from the tank, so a
motorised valve will have to be in place.  Moving water is less likely to
freeze.  Another stat senses if the water is about to freeze, then a the
pumps cuts out and the panel water empties into an open vented tank in the
attic.  Dependent on local climate of course.

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Posted by Bruce Wilson on July 11, 2003, 6:00 pm
 News, Look up "Evacuated tube collectors".


Posted by News on July 11, 2003, 10:26 pm

Missed that.

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