thermosyphoning freeze protection SOLAR.KnowledgePublications.com

thermosyphoning freeze protection
11 jul 2003
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.  

nick