[Greenbuilding] thermosiphon questions

nick_pine at verizon.net nick_pine at verizon.net
Sat Nov 1 14:46:05 CDT 2014


One way to design a thermosyphoning system:

1. Calculate the required water flow, approximately G = 0.04A gpm for an A 
ft^2 collector, eg G = 1.28 gpm for a 4'x8' collector.

2. Calculate the available thermosyphoning head, Ha = H(62-61.71)/144 = 
0.002H psi, based on a 10 F temp diff at 100 F and an H foot height 
difference, eg Ha = 0.002x8 = 0.016 psi with H = 8'.

3. Calculate the minimum supply and return pipe inside diameter d in inches, 
given their total length L in feet.

http://www.calctool.org/CALC/eng/fluid/hagen-poiseuille says dP = 
32MuLV/d^2, where V is a velocity and d a diameter. A G gpm flow rate makes 
V = cG/d^2 for some constant c, ie dP = kLG/d^4 for some constant k.

Plugging L = 8' and d = 1" and G = 1.28 gpm into the website makes dP = 
0.001775 psi = k8'x1.28gpm/1^4, so k = 1.73E-4.

In the example above, d = (kLG/Ha)^0.25 = 
(1.73E-4x16'x1.28gpm/0.016psi)^0.25 = 0.69" for L = 16'.

Nick 





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