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re: thermosyphoning freeze protection
12 jul 2003
andy  wrote:

>...the evacuated tubes are almost always plumbed for ~3/4" tube...
>this of course would aid the thermosyphon effect...

sure, since the flow increases as r^4. we might also use the lower viscosity
at 68 vs 32 f, 3.4 vs 6x10^-7, and the more rapid change in density, 0.01 vs
0.003 lb/ft^3-f.

with 16' of height and a dt temp diff, dp becomes 0.16 vs 0.048dt lb/ft^2.

a 3/4"x32' pipe around 68 f might have q = pi(0.375/12)^4dp/(8x3.4x10^-7x32')
= 0.034dp ft^3/s or 1269dt lb/h, which moves 1269dt^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 68 f water temp to outdoor air at,
say, -20 f, (68+20)4 = 352 btu/h makes dt = sqrt(352/1269) = 0.53 f. water
goes up at 68 f and returns at 67.47 f. at -40, with 432 btu/h of heat loss,
the water might return at 68-sqrt(432/1269) = 67.42 f.  

>...the idea here is ultimate simplicity and elegance leading to low cost,
>added reliability and ultimate consumer acceptance.

sounds great. steve baer says those freeze-valves are notoriously unreliable. 

but i'd still like to see a differential thermostat to lower the nighttime
collector heat loss and the pump energy and make the pump last 20 vs 5 years. 
smart electronics can be very reliable and cheap in volume. to reduce sensor
wiring, we might run the pump for a moment once in a while after dawn to
sense the collector temp via the return pipe, or sense the tiny continuous
upgoing and downgoing thermosyphoning temp diff and run the pump when it's
close to zero.
 
>"if freeze protection is the goal, keep in mind the density of water
>stops increasing with dropping temperatures when you get to about 39 degf... 

>...this is why the system would be engineered to keep the temp above 45 deg
>- and as nick points out - this may not be so difficult.

as long as there is a heat source below, eg an unfrozen house. it also needs
some bare surface to gather house heat. we might wrap some electric heater
tape around the upgoing pipe, with a thermostat, for very cold nights. 

> 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...

>r4? hmmmm. maybe conservative. maybe not. in anycase they could be
>manufactured rmore.... maybe r16 even?

looks like r4 is plenty. a little foam or fiberglass inside the header
enclosure. no insulation might be ok, except in a very strong wind, but
we also want to avoid heat loss in normal times.

nick




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