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Thermosyphoning freeze protection - Page 5

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Posted by Nick Pine on July 12, 2003, 11:32 am

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.  

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.

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.

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.


Posted by andy on July 12, 2003, 6:22 pm

This is a good idea.........

Posted by daestrom on July 12, 2003, 2:12 pm

In your paper, the drawing shows the return from the heat-pipes going to the
domestic hot-water heater outlet (i.e. the penetration that connects to the
'top' of the water in the tank).  While in the discussion under 'Basic
Circulation and Control', third paragraph, you explain that solar heat
returns, "...to the bottom of the tank via the internal tank fill pipe."  I
think you want to say it returns to the top of the tank via the hot-water
outlet pipe.  This would perhaps make the recovery time better as the top of
the tank would be heated first.  And it would allow the pump suction to
remain cooler for as long as possible.  Or maybe not, as I read it, you are
depending on the return going into the bottom of the tank to minimize losses
at night by only circulating water from the bottom of the tank.  Well,
either way, the drawing and words don't match up so you might want to change
one of them.

Your ideas for over temperature immunity are novel.  Many folks come up with
schemes to prevent boiling, whereas you allow boiling, just limit the amount
of liquid available and accomodate this.  If we use Nick's dimensions for
the header of 6"x6"x8', we have 2 ft^3 of water at an initial temperature of
say 140 degF.  That's about 123 lbm of water.  If the 'mains' pressure is 50
psig, then the steam formed from boiling will be 123 lbm of vapor at 50
psig.  That much vapor has a volume of  820 ft^3.  Allowing for the intitial
space of the header, the expansion tank will have to accomodate the
remaining 818 ft^3.  Of course, many homes fed from municipal water supplies
here in the US don't have any sort of check-valve between the street supply
and the hot-water heater so in those situations, you *could* let the steam
pressure simply force water back out to the 'mains'.  For those with pumped
wells, I don't think this option would work as they have checkvalves.  Such
a large expansion volume could be a problem, remember it must be kept
'empty' when not boiling to have room for this expansion.  And I have no
idea what boiler/plumbing code has to say about this :-)



Posted by andy on July 12, 2003, 6:18 pm
Nope - I drew the check valve backwards - I 'll fix this now.

This would perhaps make the recovery time better as the top of

I din't think a return to the top of the tank (hot water outlet) is a
good idea. 1) The returning water will only be a tad above the lower
tank temperature. 2) The stratification of the tank will be up set. 3)
The top element will now heat the entire tank....and on and on. I'll
fix the diagram.

no no no no that is the outer surface area of the header. the interal
volume is that of an 8' long 3/4" pipe.

Additionaly, even if the header was that volume you only need to
displace 2 ft^3 and not 820. The entire volume of header water is
displaced by the small volume of initial boiled water......unless
there is some minor pooling to deal with. The remaining 2ft^2 of water
vapor in the header can't boil so it can't displace any more water.

 Allowing for the intitial

Posted by andy on July 14, 2003, 1:06 am
Yes, the heat pipe recepticles brazed into the crossdrilled 3/4" tube.

It can potentially get to about 400F when the heat pipe cycle begins
to colapse. This is why all the connections are brazed and not

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