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Check valves for passive solar hot water system - Page 7

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Posted by Robert Scott on November 1, 2007, 11:48 am
 
On Wed, 31 Oct 2007 20:34:17 -0000, mark.fox@gmail.com wrote:


Seems to me that 0.1 PSI is still a fairly large pressure compared to what can
be generated by thermal siphoning.


Robert Scott
Ypsilanti, Michigan

Posted by daestrom on November 2, 2007, 1:15 am
 


Right you are!

Take for example water on the 'cold leg' of 100F (0.01613 ft^3/lbm) and
water on the 'hot leg' of 120F(0.01620 ft^3/lbm).  Even if the tank is 20 ft
above the collector, this 20F difference results in the two columns of water
with a driving head of 0.04 psi.  If the valve won't open until you get to
0.1 psi across it, then this 20F difference over a 20 ft height wouldn't
even come close to opening it.

Some folks don't grasp just how little driving pressure is involved in
thermosyphon systems.  Every little elbow, fitting and joint is important in
such a system.

daestrom


Posted by DavidMDelaney on November 11, 2007, 1:32 am
 On Oct 31, 3:34 pm, mark....@gmail.com wrote:

ofhttp://www.mcmaster.com/ctlg/DisplCtlgPage.aspx?ReqTypTALOG&CtlgPg ...).

A column of water 12 inches high exerts a pressure of about 0.433 PSI.
(62.4/144).
So 1 PSI requires a column of water 1/0.433 x 12 = 27.7 inches.
Therefore 1
PSI requires a column of water 0.1 x 27.7 inches = 2.77 inches.

This is a *huge* differential head in a thermosyphon system. The
thermal coefficient of
expansion of water is about 3e-4/C. So a column of water 2 m high
whose average
temperature was 20 C greater than that of a storage tank at the same
level would provide
a differential head of at most 3e-4/C x 20C x 2000 mm = 12 mm = 1.3 in
-- not enough
to crack the valve you mention.

Note that you would want to collect hot water even when the
temperature of the collector
was a lot less than 20C average hotter than the storage tank.

You should  put the storage tank above the collector, or use
Buckley's  fluidic check valve
(http://www.geocities.com/~dmdelaney/buckley/solar-modules-check-
valves.html), or use Larkin's idea,
(http://www.geocities.com/~dmdelaney/larkin/larkin-tap-1.html ).

If the descender from an overhead tank is in heated space it should be
well insulated to
reduce the rate at which it heats, and hence the reverse flow, to a
negligible level.  If the descender is in unheated space, it will not
heat and will not create a reverse flow.

David


Posted by DavidMDelaney on November 11, 2007, 1:51 am
 
ofhttp://www.mcmaster.com/ctlg/DisplCtlgPage.aspx?ReqTypTALOG&CtlgPg ...).

12 mm = 1.3 in was a deliberate test of alertness -- actually 0.47 in


Posted by Steve O'Hara-Smith on November 11, 2007, 11:03 am
 On Sat, 10 Nov 2007 17:32:48 -0800



___________^

    The missing 0. where indicated was another test of alertness ?

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