Hybrid Car – More Fun with Less Gas

heat exchanger capacity - Page 3

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Posted by daestrom on September 21, 2006, 9:51 pm

Well, using just the nominal dimensions (inside and outside diameters are
*not* the same), the cross-section for flow of the water (outer tube) with
just one 1" tube is 0.34 in^2.  With three 3/4" tubes, the flow
cross-section is 0.74 in^2, more than twice as much.

If you're going with thermosyphon, the placement and orientation can be
critical.  You'll want your heat exchanger as far below the tank as
possible.  And run the upper end of the heat-exchanger to the upper end of
the tank (and lower to lower).

I'd go with the three 3/4" tube design.  More surface area, more flow
cross-section.  A few more fittings though...

"as long as possible" is always a safe answer :-)  Of course without seeing
how your tank is oriented (is it on the floor, or can it be put up higher?)
or other things, it's a bit hard to work any 'real numbers'.  How far from
the tank?  How many elbows/fittings between the hx and the tank?  (i.e. is
it a straight drop, or are you thinking around the corner, down the hall,
behind the laundry kind of thing?)


Posted by Eric Yancey on September 22, 2006, 12:03 am




Thanks for the feedback.  My tank is almost 2 feet off the floor on a
platform.  It could be higher as I have 10' ceilings.

If I rely on a thermosyphon I will put the exchanger very close to the tank
(assuming I use a tank after the discussion with the others in this thread).
I have enough room to basically situate the heat exchanger wherever I'd

Thanks again,


Posted by daestrom on September 22, 2006, 8:39 pm

Well, just to be sure we understand each other.

The heat exchanger should be directly underneath the tank, not along side
it.  With the hx as low as possible, and the tank as high as possible,
you'll maximize the thermosyphon effect.  And if at all possible, the hx
outlet should go to the top of the tank, and the hx inlet come from the


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