# Nick a thermal transfer question - Page 2

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Posted by nicksanspam on December 21, 2005, 2:40 pm

Pounds or Btu/F-gallon. NTU is a dimensionless, so the units in
the numerator (ft^2 x Btu/h-F-ft^2) and denominator are the same.

Sounds fine to me. Crimp 20% of the diameter at various orientations with
vice-grips? It won't help the small still water film conductance on
the outside of the pipe.

Nick

Posted by daestrom on December 21, 2005, 11:21 pm

I wouldn't spend a lot of time at it.  When you have flow through the tube,
and stagnant, natural convecting water on the outside, the heat-transfer
coefficient to the stagnant water is the dominant thing.  I did some calcs
for 3/4" copper with 150F water on the outside, and the heat transfer
coefficient on the water side came out to 73 BTU/ft^2-hr-F.

daestrom

Posted by SolarFlare on December 22, 2005, 3:09 am
So a stirring mechanism in the stagnant body would
increase the transfer efficiency greatly?

message

flow through the tube,

outside, the heat-transfer

thing.  I did some calcs

the heat transfer

BTU/ft^2-hr-F.

Posted by daestrom on December 22, 2005, 9:46 pm

Absolutely!  Mind you, Nick is big on allowing stratification, so he would
probably advise against a mechanical stirrer.  But a round tank, with
circular tubing in it shouldn't take much energy to keep the water spiraling
around nicely.

Even a modest 0.25 ft/s velocity (less than 1 RPM in a six foot diameter
tank) past the tubing will boost the coefficient to around 164
BTU/ft^2-hr-F.  Compare this to the inside of a 3/4" pipe with a flow of 3.5
gpm, where the heat transfer coefficient is over 775 BTU/ft^2-hr-F, and the
a 1/16" thick wall of copper tubing is over 41000 BTU/ft^2-hr-F.

These numbers are for 'single pass horizontal' 3/4" thin wall tubing.
Different diameters would yield different results.  For the natural
convection case, I assumed tank water of 150F.  If you need to make multiple
turns/passes, its best to stagger them so they aren't directly over each
other.  But that can be a bit harder to build.  Also be warned, the natural
convection number will drop to nil if the water is cold (e.g. below 50F).
Ethylene or propylene glycol will also hurt these numbers quite a bit.

daestrom

Posted by SolarFlare on December 23, 2005, 3:13 am
I was thinking something like two 3/4" copper pipes in
a spiral, soldered together down their lengths (or at
regular points) and then immersed in a thermal mass
tank with water, hoping to play the statification and
transverse feed heat exchange properities.

When both pipes are active transverese feed should
raise efficiency and when only the heat draw tube is
active the stratification should increase efficiency,
or at least the temp output.

Haven't thought this all out yet, but the ideas are
starting to come.

Thanx

message

stratification, so he would

round tank, with

keep the water spiraling

a six foot diameter

around 164

pipe with a flow of 3.5

BTU/ft^2-hr-F, and the

BTU/ft^2-hr-F.

thin wall tubing.

For the natural

you need to make multiple

directly over each

be warned, the natural

cold (e.g. below 50F).

numbers quite a bit.

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