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Nick a thermal transfer question

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Posted by SolarFlare on December 19, 2005, 1:27 am
 
I know you have some experience with this and don't
just give the "yup, nope" answers so:

If I put a coil of copper tubing or even PEX tubing,
how do I calculate how much heat will be transfered for
a given length, dia, temperature diff, time in the tube
(velocity) etc.

I assume the conduction coefficient of copper or the
PEX must be known also. This calculation must be
popular with the hydronic slab people somewhere also?

Got an insight or info on this?

Thanx



Posted by nicksanspam on December 19, 2005, 12:05 pm
 


Unpressurized STSS ((717) 761-5838, sventjer at aol) heat storage tanks come
with 120 or 180 foot x 3/4" copper pipe heat exchangers. An 8' diameter x 6'
tall 1900 gallon tank with 2 120' heat exchangers in 22"x43" tall coils costs
about $K. The tall coils encourage stratification. The 120' coils are "rated
for 26,400 Btus at 120 degrees tank temperature for space heating." I'm not
sure what that means.
 
The Bucknell Small Business Development Center tested a 415 gallon tank with
2 120' coils plumbed in parallel. The whole tank was heated to 180 F and 4 gpm
of 48 F water flowed into the bottom of the coils for 68 minutes (274 gallons
total.) Over this time, the output temp dropped from 180 to 120 F.

Daestrom could probably do this better and Gary may have more numbers, but
I'd figure the still tank water limits conductance to about 60 Btu/h-F-ft^2.
With about 47 ft^2 of pipe surface, NTU = AU/Cmin = 47x60/(4x8x60) = 1.47.
With a capacity rate ratio close to 0, E = 1-e^(-NTU) = 0.77, so just after
the 6 gallons of stored coil water ran out, the output temp might have been
48+0.77(180-48) = 150 F.


I don't think we can ignore the lower PEX vs higher copper conductance.


That has significant slab and airfilm resistance.

Nick


Posted by nicksanspam on December 19, 2005, 1:50 pm
 
The STSS email address has changed to stsscoinc at comcast.net.

Nick


Posted by SolarFlare on December 20, 2005, 1:13 am
 Thanx. Will give me some figures and names to start
with.


heat storage tanks come

exchangers...

comcast.net.


Posted by Steve Shantz on December 21, 2005, 12:59 am
 Nick,

Thanks for the calculation example.  I ran your formula (as best I
could, see questions below) on my system design, and the numbers pretty
much came out as I have observed, fairly accurately predicting the
equilibrium temperatures which I observe in my glycol, based on the
heat input from my two panels and the temperature of my barrels. Your
flow equation also somewhat accurately predicts how the water
temperature changes with flow rate when I draw water through the top
copper coils in my barrels.

One part of your calculation I couldn't follow is when you calculate
NTU.  (What does this stand for anyway?  OK, I Googled it - Number of
Transfer Units)


The 47 is square feet of copper
The top 60 is your best guess for conductance
The bottom 60 is minutes per hour
The bottom 4 is flow rate in GPM???
What is the bottom 8 in the equation?

Also, do you think there is any advantage in partially crimping the
coil(s) every so often to encourage mixing in the tube(s)?  Pressure
drop coud be a problem here if crimped too much or too frequent, but I
have 8 - 1/2 x 20' copper tubes in parallel, so I don't think it would
hurt for pressure drop in my system.

Steve


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