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

*>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.*

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 assume the conduction coefficient of copper or the PEX must be known also.*

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

*>This calculation must be popular with the hydronic slab people...*

That has significant slab and airfilm resistance.

Nick

Posted by *nicksanspam* on December 19, 2005, 1:50 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...*

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.

*> >Unpressurized STSS ((717) 761-5838, sventjer at aol)*

heat storage tanks come

*> >with 120 or 180 foot x 3/4" copper pipe heat*

exchangers...

*> The STSS email address has changed to stsscoinc at*

comcast.net.

*> Nick*

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)

*>NTU = AU/Cmin = 47x60/(4x8x60) = 1.47.*

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

>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.