heat exchanger design calculations [repost]

[Apologies for reposting but I sent this one out yesterday and oddly, although I can see it on Google, it hasn't shown up on my local news server (text.news.ntlworld.com), so I'm wondering if the lack of followups is due to others not having seen it either rather than (or perhaps as well as) no-one finding it faintly interesting or being able to help :-)]
Anyone know where I can get figures for calculating (allright, guesstimating :-) rates of heat transfer through pipes for a diy heat store heat exchanger? I'm planning to hang say a 25m coil of 10mm table Y copper pipe in a tank connected to the CH primary and wondering what rates I can expect to get water out at given inlet and outlet (and tank) temperatures.
Obviously I can suck it & see but it'd be nice to have some sort of theoretical figures to start with (and encourage me to actually have a go with the idea :-).
tia
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wrote:

---8<---
Thanks, so maybe it's just NTL losing one of their own [l]users' posts :-)
Presumably it didn't appear on You-Know-Who's server ;-)
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Oh it probably did, but we all know that he deals mainly in opinions and suppositions and not in hard figures, and you specifically asked for hard figures...
...but then that doesn't *usually* stop him ;-)
Hwyl!
M.
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wrote:

But he does know what he is on about.
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On Tue, 15 Jul 2003 23:00:16 +0100, "John Stumbles"

Had you thought of contacting one or other of the cylinder manufacturers and asking them for information on their water cylinders?
e.g. you could ask for the kW rating of the coil and its water content. From the water content you can calculate the pipe surface area. The heat transfer rate will be dependent on surface area.
Another source might be the Copper Development Association who should be able to give some data at least.
Theoretically, the maximum rate of transfer is going to come from the famous equation of mass x specific heat x temperature rise.
However, I can see at least two factors that will reduce that.
1) the thermal resistance of the copper and the area.
2) the thermal gradient from the top to bottom of the tank - unless you stir the water of course.
Are you doing this as an experiment or on a low budget?
If not, I can't help thinking that using a plate heat exchanger external to the tank might be a better (or rather more predictable option.
For a slightly different application of coupling a second CH circuit that feeds my workshop to the main house CH circuit, I used one made by GEA Ecobraze. This was a small unit, a little larger than a house brick, but able to transfer up to 200kW. Since that is much larger than the amount of heat required to be moved (I need less than 10kW) the effect of the exchanger itself can be ignored and the rate is determined by the flows on each side.
.andy
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Most heat exchange properties can be calculated using Newton's Laws on Materials Cooling, The Fourier Modelling Equation and The Stephan-Boltzmann Rule, all of which give calculations for flow rate and flow restriction against surface area. I think that they all roughly state that a longer wider diameter (greater surface area), higher temperature pipe, with a slow replenishing flow gives the best results.
The same equations are used in the calculations of heat sink properties on electronics equipment, which I think a few people using the group will be able to give more details of.
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The calculate a heat exchanger a half page equation is generally used. So try this on-line calc software: http://www.freecalc.com/hxfram.htm
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