How much heat through copper?

[Gyn_Paul]

Hi,

Trying to work out the the length of copper needed to make an effective

heat exchanger, but cannot find a table of the thermal conductivity of

2mm thick copper.

I will have 2 x 1500 L of solar-heated water through which the incoming

cold main will flow en route to the chauffeau via 2 (parallel) coils of

12/14 copper.

I know that a 10m roll of 12/14 has a surface area of 4.5m, and I'm

assuming the incoming cold main has a temperature of around 15C and the

tanks have a temp of - say - 65C (ever hopeful!) ...... now I'm stuck !

Can anyone help ?

paul

[edited to correct stupid errors]

[LesFlamands]

When you say heat exchanger, is it for a solar collector?

[condorman]

Bonjoooouuurrr,  Interesting this, i supose it depends on the quantity of water you will require, the 2 coils will hold x amount of liters, so your needing to know if the water at 35 deg. will after traveling up the 10 meters at what ever speed it needs to run at, be  close to the temp of the water jacket around it, humm its weather, if not at full flow, you can turn it down to get an aceptable flow rate at the temp you require,  I would as a test take your 10 meter coil and fix a hose pipe to one end, put the coil into a bath of water at the 65 degres, have a temp guage handy now turn on the tap, see what temp the water is as it exits the end of the copper tube, I make heat exchangers for fuel in cars and usualy run some mock up like this first, Ide be intersested in the results as ill soon be making some solar heating myself, best of luc Matey... condi

[powerdesal]

Incoming cold water main at 35 deg C ??????????? Where on earth do you

live ? Our incoming supply in Normandy, even in the middle of summer is

closer to 15 than 35 (guesswork),  althought, to be honest, I

haven't actually measured it, it just feels very cold.

Regards

[Gyn_Paul]

[quote user="powerdesal"]Incoming cold water main at 35 deg C ??????????? Where on earth do you

live ? Our incoming supply in Normandy, even in the middle of summer is

closer to 15 than 35 (guesswork),  althought, to be honest, I

haven't actually measured it, it just feels very cold.

Regards

[/quote]

Of course you're right: for a moment I slipped into a parallel world of

oF above oC . So I suppose I meant something like 10 - 15C

paul

[Gyn_Paul]

 

[Gyn_Paul]

[quote user="LesFlamands"]When you say heat exchanger, is it for a solar collector?[/quote]

Les,

Because of the linear nature of our house and barn, I shall be adding

adding a couple of chauffeaux as we finish bathrooms. So we will end up

with 3 full of hot water (the alternative is one large one which will

then be 8 - 10 m from any of the taps: much too far. I've had a house

where you ran the tap for half a minute before any hot appeared. Never

again!). So what I plan to do is feed the chauffeaux with cold mains

pre-heated en route.

I have a large delapidated hanger roof facing SSW which I shall have to

replace before the next gale carrys it off so since I'm replacing it I

shall put in several large solar panels, the heated water from which

will reside in 2 x 1500 L foam jacketed (oil) tanks. Turn on a hot tap

and the cold feed to the chauffeaux flows through the coils in the

tanks picking up heat en route. The elements in the chauffeaux being

required simply to 'top up' the temperature to 65C.

Yes, there will still be problems with dead legs, but at least it will

be free hot water between the coils and the chauffeaux going cold

rather than the paid-for hot between the chauffeaux and the taps, plus

there'll be no water wasted waiting for it to run hot.

I'm estimating 12 L/min flow rate through the coils 6 L/min/coil, or 12 L/min across 9 sqm

paul

[bejay]

Hi there

Thermal conductivity of copper =  380 W/m/degK but I dont think it will help you much.  I feel that the heat take up by the fast flowing mains water will be too small to be of much value, neither the surface area nor the temperature gradient is great enough. But do the sums and prove me wrong!

My I suggest the following alternative solution to your situation.

To avoid the problem of hot taps running cold, run the hot water from a single large Chauffeau in a pumped recirculating loop and tap off hot water where you nned to, rather like a ring main. The circuit would need to be well insulated.

To use the solar heated water get a chauffeau with a coil in it (its got a name which I can't remember as I write) and pump the heated water through the coil, again using a recirculating system. You will need a device to switch off this pump when the water in your solar tanks is cooler than the water in your hot storage tank. These are obtainable.

bj

 

[condorman]

Thats brilliant, Thank you all very much for your input, gratefully recieved, m

[Gyn_Paul]

[quote user="bejay"]

Hi there

Thermal conductivity of copper =  380 W/m/degK but I dont

think it will help you much.  I feel that the heat take up by

the fast flowing mains water will be too small to be of much

value, neither the surface area nor the temperature gradient is

great enough. But do the sums and prove me wrong!

My I suggest the following alternative solution to your situation.

To avoid the problem of hot taps running cold, run the hot water

from a single large Chauffeau in a pumped recirculating loop and tap

off hot water where you nned to, rather like a ring main. The circuit

would need to be well insulated.

To use the solar heated water get a chauffeau with a coil in it (its

got a name which I can't remember as I write) and pump the

heated water through the coil, again using a recirculating

system. You will need a device to switch off this pump when the water

in your solar tanks is cooler than the water in your hot storage tank.

These are obtainable.

bj

 

[/quote]

a CHAUFFEAU MIXTE is what you're thinking of I suspect. The only

drawback with this excellent idea is that - once having heated the 300

or so litres - there is nowhere else to store more potential hot

water. Here in La Creuse where the weather is currently vile, with

wall-to-wall grey,  p##sing rain and blowing a gale, it hard to

remember that for great stretches of the other 3 seasons we can have

days of  10 - 12 hours of flag-cracking sunshine.  Also, to

circulate round the heat main loop I'd need a hidiously expensive

bronze pump. I think I'll try a few tests and get back to you. First

thing I need to know is how much water a 10m x 12mm pipe contains.....

p

[Nick Trollope]

Unless my maths is miles out, you will need 120M of 12/14 pipe to get 4.5Msq of surface area (SA = Length x circumference = L x PI x diameter = 120 x PI x .012 = 4.52Msq)

The volume of this is SA of the end x the length = PI x r(adius)2(squared, can't do superscript) x 120M = .01357Mcubed = 13.57Litres

Assuming my maths in OK!!

[PeterG]

Iv'e read all this thread and now would somebody  like to ask me the sports question????????

[Gyn_Paul]

[quote user="nicktrollope"]Unless my maths is miles out, you will need

120M of 12/14 pipe to get 4.5Msq of surface area (SA = Length x

circumference = L x PI x diameter = 120 x PI x .012 = 4.52Msq)

The

volume of this is SA of the end x the length = PI x r(adius)2(squared,

can't do superscript) x 120M = .01357Mcubed = 13.57Litres

Assuming my maths in OK!!

[/quote]

Thanks Nick, you sure know how to rain on someone's parade!

I'd go for 100m as I seem to be out by a factor of 10! My paper tape

gives me a circumference of 4.5cms.  Ringing in my ears is the voice of

one of my maths master's saying, "calculators are all well and good,

always provided you know the order of the answer you are expecting."

If I had imagined, for a moment, chopping the roll up into 1 m lengths

then splitting and flattening the strips, it would have been obvious the strips wouldn't get to halfway across a square.

As to the volume, I'll let you know when I've filled it with water

tomorrow , too wet to paddle to the barn to get the roll now, dodging

through the curtain of rain sluicing off the (no gutter) roof!

paul

[bejay]

My you do paint a miserable picture of the the creuse.  I only know it in the summer months.

I take your point about the DHW circulating pump.

My suggestion was that you could circulate the water through the coil of the chauffeau mixte (thanks for that) from your larger tanks so you still have that heated water. The snag with his system is that it will only be working when the water in the large tanks is hotter than the water in the chauffeau and these occasions will be deeply unpredictable.You could of course send the water directly from the solar panels (which will be hotter more often then the water in the large tanks) through the chauffeau  and then divert it through the large tanks when it is cooler. This should be  possible with a mixture of C/H diverter valves and imagination.

I persevere with this recirculating principle because it is well used standard practice.

Your 10 m of pipe will hold 1.1 litre of water  At 12 l/min your it will be through your tank in 5 seconds when it is flowing.

The mains water will pick up heat when it is stationary  but littlle when it is on the move.

You can multiply this up for 100m of pipe but that is a lot of copper.

The conductivity of copper is pretty irrevelant here, you can regard the pipe as thermally transparent,  its more to do with volume/surface area ratios, type of flow, laminar or turbulent and the low conductivity of water itself.

This is, of course, why heat exchangers such as car radiator cores have many very narrow and flattened water channels and dozens of fins.

Alternatively!

Forget your storage tanks, build a swimming pool, put a collector pipe circuit in the pool connect it to a heat pump, circulate the pool water through the solar panels. You can extract the heat energy from the pool as you need to. If you overdo the heat extraction in the winter you might even get a bit of skating in, that is if it stops raining on you.

Shouldn't cost much!

bj

[bejay]

Sorry just testing something that doesn't seem to be working