Construction of a copper coil heat exchanger

[jonule]

you find plenty of them at scrap yards at the price of aluminum, but some are no longer very waterproof, they must be repaired.

but it must be able to be approached with car radiators yes, since any system that can emit heat can receive it (I am thinking of house radiators with fins painted in black serving as solar collectors) ;-)

[Christophe]

I got the doc.

For a simple copper pipe of 22 it takes a K of 700 to 1000 (to be defined).

It seems a little high (some industrially designed exchangers have a K of 600) but they probably have more experience than me

Example in our case:

Solar exchanger calculation

Sensor area = 65 m2
PST = 55,25 (heat exchanger power in kW = Solar power below)
Number of sensors = 1
Total collector area = 65 m2
Solar power (recoverable solar yield is included) = 850 watt per m2
Total solar power = 55,25 Kw
Delta T ° = 10 degrees
Pipe diameter in m = 0,016 m
P (Kw) = (CT x 3,14 x diameter in mx DT ° x length in m) / 1000 L (m) = (P (Kw) x 1000) / (CT x 3,14 x diameter in meter x DT °)

CT (from 700 to 1000) = 700

Coil area required = 1,97 m² (I added this line for information)

Length of the coil of diameter 16 mm = 157,10 meters

Please note I was not clear before: the exchangers in the photo below are not the solar exchangers (there is no buffer water between them directly in the panels). These are the exchangers to recover the calories from the buffer for use (Domestic Hot Water).

I think that our exchangers are largely oversized (exchange at low delta of T °), so the DHW which leaves is at the T ° of the buffer at 1 ° C ready.

[Christophe]

Looping, I think there is a small error in the 2nd .xls

The power (line 12) is in W while these are kW.

In the end it changes nothing in the results but it was to signal it, a mistake by a coef. 1000 is a task ...

It shows that with the thermal exchanges of water, we very quickly increase in Power!

[Christophe]

Second error (or rather too large an approximation) in the 2nd doc: the exchanger delta is calculated between the T ° of cold water and the T ° of the buffer. Whereas a calculation with the average T ° (cold DH + hot DH) / 2 would have been more judicious.

It greatly increases the length of the DHW exchanger which requires more surface than the main solar exchanger!

I fix it all and put it online.

[loop]

Hello

The file you call 2nd .xls (1st link cited above) allows you to calculate the heating coil for DHW
Indeed I confirm the first error on the units and your remark on the average temperature.
Well seen

The first spreadsheet (2nd link) calculates the length of the coil for transferring the power of the sensors to the water in the buffer tank.

For my project I will consider 15 m2 of sensors, 12.75kW of power (maximum I think), delta T of 5 °
Which gives me about 50M of D22 tube
We can add a line for the calculation of the exchange surface: 3.64 m2
Going to 30m2 of sensors my coil is still suitable but the T ° delta would go to 10 °, which is reasonable.

Commercial tanks often have large-sized exchangers. Are the coils made of copper? For enameling I think steel is more common.
They are often of a large diameter which makes it possible to make fewer turns.

A+

[Christophe]

Here I finished the correction / improvement and uped the 2 files in question:

1) The solar exchanger: https://www.econologie.com/calcul-d-un-e ... -3913.html
2) The DHW exchanger in thermal buffer:
https://www.econologie.com/calcul-d-un-e ... -3914.html

I noticed that the 2nd file can very well be used to size a heat exchanger of a low temperature heating floor (just adapt the T ° inlet and outlet and the flow according to the average heat loss of your house or more simply the flow of your circulator (but hard to be precise in this case))

In addition, the source is not necessarily solar: so a wooden pad can be very suitable (obviously)!

Contrary to what one might think: we quickly arrive at a very high power value for DHW of the order of 10 times that required for heated floors !! Funny huh?

If remarks or errors, I can obviously still modify these 2 files ...

[Christophe]

For my project I will consider 15 m2 of sensors, 12.75kW of power (maximum I think), delta T of 5 °
Which gives me about 50M of D22 tube
We can add a line for the calculation of the exchange surface: 3.64 m2
Going to 30m2 of sensors my coil is still suitable but the T ° delta would go to 10 °, which is reasonable.

Commercial tanks often have large-sized exchangers. Are the coils made of copper? For enameling I think steel is more common.
They are often of a large diameter which makes it possible to make fewer turns.

A+

Yes, it is in enamelled steel (but generally a simple enamelling on the tank side, ie on the running water side).
The diameter is to their advantage since it is the surface that counts.

To my knowledge there is no thermal buffer on the market which uses a DHW coil: the DHW is always rated balloon (large volume). And for good reason it should be very large (4 m² according to the file) and above all enamelled inside (or copper and still some too acidic water would eat copper!) ...

I paid for it during my first assembly of our wooden boiler, see the 1 evolutions of the assembly: https://www.econologie.com/forums/schema-cha ... t4266.html

ps: pkoi you want a delta of only 5 ° C? The larger the delta, the better the solar COPA, right?

[tigrou_838]

Hello everyone,

so after having redone my calculations with christophe's table, i arrive more or less at my 25m of copper in serpentine diam 20/22 that i had put in the tank.

when the floor heating, I do not have an exchanger, I take the hot water directly from the tank to send it to the floor heating, of course with thermostatic valve set to 24degrees.

thank you christophe.

tigrou

[Christophe]

Ah well well and with the original file of Apper you found how much? No doubt a little less right?

[tigrou_838]

hmmm

I was about in the same section at the level of diameter exchanger length calculation with the spreadsheet of the apper site, that's why I put 25m in copper 20/22

but it was three years ago. me and math we don't love each other too much and my memory less.