design of the heat exchanger...in chimney 
I want to design a heat exchanger in a chimney in order to utilize heat from chimney. I have done several experiments, but I could not determine the exact length of tube (carrying water), such that its inlet temp is ambient temperature and outlet temperature is expected to be  100 degree Celsius. I would be very thankful if some one would help me determine the length of the tube. tube diameter is 10mm (almunium), chimney diameter is(120 mm). 
Can anyone help me with formulas involved in calculating the heat transfer and length of the tube...
I already calculated the lenght experimentally but I could not do it mathematically.
 A: Briefly put: you need to know the inlet flow rate of exhaust gasses and the temperature and the power you want to extract. From the power (and your ambient temperature) you also get the flow rate for the water.
From this caclulate the log mean temperature difference (LMTD) along the heat exchanger, look for the heat transfer numbers for the configuration your using (one list can be found here - you'll see that there is a wide range of possible transfer values to consider) and you should be able to calculate the surface area you need.
A very brief explanation is also found on the wikipedia page for LMTD
Please also note the following: by cooling the exhaust, you may have less natural convection in your chimney, make sure your oven will still work. Also consider a margin of safety in sizing the exchanger, fouling will surely lower the heat transfer over time.
Last not least, this is an engineering question - support engineering stack exchange!
A: Be aware that the smoke to raise needs a temperature gradient between the top and bottom of the cheminey. If you extract too much power along the path you may end up smoked like a fish. An other problem one often encounters is tars liquefaction: below a critical temperature tars appear and flow on the cheminey walls. They are known to be acid, toxic and produce skin cancer.
To answer your question, you need to know the temperature at the bottom of the cheminey (it depends on the moisture in your wood), the flow of water, the distance between the heat exchanger and the flame, etc...
