Cross-flow Heat Exchanger 2 phase calculation

1. May 6, 2015

cheetahunter

Hi all!
I'm working on a project of heating water from exhaust gases of kamin's chimney.
I have a cross flow heat exchanger (see pdf).
we have done calculations of heat transfer rate and the outlet temerature of the water, and got 150C.
so it probably gets boiled somewhere inside the HX. I need to do further calculation which will consider 2 phases of the water and new outlet water temperautre and heat transfer rate.
Does anyone can give me a direction?

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2. May 6, 2015

Staff: Mentor

Tell us a little bit about how you did the analysis (assumptions and equations) when you assumed that the water wasn't boiling. Is your objective to heat the water or to cool the exhaust gas? Are you trying to transfer as much heat as possible? How accurate does your answer have to be?

Chet

3. May 6, 2015

cheetahunter

I have used the e-ntu method. I have assumed that the water remains liquid (single phase) but then I got that the outlet temperature of the water reaches 150C .
I am trying to heat the water, and to transfer as much heat as possible.
I need to get atleast a range of outlet water temp's, but accurate answer will be great also

4. May 6, 2015

Staff: Mentor

Is the pressure 1 atm (roughly)?

Chet

5. May 7, 2015

yes.

6. May 7, 2015

Staff: Mentor

So, if the pressure is 1 atm., what is the temperature of the liquid/gas mixture once it starts boiling? So, recalculate the heat exchanger, for the part of the tubing up to this temperature, and find the corresponding length. Then recalculate the remainder of the heat exchanger holding the temperature fixed at this value. But, if all the water boils away before the end of the tube, you also need to consider a third section in which the water vapor is superheated.

Chet