Cross-flow Heat Exchanger 2 phase calculation

AI Thread Summary
The discussion centers on calculating the performance of a cross-flow heat exchanger used to heat water from exhaust gases. The initial calculations indicate an outlet temperature of 150°C, suggesting that boiling may occur within the heat exchanger. Participants discuss the need to account for two phases of water—liquid and vapor—and the implications for heat transfer rates. The e-NTU method is mentioned as a calculation approach, with emphasis on the importance of accurately determining the outlet temperature and the heat transfer efficiency. The conversation highlights the necessity of recalculating based on the boiling point at 1 atm pressure and considering the potential for superheating if all water vaporizes.
cheetahunter
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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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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
 
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
 
cheetahunter said:
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
Is the pressure 1 atm (roughly)?

Chet
 
yes.
 
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
 

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