Cross-flow Heat Exchanger 2 phase calculation

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Discussion Overview

The discussion revolves around the calculations involved in a cross-flow heat exchanger used to heat water from exhaust gases. Participants are exploring the implications of water transitioning from a single phase to two phases (liquid and gas) and the resulting effects on outlet temperature and heat transfer rate.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant describes their project involving a cross-flow heat exchanger and notes that their calculations indicate an outlet temperature of 150°C, suggesting boiling may occur.
  • Another participant requests details about the initial analysis, including assumptions and objectives, and questions the accuracy required for the results.
  • A participant mentions using the e-NTU method and initially assuming the water remains liquid, but acknowledges the outlet temperature reaching 150°C.
  • There is a query about the pressure being at 1 atm, which is confirmed by another participant.
  • A suggestion is made to recalculate the heat exchanger considering the boiling point of water at 1 atm and to account for the length of tubing needed for different phases of water (liquid and vapor).

Areas of Agreement / Disagreement

Participants generally agree on the need to consider the phase change of water in their calculations, but multiple approaches and assumptions are presented without a consensus on the best method to proceed.

Contextual Notes

Participants have not fully resolved the assumptions regarding the heat exchanger's operation under two-phase conditions, and the implications of boiling on the calculations remain open for further exploration.

Who May Find This Useful

Individuals interested in heat exchanger design, thermodynamics, and phase change phenomena in fluid systems may find this discussion relevant.

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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