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In a concentric double-pipe heat exchanger where one of the fluids undergoes phase change 

In a concentric double-pipe heat exchanger where one of the fluids undergoes phase change  Correct Answer The directions of flow of the two fluids are of no consequence 

Concept:

When one of the fluid in Heat Exchanger is undergoing a change of phase like,

1) The profile for Steam Generator is, 

[ alt="Heat Transfer ISRO Upload (1-51) images Q23a" src="//storage.googleapis.com/tb-img/production/17/05/Heat%20Transfer%20ISRO%20Upload%20%281-51%29_images_Q23a.PNG" style="height: 134px; width: 148px;">

2) The profile for Evaporator is,

 [ alt="Heat Transfer ISRO Upload (1-51) images Q23b" src="//storage.googleapis.com/tb-img/production/17/05/Heat%20Transfer%20ISRO%20Upload%20%281-51%29_images_Q23b.PNG" style="height: 134px; width: 150px;">

In both the cases, the temperature of one fluid remains constant. 

So temperature differences interchange at inlet and outlet if changed direction but they do not change in magnitude.

Hence, both counter flow and parallel flow will have same LMTD. 

LMTDcounter = LMTDparallel

  • For a concentric double pipe heat exchanger where one of the fluids undergo a phase change the direction of flow of the two fluids is of no consequences.
  • Because the (LMTD) logarithmic mean temperature difference of the two fluids in both cases i.e. parallel and counter flow are the same so that the heat transfer remains the same in both the cases.

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