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Consider a counter flow double pipe heat exchanger inside which the cold fluid enters at 20 degrees Celsius and hot fluid enters at 100 degrees Celsius. Initially the length of the heat exchanger is L1 and the temperature profiles are shown in the figure. If I continue to increase the length of the heat exchanger the temperature of the hot fluid at exit decreases and of the cold fluid at the exit increases. Let us say, at some length L3 the exit temperature of the hot fluid becomes equal to the inlet temperature of the cold fluid. What happens to the exit temperatures and temperature profiles once I continue to increase the length futher?

enter image description here

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Nothing will change beyond L3.

As you increase the length of the heat exchanger, the hot will reach the cool temperature, and beyond that point there will be no change. So beyond L3, the temperature of the cool flow and the hot flow will be equal to the cool flow inlet temperature $t_{c,i}$.

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  • $\begingroup$ Oh. What stops 80 degrees to not reach 100, as I continue to increase the length? $\endgroup$ Aug 17 at 14:19
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    $\begingroup$ Where is the extra heat coming from to raise the temperature higher? There is no more heat flow from the hot stream when its temperature has fallen to 20. However if you reduce the mass flow of the cold stream, it could then reach a higher temperature than 80. $\endgroup$
    – alephzero
    Aug 18 at 17:07

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