# In this scenario, if I continue to increase the length of heat exchanger how will the temperatures change?

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?

• Are you asking if the temperatures change after reaching equilibrium? Aug 17, 2021 at 13:27
• If you work it out, you'll find that L3 is infinite. Aug 17, 2021 at 13:51
• @AdrianHoward I was thinking that may be the 80 degrees will also rise to 100 if I keep of increasing the length. Aug 17, 2021 at 13:57
• @ChetMiller So increasing beyond L3 doesn't change the exit temperatures at all? Temperatures still remain 20 and 80. However, what limits 80 to not become 100, upon further increase. Aug 17, 2021 at 14:03
• If the length is infinitely long, what does increasing it even mean? Aug 17, 2021 at 14:07