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If we have tube in tube heat exchanger, in classic situation surface area in contact with both fluids is the same (if we ignore wall thickness of the tube).

If fluids have significantly different flow rates, for instance hot fluid 100 l/min and cool fluid 20 l/h, how would changing surface area in contact with one of those fluids while keeping surface area in contact with another fluid the same (for instance by adding fins to one side of tube), affect exit temperature of the cold fluid?

Fluids do not change phase in exchanger, and let us pretend that fins do not change the nature of the flow.

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The thing that really matters is not the heat transfer coefficient on each fluid side or the area on each fluid side, but the product of heat transfer coefficient and area on each fluid side. So say, if you add fins to the cold fluid side, you can model this either by considering and increase in area on the cold side, or an increase in heat transfer coefficient (as long as the the product properly accounts for the change). This resistance is then summed with the resistance on the hot fluid side to obtain the overall resistance.

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  • $\begingroup$ So, are you saying that adding fins can increase output temperature of the cold/slow side? Where should fins be, in the cold/slow or hot/fast side? $\endgroup$
    – Davorin Ruševljan
    Mar 16, 2016 at 22:47
  • $\begingroup$ You don't want to have fins on the inside of a tube. Cleaning becomes a problem. $\endgroup$
    – Chet Miller
    Mar 16, 2016 at 23:06
  • $\begingroup$ The total heat transferred between the hot and cold fluids would increase. $\endgroup$
    – Chet Miller
    Mar 16, 2016 at 23:13
  • $\begingroup$ Yes but I could switch circuits. What am I asking this, will fins help, and in which fluid they need to be imersed in order to do so. $\endgroup$
    – Davorin Ruševljan
    Mar 16, 2016 at 23:16
  • $\begingroup$ They will help most on the fluid side with the largest heat transfer resistance. If the heat transfer coefficient on one of the sides were very high, adding fins on that side wouldn't help much, right? $\endgroup$
    – Chet Miller
    Mar 16, 2016 at 23:19

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