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.