i do not know their shape i hope you will attach a picture thanks in advance please it is urgent
plain tubes in heat exchangers does not have any thing attached to their exterior wall to dissipate energy. The only area they use to dissipate energy is their exterior area. While finned tube have some extra mass attached to their wall so as to increase area to increase heat dissipation. Car radiator has fins attached to their wall , the silvery thing you see, motor cycle engine is also finned to increase heat dessipation. while plain tube is very rarely seen.