Section 178 outlines a number of passive and active methods for the enhancement of heat transfer, and this section will deal with one of the most commonly-used passive methods, the tube insert. Inserts are becoming more widely used in both single- and multi-phase applications, although are more prominent in the former. Of the various passive enhancement methods that are suitable for single-phase applications, displaced enhancement devices, swirl-flow devices, or hybrids of the two are in common use.

In two-fluid heat exchangers, it is the ratio of thermal resistances that will be of primary importance in determining to which side of the exchanger an enhancement method should be implemented in order to be of benefit. Enhancement is employed in order to reduce the controlling thermal resistance and thus improve the overall heat transfer coefficient across the interface. Consider, for the purpose of illustration, a shell-and-tube exchanger running water on the shell side and oil on the tubeside, with the heat transfer coefficients for the shell and tubeside being 2.5 and 0.5 kW/m²K, respectively. Assuming a thin tube wall with negligible resistance and no resistance due to fouling, the overall thermal resistance would be given by

\(\dfrac{1}{U}=\dfrac{1}{\alpha_i}+\dfrac{1}{\alpha_o}=2.0+0.4=\) 2.4 m² K/kW

The proportion of the overall thermal resistance contributed by the tubeside would therefore be 2.0/2.4 = 83%. This system is clearly tubeside controlled, so would be a candidate for the use of tube inserts. Use of a shell side enhancement method (external fins, for example) would not have a significant impact on the overall heat transfer performance. Fundamentally, the use of any enhancement method serves to reduce the thermal resistance per unit length on the side of the exchanger in question. Any one of the following objectives may justify the use of tube inserts: