The following conclusions may be drawn from the discussion made in this research monograph:

  • Insert devices are not good competitor with internally finned tubes or roughness for turbulent flow.

  • However, insert devices are good for laminar flow of viscous liquids.

  • Integral roughness is not good for laminar flow since the roughness is too small to enhance a laminar flow.

  • Insert devices are used to upgrade the performance of an existing heat exchanger having a plain inner tube surface

  • Major insert devices are twisted tapes, wire coil inserts, extended surface inserts, mesh or brush inserts and displaced inserts

  • Many design correlations are available for inserts, but this is not enough.

  • More research on tangential swirl injection is needed.

  • More information on performance characteristics with various insert devices is needed.

  • Research on fouling with insert devices is necessary.

  • Significant progress has been made in the design and manufacture of roughened surfaces. However, more advanced research is necessary.

  • A properly designed rough surface must have good efficiency index.

  • More and more advanced manufacturing techniques are likely to give better rough surfaces for enhancement.

  • Geometrical similarity makes the design of rough surfaces more convenient.

  • Although predictive methods have already made significant inroads, better predictive models are required for rough surfaces.

  • Heat transfer surface fouling is a matter of concern for rough surfaces.