Heat transfer and isotermal friction pressure drop results have been obtained experimentally for two single start spirally corrugated tubes combined with five twisted tape inserts with different relative pitch in the range of Reynolds number 4×10~3 - 6×10~4. The characteristic parameters of the tubes are: height to diameter ratio e/D_I = 15.4, 12.5, 7.9, 5.9, 4.8. Extended performance evaluation criteria equations for enhanced heat transfer surfaces have been used to assess the multiplicative effect. Thermodynamic optimum can be defined by minimizing the entropy generation number compared with the relative increase of heat transfer rate or relative reduction of heat transfer area.
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