Heat transfer and isothermal friction pressure drop results have been obtained experimentally for two three-start spirally corrugated tubes combined with five twisted tape inserts with different relative pitches in the range of Reynolds number 3 x 10(3)-6 x 10(4), The characteristic parameters of the tubes are: height to diameter ratio, el D-i=0.0407 and 0.0569; and relative pitch, H/D-i=15.3, 12.2, 7.7, 5.8, 4.7. Significantly, higher friction factor and inside heat-transfer coefficients than those of the smooth tube under the same operating conditions have been observed. Extended performance evaluation criteria (PEC) 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. (C) 2001 Elsevier Science Ltd. All rights reserved.